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Revision 1.00
FS752 G NSS Disciplined
Time and Frequency
Reference
User Manual
FS752 GNSS Time and Frequency Reference Stanford Research Systems
Certication
Stanford Research Systems certifies that this product met its published specifications at the time
of shipment.
Warranty
This Stanford Research Systems product is warranted against defects in materials and
workmanship for a period of one (1) year from the date of shipment.
Service
For warranty service or repair, this product must be returned to a Stanford Research Systems
authorized service facility. Contact Stanford Research Systems or an authorized representative
before returning this product for repair.
Information in this document is subject to change without notice.
Copyright © Stanford Research Systems, Inc., 2019. All rights reserved.
Stanford Research Systems, Inc.
1290-C Reamwood Avenue
Sunnyvale, California 94089
Phone: (408) 744-9040
Fax: (408) 744-9049
ww w.thi nkS RS .c om
Printed in the U.S
FS752 GNSS Time and Frequency Reference Stanford Research Systems
i Table of Contents
FS752 GNSS Time and Frequency Reference Stanford Research Systems
Contents
Contents i
Revisions v
Safety and Preparation for Use vii
Symbols You May Find on SRS Products viii
Specifications ix
Remote Interface Commands xiii
Quick Start Instructions 1
Installing the FS752 1
Installing the Antenna 1
Selecting a Location for the Antenna 1
Antennas Offered by SRS 2
Active Antenna Required 2
Antenna Warning LED 2
Antenna Delay Correction 2
GNSS-Outdoor Antenna Kit (Model O740ANT2) 3
Outdoor Antenna Kit Contents 3
Design Considerations 4
Mounting the Antenna 5
Lightning Protection 5
Troubleshooting 7
Global Navigation for Timing 9
FS752 Feature Overview 9
Theory of Operation 10
Global Navigation Systems 10
How Does GPS Work 10
Using GPS Satellites for Timing 11
Position Survey for Improved Timing 11
Locking to GNSS Satellites 12
Phase Lock Loop Design 12
Predictive Filtering 13
Timebase Stability 13
Adaptive Bandwidth 14
Losing Lock to GNSS 14
Monitoring Lock to GNSS 14
FSM States 15
Power On 15
Search 16
Stabilize 16
Table of Contents ii
FS752 GNSS Time and Frequency Reference Stanford Research Systems
Validate 16
Lock 16
Holdover States 17
Recovering From Holdover 17
System Alarm 17
Timebase Events 18
Instrument Operation 19
Front Panel 19-
Display 19
Rear Panel 19-
AC Power 19
USB 20
Alarm Relay 20
Antenna Input 20
1 PPS and 10 MHz Distribution 20
Standard Distribution 20
Option 20al Distribution
Front Panel Operation 21
Installation and Power 21
Navigating the Display 21
Main Displays 21
TIME / DATE 21
∆1 PPS 22
SATS / SNR 22
Alternate Displays 22
Position 22
Alarm 22
Timebase 23
Startup Displays 24
Timebase Status 24
USB Status 24
Error Reporting 25
GnssDO Application 25
Installation Requirements and Setup 25
Instrument Status 25
Configuration 26
Console 27
Configuration 29
Timebase Configuration 29
Lock to GNSS 29
Loop Bandwidth 29
Manual Loop TC 30
Criteria to Enter Holdover 30
Criteria to Leave Holdover 31
Wait for Good 1 PPS 31
Table of Contents iii
FS752 GNSS Time and Frequency Reference Stanford Research Systems
Jump to Good 1 PPS 31
Slew to Good 1 PPS 31
GNSS Receiver Configuration 32
Constellations Tracked 32
Timing Alignment 32
Timing Quality 33
Survey 34
Disabled 34
Redo Survey at Power On 34
Remember Survey Results 34
Position Fixes in Survey 34
Antenna Corrections 34
Local Time Offset 35
Alarm 35
Alarm Mode 35
Tracking Current Condition 35
La 35tch Alarm Condition
Manually Set State 36
Alarm Conditions 36
1 PPS Output 36
Time Offset 36
Factory Default Settings 37
Forcing Instrument Settings to Factory Defaults 37
Remote Programming 39
Introduction 39
USB 39
Virtual RS- 232 COM Port 39
Front 39-Panel Indicators
SCPI Command Language 40
SubSystems 40
Understanding Command Syntax 40
Keyword Case 41
Punctuation Used in Definitions 41
Examples 41
Queries 41
Separators 42
IEEE 488.2 Common Commands 42
Parameter Types 42
Numeric Values 42
Units 43
Discrete Parameters 43
String Parameters 43
Command Termination 43
Status Reporting 44
Architecture 44
Condition Register 44
Table of Contents iv
FS752 GNSS Time and Frequency Reference Stanford Research Systems
Event Register 45
Enable Register 45
FS752 Status 46
Serial Poll Status Byte 46
Standard Event Status Register 48
Questionable Status 49
Operation Status 50
GPS Receiver Status 51
Common IEEE-488.2 Commands 52
GPS Subsystem 57
Source Subsystem 63
Status Subsystem 64
System Subsystem 67
Timebase Subsystem 74
Error Codes 81
Command Errors 81
Execution Errors 82
Device Specific Errors 82
Query Errors 82
Instrument Errors 83
FS752 Circuit Description 85
Overview 85
Timebase and Oven 85
GNSS Receiver and Time Tagging 86
1 PPS Outputs 87
10 MHz Outputs 87
Microcontroller, Communications, and DACs 88
Power Supplies 88
Appendix A: Parts List 91
Appendix B: Schematic Diagrams 97
v Revisions
FS752 GNSS Time and Frequency Reference Stanford Research Systems
Revisions
Rev
Date
Changes
1.00
1/1/19
First release
Safety and Preperation for Use vii
FS752 GNSS Time and Frequency Reference Stanford Research Systems
Safety and Preparation for Use
Line Voltage
The instruments operate from a 90 to 132 V
AC or 175 to 264 V
AC power source having a
line frequency between 47 and 63 Hz. Power consumption is less than 80 VA total. This
instrument is intended to be powered at all times. Therefore, there is no power switch.
Power is applied to the instrument as soon as the line cord is plugged in.
Power Entry Module
A power entry module, labeled AC POWER on the back panel of the instrument,
provides connection to the power source and to a protective ground.
Power Cord
The unit is shipped with a detachable, three-wire power cord for connection to the power
source and protective ground.
The exposed metal parts of the box are connected to the power ground to protect against
electrical shock. Always use an outlet which has a properly connected protective ground.
Consult with an electrician if necessary.
Grounding
BNC shields are connected to the chassis ground and the AC power source ground via
the power cord. Do not apply any voltage to the shield.
Line Fuse
The line fuse is internal to the instrument and may not be serviced by the user.
Operate Only with Covers in Place
To avoid personal injury, do not remove the product covers or panels. Do not operate the
product without all covers and panels in place.
Serviceable Parts
There are no user serviceable parts. Refer service to a qualified technician.
Safety and Preperation for Use viii
FS752 GNSS Time and Frequency Reference Stanford Research Systems
Symbols You M ay Find on SRS Products
Symbol Description
Alternating Current
Caution – risk of electrical shock
Frame or Chassis terminal
Caution – refer to accompanying document
Earth (ground) terminal
Battery
Fuse
Power On
Power O
Power Standby
Specifications ix
FS752 GNSS Time and Frequency Reference Stanford Research Systems
Specifications
OCXO Timebase
Oscillator type Double oven controlled
Temperature stability < ×101-9 (2 0 to 30°C)
Aging (undisciplined to GNSS) <0.05 ppm/year
Phase noise at 10 Hz offset 25 (SSB) <−1 dBc/Hz
Stability <5×10-11
(1 second) See graphs next page
Holdover <40 µs / 24 hr
x Specifications
FS752 GNSS Time and Frequency Reference Stanford Research Systems
Specifications xi
FS752 GNSS Time and Frequency Reference Stanford Research Systems
G NSS Receiver
Model u-blox -M8T , NEO
Power on to satellite acquistion <1 minute (typical)
Time to acquire almanac ~15 minutes when continuously tracking satellites
Optimized for static applications Over determined clock mode enables receiver to use all
satellites for timing.
Accuracy of UTC < 100 ns
Timing wander 20 < ns rms, clear sky
Antenna dela 32.767 µs y correction range ±
1 pps Output
Period 1 s
Width 10 µs
Phase Offset Range 1 second ±
Phase accuracy to internal reference <2 ns
Jitter <50 ps (rms)
Level +5 V CMOS logic
Transition time <2 ns
Source impedance 50 Ω
Reverse protection ±5 VDC
10 MHz Output (50 Ω load)
Amplitude 13 dBm
Amplitude accuracy 1 dB ±
Harmonics <–40 dBc
Spurious <–90 (100 kHz BW) dBc
Phase settability Phase can not be adjusted.
Output coupling 2 % DC, 50 Ω ±
User load 50 Ω
Reverse protection ±5 VDC
Computer Interfaces (standard)
USB Virtual COM port with FTDI drivers
115.2k baud, 8 bits, no parity, 1 stop bit, RTS/CTS flow
Option A: 10 MHz Distribution
Number of outputs 4
Specifications Same as standard 10MHz output
Option DistributionB : 1 PPS
Number of outputs 4
Specifications Same as standard 1 PPS outputs
Specifications xii
FS752 GNSS Time and Frequency Reference Stanford Research Systems
General
Line power 0 W, 90 to 264 V<3 AC, 47 to 63 Hz with PFC
EMI compliance -FCC Part 15 (Class B), CISPR 22 (Class B)
Case dimensions 17 2 2” × ” × 1 ” (W × H × D)
Weight 10 lbs <
Warranty on parts and labor One year
Commands xiii
FS752 GNSS Time and Frequency Reference Stanford Research Systems
Remote Interface Commands
Common Commands
Page
*CLS
52
*ESE
52
*ESR?
52
*IDN?
53
*OPC
53
*OPT?
53
*PSC
54
*RCL
54
*RST
54
*SAV
54
*SRE
55
*STB?
55
*WAI
56
GPS Subsystem
Page
GPS:CONFig:CONStellation
57
GPS:CONFig:MODe
57
GPS:CONFig:SAVe
58
GPS:CONFig:SURVey:Mode
58
GPS:CONFig:SURVey:FIXes
58
GPS:CONFig:ALIGnment
59
GPS:CONFig:QUALity
59
GPS:CONFig:ADELay
59
GPS:POSition?
60
GPS:POSition:HOLD:STATe?
60
GPS:POSition:SURVey
60
GPS:POSition:SURVey:DELete
61
GPS:POSition:SURVey:PROGress?
61
GPS:POSition:SURVey:SAVe
61
GPS:POSition:SURVey:STARt
62
GPS:POSition:SURVey:STATe?
62
GPS:SATellite:TRACking?
62
GPS:SATellite:TRACking:STATus?
62
GPS:UTC:OFFSet?
63
Source Subsystem
Page
SOURce:PHASe
63
SOURce:PHASe:SYNChronize
64
SOURce:PHASe:SYNChronize:TDELay
64
Status Subsystem
Page
STATus:GPS:CONDition?
64
STATus:GPS:ENABle
65
Commands xiv
FS752 GNSS Time and Frequency Reference Stanford Research Systems
STATus:GPS:EVENt?
65
STATus:OPERation:CONDition?
65
STATus:OPERation:ENABle
66
STATus:OPERation:EVENt?
66
STATus:QUEStionable:CONDition?
66
STATus:QUEStionable:ENABle
67
STATus:QUEStionable:EVENt?
67
System Subsystem
Page
SYSTem:ALARm?
67
SYSTem:ALARm:CLEar
68
SYSTem:ALARm:CONDition?
68
SYSTem:ALARm:ENABle
68
SYSTem:ALARm:EVENt?
69
SYSTem:ALARm:FORCe:STATe
69
SYSTem:ALARm:MODe
69
SYSTem:ALARm:GPS:TINTerval
70
SYSTem:ALARm:HOLDover:Duration
70
SYSTem:COMMunicate:SERial:BAUD
70
SYSTem:COMMunicate:SERial:RESet
71
SYSTem:COMMunicate:LOCK?
71
SYSTem:COMMunicate:UNLock?
71
SYSTem:DATe
72
SYSTem:DISPlay:SCReen
72
SYSTem:ERRor?
73
SYSTem:SECurity:IMMediate
73
SYSTem:TIMe
73
SYSTem:TIMe:LOFFset
73
SYSTem:TIMe:POWeron
74
Timebase Subsystem
Page
TBASe:CONFig:BWIDth
74
TBASe:CONFig:HMODe
75
TBASe:CONFig:LOCK
75
TBASe:CONFig:TINTerval:LIMit
75
TBASe:EVENt:CLEar
76
TBASe:EVENt:COUNt
76
TBASe:EVENt:NEXT?
76
TBASe:FCONtrol
77
TBASe:FCONtrol:SAVe
77
TBASe:STATe?
77
TBASe:STATe:HOLDover:DURation?
78
TBASe:STATe:LOCK:DURation?
78
TBASe:STATe:WARMup:DURation?
78
TBASe:TCONstant
79
TBASe:TINTerval
79
Quick Start Instructions 1
FS752 GNSS Time and Frequency Reference Stanford Research Systems
Quick Start Instructions
Installing the FS752
To setup and install the FS752 follow these steps:
1. Install the GPS antenna in a location that has a clear view of the sky, such as the
roof of a building. See below for details.
2. Connect the cable from the antenna to the rear panel input of the labeled FS752
ANTENNA INPUT.
3. FS752 by plugging in the AC power cord. Power on the
If you don’t care about the absolute accuracy of the FS752’s phase relative to UTC then
setup is complete (otherwise, see section Conguration) . The FS752 will automatically
search for the G NSS satellites and lock its internal timebase to them. Once locked, the
FS752 may take up to 1 hour or more to fully stabilize. When the is fully FS752
stabilized the front panel locked and stable LEDs should be on.
Installing the Antenna
Selecting a Location for the Antenna
The signals broadcast by the GNSS satellites are extremely weak and difficult to detect.
Generally speaking, you will get best results if the antenna has a clear unobstructed view
of the sky. This is commonly on the roof of the building within which the FS752 is
located. If this is not possible, the user can try locating the antenna at a window. Doing
so, however, may degrade the quality and reliability of the GPS signal as fewer satellites
will typically be visible and with less SNR. This can degrade the long-term stability of
the bFS752 y a factor of three.
Quick Start Instructions 2
FS752 GNSS Time and Frequency Reference Stanford Research Systems
Antennas Oered by SRS
SRS oers two antenna solutions, an indoor antenna and an outdoor antenna. The indoor
antenna includes the antenna and 23 feet of RG58 cable to connect the antenna directly
to the FS752. The outdoor solution includes the antenna, 75 feet of low loss cable, and a
complete solution for protecting the equipment from lightning strikes. The outdoor kit is
described in detail below.
Active Antenna Required
The antennas provided by SRS are active antennas. An active is requiredantenna with
the FS752 to ensure sufficient signal is available at the receiver input. The FS752
supplies 5 VDC to the antenna cable to power the active antenna. Active antennas
typically have more gain and improved signal detection capability over passive antennas.
They also help to overcome the signal loss introduced by long antenna cables and,
therefore, are necessary.
Antenna Warning LED
The orange LED on the front panel labeled ANTENNA warns if the FS752 detects that
the antenna input is either an open or short circuit. It typically goes o when the antenna
is properly connected to the . FS752
The state of the LED, however, is based solely on the DC current draw on the antenna
cable, not on measured signal levels. If the current draw is less than 5 mA, the FS752
will warn of an open circuit. If the current draw is greater than 75 mA, the FS752 will
warn of a short circuit. The total current draw for the antenna is limited to 300 mA.
Please note that the performance of the is not impacted at all if the power draw of FS752
your antenna falls outside this range, The FS752’s GPS receiver will continuously
search for GPS satellites regardless of the current draw on the antenna. The warning
LED is merely meant to be a helpful diagnostic to consider when troubleshooting a lack
of signal for the most common installations.
Antenna Delay Correction
The FS752’s estimate of UTC is based on when the receiver detects the signal, not when
the signal arrives at the antenna. If a long cable is needed to connect the antenna to the
FS752, this delay can be signicant. The typical delay for most BNC cables is 1.5417 ns
per foot. Thus, for a 30 = ft cable, the delay would be 1.5417×30 46.25 ns. If
uncorrected, the FS752’s estimate of UTC would be 46. ns later than a properly 25
calibrated unit. See section Conguration for details on how to incorporate this
correction into the instrument operation.
Quick Start Instructions 3
FS752 GNSS Time and Frequency Reference Stanford Research Systems
GNSS-Outdoor Antenna Kit (Model O740ANT2)
The GNSS-Outdoor Antenna Kit consists of components for the construction of a robust
GPS/GLONASS antenna system. The kit includes a Trimble Bullet III GNSS
omnidirectional antenna with LNA and TNC connector on a short aluminum mast. The
mast has a cable access slot, silicone weather plug, and a grounding lug for lightning
protection. The cap at the bottom of the mast is tapped for ¼” 20 and bolted to a -
magnetic mount which has a 90 lbs. (40 kg) pull rating.
The magnetic mount may be removed to allow for mounting to a shelf, bracket or
cabinet. The included die cast aluminum bracket allows mounting to a wall or pole. A
lightning surge arrestor and 100’ (30 m) of 10 AWG copper wire is provided for
lightning protection. Two lengths (25’ and 50’) of low loss, 0.400” diameter, 50 Ω TNC
extension cables (male-female, not RP), and a TNC to BNC adapter to connect to the
SRS GNSS receiver are included.
The +32 dBi gain antenna will provide a +20 dBi signal to the receiver, allowing for up
to 12 dB of cable loss. The 0.400” cables allow cable lengths up to 200’ (60 m). Inline
GPS amplifiers are available from third party vendors if the antenna is more than 200’
from the receiver.
Outdoor Antenna Kit Contents
The contents of the outdoor antenna kit are detailed in Table 1. The main components
are pictured in 1. Figure
Table 1 : Outdoor antenna contents
Qu antity in Kit Description
1
1
1
1
4
4
1
1
1
1
20
1
2
1
Quick Start Instructions 4
FS752 GNSS Time and Frequency Reference Stanford Research Systems
*** Hardware item is located in polybag.
Figure 1: Outdoor antenna kit
Design Considerations
There are many considerations for the design of an outdoor GNSS antenna, including:
1. An unobstructed view of the sky. If obstructions a re unavoidable, a clear view of
the southern sky is preferred. Also, avoid antenna placement with multipath
opportunities (reflections from other structures).
2. Use of cable types and lengths with less than 12 dB of loss at 1.6 GHz between
the antenna and the timing receiver. (The cables included with the kit have a
total loss of 5.6 dB.)
Quick Start Instructions 5
FS752 GNSS Time and Frequency Reference Stanford Research Systems
3. Sufficient height so that the antenna will not be buried by more than 1 foot of
snow.
4. Strategies to avoid lightning strikes. Avoid being the highest metal object
(which, unfortunately, conflicts with a clear sky view and avoiding multipath).
5. A strategy to handle a lightning strike. This is a complicated and important topic
which must be addressed to insure the safety of personnel and reduce equipment
damage. The antenna mast and inline lightning surge arrestor, included with the
antenna kit, must be attached to a grounded structure, or connected to earth
ground via a grounding rod.
6. Compliance with local building and electrical codes.
7. Compliance with building lease term and easements.
When designing the outdoor GNSS antenna system, site specific designs and the use of
other materials will be required. If additional cables are needed they should have a TNC
male on one end and a TNC female connector on the other, so that they may be used as
extension cables without coax barrels. These are RP cables (which reverse the pin not
and socket of conventional connectors). The cables sold by SRS are not plenum rated.
For additional TNC cables we suggest part number 28-463-050 (a 50’ cable with a
typical loss of 4.2 dB) available from http://www.showmecables.com/
Mounting the Antenna
The required connections for installing the antenna are highlighted in Figure 2. The
simplest semi-permanent installation uses the magnetic antenna mount placed on top of a
HVAC unit on the roof. If HVAC placement is not available, the magnetic mount can be
attached to a 10 lbs. weight (barbell weights work well for this purpose) and placed on
the roof.
Alternatively, the magnetic mount can be removed allowing the antenna to mount to a
wall or pole using the included mounting arm. The antenna may also be mounted to any
horizontal surface or brace which has a ¼” diameter hole. A 3/16” hex wrench is
included in the kit to remove the magnetic mount from the antenna mast. The hardware,
including fiber washers to break galvanic contact, should be reused.
To connect the antenna to the coax cable, first remove the antenna from the mast.
Observing the gender of both ends of the cable, thread the male end of the cable through
the oval slot in the antenna mast. Remove the plastic protector from the antenna
connector. Screw the cable on to the antenna connector finger tight. Push the cable down
into the mast and screw the antenna onto the mast. (The TNC connector is free to rotate,
and the direction of rotation will not cause the TNC to loosen.)
Lightning Protection
There are two important components for lightning protection: A ground wire attached
directly to the aluminum antenna mast, and a lightning arrestor/surge absorber located
where the coax cable enters the building. The arrestor has a TNC male connector on one
end and a TNC female on the other and a lug for earth ground in the center. Both the
antenna mast ground and the lightning arrestor must be separately connected to earth
ground with #10 AWG copper wire (included with kit). All outdoor TNC connectors,
Quick Start Instructions 6
FS752 GNSS Time and Frequency Reference Stanford Research Systems
including those attached to the lightning arrestor, should be protected from weather and
sunlight exposure with the included silicone tape wrap.
Figure 2: Antenna installation
Quick Start Instructions 7
FS752 GNSS Time and Frequency Reference Stanford Research Systems
Troubleshooting
Symptom
Solution
No satellites found, or stuck
searching for GPS.
Most likely caused by low SNR due to a poorly
positioned antenna. Verify that the connection to
the antenna is good. Then try moving the antenna to
a location with a clear view of the sky to see if this
improves the situation. For good results, the unit
should have an SNR > 40 once satellites are found.
Instrument is tracking satellites
but reports “no pps”.
When locked to only GPS satellites, the receiver
may take up to 12.5 minutes to receive the
information needed to align the 1 PPS pulse output
to UTC. Until the UTC information is received, the
receiver will hold off generating the 1 PPS timing
pulses. This is generally not a problem when
simultaneously atellites as locked to GLONASS s
their timing is already aligned to UTC.
Global Navigation for Timing 9
FS752 GNSS Time and Frequency Reference Stanford Research Systems
Global Navigation for Timing
F Feature S752 Overview
The Frequency FS752 GNSS Disciplined Time and Reference is designed to provide
continuously calibrated time and frequency distribution to a local laboratory. Calibration is
maintained by locking its internal reference to the time-of- the day signals broadcast by one of
various GNSS constellations of satellites. The satellites which make up the GNSS network are
equipped with rubidium and cesium clocks which are monitored and controlled by government
agencies to ensure that the system maintains synchronicity with UTC. G The FS752’s NSS
receiver can track any of the four major GNSS constellations: GPS, GLONASS, BEIDOU, or
GALILEO. In fact, two constellations can be tracked simultaneously. By default, the FS752 is
configured to track both GPS and GLONASS. Simultaneous tracking improves quality and
reliability by increasing redundancy and enabling the receiver to incorporate independent timing
solutions into its estimate of UTC.
The FS752’s receiver specifically designed to generate precise timing is also . by It achieves this
performing an extended survey of its position. Once the position is accurately known, all satellite
information may then be dedicated toward improving the timing solution. This not only
improves the quality of the timing, but also improves overall robustness by enabling the receiver
to generate timing pulses with as little as 1 satellite in view. The FS752 can align its 1 PPS to
UTC provided by the receiver with a relative precision of 50 ps, an RMS deviation of less than
15 ns and an absolute accuracy of 100 ns.
The FS752 10 outputprovides five buffered MHz s and two buffered 1 PPS digital outputs
aligned to UTC. 10 The MHz outputs generate 1 Vrms into 50 Ω and may be used as frequency
references for laboratory equipment. The 1 , with 5 s 10 sPPS output generate µs pulse V CMOS
logic and s rising edge aligned to UTC. The 1 PPS outputs can be arbitrarily advanced or delayed
(as a group) to account for cable delays if desired.
Still more distribution may be installed with the purchase of two distribution boards, with one or
four additional outputs each. Option A provides four 10MHz outputs. Option B provides four
1 PPS outputs. Any combination of option boards may be installed to better match customer
application requirements. A unit with two Option A boards installed will have the ability to
distribute thirteen buffered copies of the 10 MHz: five standard copies, and eight additional
copies included on the two option boards.
Global Navigation for Timing 10
FS752 GNSS Time and Frequency Reference Stanford Research Systems
Theory of Operation
G lobal Navigation Systems
Global navigation systems are satellite-based systems designed to provide a user with
inexpensive, yet precise, position and timing information. There are four major
constellations in operation today as detailed in 2. Table
Table 2: GNSS Navigation Systems
GNSS System
Controlling Governments
GPS
United States
GLONASS
Russia
BEIDOU
China
GALILEO
European Union
GPS was the first navigation system deployed in 1995. It is the oldest and most mature
navigation system in operation today. Russia soon followed with GLONASS. More
recently, China’s BEIDOU and the European Union’s GALILEO navigation systems are
coming online.
How Does GPS Work
All four systems have similar performance and specifications. While the details between
the various systems differ, the basic theory of operation is the same. For simplicity of
presentation, we will highlight the various components that make up the GPS system.
However, the basic conce systemspts apply to other as well.
The Global Positioning System, or GPS, is a radio-navigation system which allows users
with a clear view of the sky to identify their current position and time of day from any
location around the globe. The system was originally designed for the military, but has
been used for a wide variety of civil applications since its deployment, particularly in ian
automobile and marine navigation. The system is managed and controlled by the United
States Air Force (USAF). It consists of three parts: a space segment, a ground-based
control segment, and the user segment.
The space segment consists of 24 satellites orbiting earth in 12-hour orbits at an altitude
of approximately 20,200 km. The satellites are arranged into 6 equally spaced orbital
planes with 4 satellites allocated to each plane. The organization is designed so that a
user will always be able to view at least 4 satellites from virtually any location around
the world. Atomic clocks on board the satellites maintain precise time of day
information. The satellites then broadcast ephemeris and time of day information to the
user segment. Using trilateration, a receiver with a view of at least 4 satellites can use
this information to locate position and time. its
The control segment monitors the health of the satellites and uploads corrections and
updates to the navigational information stored in the satellites. It is also in charge of
decommissioning satellites when they have reached end of life, and the commissioning
of new satellites to replace them. It consists of a master control station and a number of
antennas and monitoring sites spread throughout the world.
Global Navigation for Timing 11
FS752 GNSS Time and Frequency Reference Stanford Research Systems
The user segment consists of an antenna and a receiver which extracts the signals
broadcast by the satellites. If the user can track at least 4 satellites, it has enough
information to locate its position (latitude, longitude, height), and time of day.
If the receiver has an accurate clock, it can compute the distance to a known satellite by
comparing the time of day broadcast by the satellite to its own clock. The difference
between the two clocks represents the amount of time it took for the radio signals to
travel from the satellite to the receiver. Since radio signals are known to travel at the
speed of light, the distance to the satellite is computed simply by multiplying the time
difference by the speed of light. By computing the distance to three different satellites,
the receiver can infer its own position by finding the intersection of three spheres
centered at the location of each satellite with radii equal to the computed distance to the
given satellite.
Unfortunately, most receivers do not have sufficiently accurate clocks to make this
measurement possible. The satellites, on the other hand, do have accurate clocks on
board. By tracking a fourth independent satellite, therefore, the receiver can extract the
correct time of day needed in the computation of its position. In the end, the signals from
four independent satellites describe four equations which enables one to solve for four
unknowns: latitude, longitude, altitude, and time. If still more satellites are visible and
tracked, more information can be incorporated into the solution and the overall error is
correspondingly reduced.
Using Satellites f GPS or Timing
While most people are aware of the positioning benefits of GPS, it is really precise
timing that makes the whole endeavor possible. GPS time is monitored and maintained
by a network of atomic clocks on the ground and aboard the satellites which are
collectively steered to follow universal coordinated time (UTC) as maintained by the
United States Naval Observatory (USNO).
For the FS752, it is this last point which is of significant importance. By locking its
internal timebase to GPS or any of the other GNSS constellations, the FS752 can
provide long term frequency stability on par with the best time keeping instrumentation
in world, but at a fraction of the cost.
Position Survey for Improved Timing
Further improvements in timing can be achieved if the receiver is located at a fixed
position and is not expected to move. This is assumed to be the typical case for users of
the FS752. In this scenario, the timing produced by the receiver can be improved by
performing an extended survey of its position. Once the position is accurately known,
the receiver need not compute its position any longer. Rather, it can supply its known
position into the equations and use toward improving the timing all satelliteavailable s
solution. This not only improves the quality of the timing, but also improves overall
robustness by enabling the receiver to generate timing pulses with as little as 1 satellite
in view.
Global Navigation for Timing 12
FS752 GNSS Time and Frequency Reference Stanford Research Systems
Locking to GNSS Satellites
Phase Lock Loop Design
When the FS752’s GNSS receiver starts tracking satellites it outputs a 1 PPS aligned to
UTC. The FS752 uses a sophisti phase lock loop to cated lock its OCXO timebase to that
signal. The basic loop design can be described by the diagram in Figure 3.
Figure 3 : GPS phase lock loop
The symbols in the diagram have the following definitions:
s Laplace frequency
Ap Proportional gain
τi Integral time constant
τp -Pre filter time constant
Kdet Phase detector gain
Kvco VCO gain
The basic architecture is that of a 2nd order phase lock loop with proportional and
integral gain. This basic loop is then augmented with the insertion of a pre-filter
designed to reduce the loop’s sensitivity to the broadband noise of GPS. Given the
definitions above, we define the natural loop time constant, τn, and use it to compute the
following variables in the phase lock loop:
=2
 
=  
=
6
Given these definitions and igno filter for the moment, the loop would have ring the pre-
the following response to a step in frequency and phase:
 () = [  (0)/ ]
+ (0)
with the following symbol definitions:
t Time
∆T(t) The phase deviation as a function of time
1
+ 1
+1

1
10 000, ,000
1
Pre-filter
Proportional and
Integral Gain
Time Stamp
Circuit
GPS 1pps
10 MHz
Output
Kdet
Global Navigation for Timing 13
FS752 GNSS Time and Frequency Reference Stanford Research Systems
∆T(0) The initial phase deviation at time t = 0
F0 The initial frequency offset at time t = 0
τn The natural loop time constant for the phase lock loop
Graphically, the step responses for phase and frequency are shown in 4. One can Figure
see that it takes about one to two time constants before the bulk of the error is corrected.
It takes five to six time constants before the phase to stabilized. The insertion of the pre-
filter perturbs this solution slightly, but the overall response is very similar.
The speed of the phase lock loop is controlled by the natural loop time constant, τn.
Shorter time constants enable the loop to follow the reference more faithfully, including
its broadband noise. Longer time constants will follow the reference more loosely and
may provide better performance, particularly if the local reference is more stable in the
short term. The goal is to select a time constant that reflects the best balance between
short-term and long-term performance for the loop.
Figure 4: Phase lock loop step response
Predictive Filtering
The superior short- y term stabilit of the FS752’s OCXO enable the usage of predictive
filtering to improve the stability of the FS752 even further over traditional methods.
Predictive filtering uses state space methods to predict the phase of the local timebase
relative to GPS. The technique is quite similar to Kalman filtering. The benefit is that the
FS752 can average the GPS signal more effectively, resulting in a significantly more
stable signal with a shorter time constant than would be possible with traditional
filtering.
Timebase Stability
While the long-term stability of GPS is excellent, its short-term stability is rather poor in
comparison to modern oscillators. The FS752 incorporates an ovenized crystal oscillator
with a short-term stability that out performs GPS for time intervals up 250 seconds or so.
Figure 5 shows the typical measured stability of the FS752 locked to GNSS satellites at
various time intervals. The stability of the GNSS receiver is also shown for comparison
purposes. Notice that at intervals of 1 second, the stability of the FS752’s OCXO is
Global Navigation for Timing 14
FS752 GNSS Time and Frequency Reference Stanford Research Systems
nearly 100 times better than that of the receiver. Out at intervals beyond 1000 seconds,
however, the GNSS receiver is clearly better. By locking the OCXO to the GNSS
receiver with an appropriate time constant, we gain the best of both worlds: superior
short-term stability and matched long-term stability.
Figure 5: Frequency stability of the FS752.
Adaptive Bandwidth
The FS752 is designed to provide the best overall trade-off between short-term and long-
term performance, once its timebase has warmed up and stabilized. At start-up, however,
the timebase is cold and its frequency is changing rapidly. In an effort to provide decent
timing as soon as possible, the time constant of the loop is shortened up significantly so
that it can follow the GPS signal in spite of its changing frequency. When the instrument
detects that the timebase has fully warmed up and stabilized, it will gradually increase
the time constant to its optimum value. This may take an hour or more.
Losing Lock to GNSS
With a well-placed antenna that has a clear view of sky, the receiver should rarely lose
lock to the GNSS satellites. A poorly placed antenna, on the other hand, will potentially
degrade the stability of FS752 in addition to making it more vulnerable to losing lock to
the GNSS signals altogether. Therefore, careful consideration should be given to antenna
placement during the installation of the FS752. However, since ideal antenna placement
is not always possible, it is important that the timebase be designed to handle unlock
events gracefully.
Monitoring Lock to GNSS
The FS752 uses a finite state machine (FSM) to continuously monitor the GNSS
receiver and the PLL. The basic operation of the FSM is illustrated in Figure 6
Global Navigation for Timing 15
FS752 GNSS Time and Frequency Reference Stanford Research Systems
Figure 6: A simplified timebase state diagram showing the most common transitions.
The FS752 starts out in the power on state. It then progresses through a number of
startup states towards its goal of extracting time of day information from the GNSS
satellites and locking its timebase to the 1 PPS generated by the receiver. Once locked
the FS752 continuously monitors the quality of the 1 PPS and transitions to a holdover
state if problems are detected. When good timing is recovered, the FS752 will transition
back to the locked state and lock its timebase to the 1 PPS generated by the receiver.
FSM States
More detailed information on each of the FSM states is provided below.
Power On
The FS752 always starts out in the power on state. There is no battery back up for
maintaining a real time clock, so the FS752 has no estimate for the current date and time.
Furthermore, the timebase and electronic components may be cold and their frequencies
will be significantly off until the electronics and the timebase warm up. During this time
all instrument initialization is performed and the receiver is configured to start searching
for GNSS satellites.
Global Navigation for Timing 17
FS752 GNSS Time and Frequency Reference Stanford Research Systems
While locked to the GNSS 1 PPS, the FS752 will continuously monitor the PLL and
adjust its configuration, if necessary to maintain lock. In particular, if the 1 PPS appears
to be ‘walking away,’ the loop time constant will be reduced to regain good alignment.
If this is not possible the FS752 will transition to one of the holdover states.
Holdover States
The FS752 transitions to one of the holdover states whenever it loses lock to the GNSS
satellites. While in holdover, the FS752 will hold its frequency steady at the last value it
had before losing lock. It will also continuously monitor GNSS time in hopes of
relocking to it the same validation criteria used at startup to initiate lock., ing us The
FS752 will enter holdover for one of three reason: no 1 PPS, a bad 1 PPS, or by user
request.
No GPS
The most common reason for entering holdover is losing track of the GNSS satellites. In
this scenario, the receiver will stop generating a 1 PPS, and the FS752 has no choice but
to wait for the 1 PPS to return when satellites are recovered.
Bad Timing
The second reason for entering holdover is for bad timing. Bad timing is defined to be a
discrepancy between the FS752’s estimate of UTC and the GNSS estimate of UTC that
exceeds a preset configuration threshold, which is 1 µs (by default). Under normal
circumstances, this should never occur, so when it is encountered, the FS752 assumes
that something must be wrong and enters holdover.
Manual Holdover
The last way of entering holdover is by user request. This can only be done over the
remote interface, and so will not happen in normal operation.
Recovering From Holdover
Once the FS752 enters holdover it will continue to monitor the 1 PPS generated by the
receiver in hopes of re-locking to the signal. It uses the same criteria used at startup for
assessing when it can initiate lock again except for one detail. At startup the FS752 will
jump the phase of its 1 PPS to match that of the GNSS 1 PPS to get good alignment
immediately. This is also the method used to recover from bad timing. or the However, f
case when the discrepancy between the FS752 and the GNSS 1 n 1 aPPS is less th µs, the
FS752 will not jump phase, but rather slew the phase by adjusting the frequency to
gradually bring the 1 PPS back into alignment using the standard PLL. In this case, there
will be no phase discontinuity in the 1 PPS at the time of re-lock, but the timebase will
necessarily run off frequency for a while in order to bring the phase back into alignment.
The 1 µs threshold between good and bad timing is a configuration parameter which can
be adjusted by the user over the remote interface via the if desired. GnssDO application
See the chapter on Configuration.
System Alarm
The FS752 includes a SPDT switch on its rear panel which can be used to switch
external instrumentation based upon the state of timebase. A front panel LED in the blue
STATUS section of front panel indicates the current state of the alarm. By default, the
alarm asserts whenever the FS725 is not locked to the GNSS 1 PPS. The conditions
under which the alarm is asserted may be configured over the remote interface via the
Instrument Operation 19
FS752 GNSS Time and Frequency Reference Stanford Research Systems
Instrument Operation
Front-Panel
Figure 8: The FS752 front panel
Display
The FS752 front panel consists of a 6-digit LED display, four columns of indicator
LEDs, two distribution outputs, and one button. The location of each of these items is
highlighted in Figure 8. The display is divided into three sections: satellite receiver,
timebase, and USB. Each section is color coded to assist the user in identifying related
functionality and status. The display button, located in the receiver section enables the
user to toggle between five displays: time, date, 1pps offset, number of satellites tracked,
and average SNR. Under the display button are three indicator LEDs to highlight the
information currently being displayed. The remaining LEDs provide dedicated status
information about the state of the receiver, timebase, or USB remote interface.
Rear Panel-
Figure 9: The FS752 rear panel
The rear panel provides connectors for remote interface communication via USB, a switch tied
to the system alarm, a GPS antenna input, distribution of the 10 MHz and 1 PPS outputs and AC
power. Space is available for the installation of two optional distribution boards, each with four
connectors a piece. Two types of option boards are available: one for extra 10 MHz distribution,
and on for extra 1 PPS distribution. The customer may install whatever combination best fits his
or her application needs. The location of all these components is highlighted in 9. Figure
AC Power
Connect the unit to a power source through the power cord provided with the instrument.
The center pin is connected to the chassis so that the entire box is earth grounded. The
unit will operate with an AC input from 90 to 264 V, and with a frequency of 47 to 63
Hz. The instrument requires 30 W and implements power factor correction. Connect
Instrument Operation 20
FS752 GNSS Time and Frequency Reference Stanford Research Systems
only to a properly grounded outlet. Consult an electrician if necessary. There is no power
on/o switch, as the FS752 is intended to be operated continuously.
USB
The USB port accepts a USB Type B connector for interfacing to a host computer. The
FS752 will enumerate as an FTDI virtual COM port with the following conguration:
115,200 baud, 8 data bits, 1 stop bit, no parity, and RTS/CTS hardware ow control.
Drivers for the port should install automatically on modern Windows computers.
Alarm Relay
This connector is tied to a 3A, SPDT switch, with normally open and normally closed
connections. By default, the alarm will assert when the receiver loses lock to the GNSS
satellites. However, the conditions under which it asserts may be congured by the user.
Antenna Input
In order to lock to GPS, the FS752 must be connected to a GPS antenna. The FS752
provides 5V power on the antenna input to support active antennas with more gain. The
FS752 tries to detect fault conditions related to the antenna to alert the user of potential
problems. If the current draw is too large, an antenna short fault is reported. Alternately,
if the current draw is too small, an antenna open fault is reported.
1 Distribution PPS and 10 MHz
Standard Distribution
The FS752 provides distribution for both 1 PPS and 10 MHz. The base instrument
includes two 1 of distribution: one on the front panel, and one on the rear PPS outputs
panel. The phase of the 1 PPS outputs is aligned to UTC. If desired, the phase of the
1 PPS may be adjusted to account for cable delays when attempting to synchronize with
external instrumentation. However, since the 1 PPS distribution consists of buered
copies of a single signal, any phase adjustment will affect all 1 PPS outputs equally.
The base instrument also includes five 10 MHz outputs of distribution: one on the front
panel and four on the rear panel. The phase of the 10 MHz is NOT correlated with UTC
in any way, and cannot be adjusted either. 10 The MHz distribution is intended to be
used as a frequency reference to synchronize laboratory equipment.
Optional Distribution
The rear panel has space for the installation of up to two option boards for expanded
distribution. Two different types of option boards are oered: 10 MHz distribution, and
1 PPS distribution. Each board provides four buered outputs of the desired signal. The
user may mix and match the installed options to meet application needs.
Instrument Operation 21
FS752 GNSS Time and Frequency Reference Stanford Research Systems
Front Panel Operation
Installation and Power
In order for the FS752 to nd GNSS satellites, it must be connected to a suitable GNSS
antenna and plugged into an AC power source. Connectors for the antenna and AC
power are available on the rear panel. For best results the GNSS antenna should be
installed in a location with a clear view of the sky. Please refer to the Quick Start
Instructions 1 on page for detailed instructions.
Apply power to the rear panel AC power input. Once power is applied, the FS752 will
show the model number (752) and the version of firmware executing. Operation then
commences immediately. There is no on/off switch on the FS752 as it is intended to be
operated continuously.
Navigating the Display
The FS752 has one button on the front panel for navigating between displays and three
indicator LEDs for identifying what information is being displayed in the 6-digit LED
display.
Figure 10: The FS752 display button and indicator LEDs.
There are five main displays: time, date, delta 1 PPS, number of satellites, and average
SNR. Navigation between displays is simple. Briey pressing the display button
advances the display to the next sequential option. The indicator LEDs help to identify
the information being displayed.
Main Displays
TIME / DATE
UTC time is displayed in the form HH:MM:SS, where HH is hours, MM is minutes, and
SS is seconds. Hours is displayed in 24-hour format with midnight represented as
00:00:00, and the second before midnight as 23:59:59.
When the UTC date is displayed, the year will be presented briefly, followed by the
month and day. The displayed month is the common three letter abbreviation for the
month, such as “Jan” for January and “Feb” for February.
Note that time and date will not be displayed until the FS752 has successfully aligned its
output with UTC.
Instrument Operation 22
FS752 GNSS Time and Frequency Reference Stanford Research Systems
∆
∆
∆
∆∆1 PPS
The ∆1 PPS display shows the difference (delta) between the FS752’s output 1 PPS and
the 1 produced by the GNSS receiver in nanoseconds. 752 is locked to PPS When the FS
GNSS satellites, the phase lock loop is working to make the average of this difference
zero.
Note that the ∆1 PPS display will not be shown until the FS752 has successfully aligned
its output with UTC and its absolute offset is less than 1 µs.
SATS / SNR
The last two displays provide diagnostic information. SAT shows the number of
satellites being tracked. SNR shows the average detected signal to noise ratio of the 4
strongest satellite signals. SNR is particularly helpful when verifying good placement of
the GNSS antenna. For best results, the SNR should be above 40, though the FS752 will
continue to operate with the SNR as low as 20.
Alternate Displays
In addition to the main displays, there are 3 alternate displays. Alternate displays are
accessed by pressing and holding the front panel display button for longer than
2 seconds. While the button is pressed the display will cycle between the three options.
Release the button to select the desired alternative display.
Position
Select position to view latitude, longitude, altitude, and survey progress. Latitude is
displayed rst. Press the display button briey to move successively to each parameter.
Once survey is viewed, the display will revert back to the previous main display.
Latitude is displayed in degrees with positive values referring to locations north of the
equator and negative values referring to locations south of the equator. Longitude is also
displayed in degrees, but with positive values referring to locations east of the prime
meridian and negative values referring to locations west of the prime meridian. Due to
lack of display space, values shown are rounded to the second digit after the decimal.
Many more digits are available over the remote interface via the GnssDO application.
Altitude is shown in meters above the GPS reference ellipsoid (WGS 84). Note that the
reference ellipsoid may deviate by up to 100 m from the more conventional denition of
altitude as the height above mean sea level. Bear this in mind when trying to interpret
altitude values reported by the receiver relative to those shown on printed maps.
The last position parameter shown is survey progress. Survey progress is shown as a
percentage from 0 to 100. When the survey is complete, the unit will display “done.”
And the “SURVEYED” LED in the STATUS section of the front panel will turn on.
Alarm
The alternate display for identifies the cause of an alarm assertion. When an alarm alarm
asserts, the rear panel SPDT switch asserts and the front panel ALARM LED in the
status section of the front panel highlights 11. as shown in Figure For the example
shown in the gure, the display indicates that the alarm is being asserted because the
timebase is in holdover.
Instrument Operation 23
FS752 GNSS Time and Frequency Reference Stanford Research Systems
Figure 11: An example alternate display for alarm.
The alarm display will identify the cause of any alarm assertion as detailed in T 3. able
Table 3: Alarm Displays
Alarm Display
Cause
None
No alarm is being asserted
No tm
Time and date has not been set by GNSS satellites
In hold
In holdover. Timebase lost lock to GNSS 1 PPS
Offset
Timebase deviates from UTC by more than the
congured amount (1 µs by default).
By default, the FS752 is congured to track the current state of the timebase and assert
the alarm if time is not set by GNSS or the timebase is in holdover. The FS752 also
supports the latching of alarm conditions. In this case, the alarm will assert until the user
explicitly clears the alarm. The user clears an alarm by pressing the display button while
viewing the cause of the alarm. The alarm will only clear, of course, if the condition
which caused the alarm is no longer true. See section Conguration for instructions on
how to change the alarm conguration.
Timebase
The timebase display provides information on the current state of the timebase and
whether it is locked to the GNSS satellites. The possible timebase displays and their
meaning is detailed in Table 4.
Table 4: Timebase Displays
Timebase Display
Meaning
Search
The GNSS receiver is searching for satellites
No PPS
Satellites have been found but the receiver is still unable to
generate an accurate 1 PPS
Verify
The unit is verifying that its timebase has stabilized and that the
timing generated by the receiver is good.
Good
The timebase is locked to the 1 PPS generated by the GNSS
receiver
Holdover no PPS
The timebase has lost lock to the GNSS satellites and is
currently in holdover because no 1 PPS is being generated.
Holdover bad PPS
The timebase has lost lock to the GNSS satellites and is
currently in holdover because the 1 PPS being generated is
either unstable or exceeds the 1 µs (default) threshold.
Holdover Forced
The user has manually requested the FS752 to unlock from
GNSS via the remote interface.
Instrument Operation 24
FS752 GNSS Time and Frequency Reference Stanford Research Systems
The first three timebase displays (search, no PPS, and verify) are only possible at
startup. Once the FS752 has set its time, and locked to the GNSS satellites, the timebase
will only toggle between locked (good) and one of the holdover states.
Startup Displays
At startup, the FS752 briefly shows the model number (752) followed by the version of
rmware executing. Initially, the main displays for time, date, and 1 ∆PPS will not be
shown because these values are undened until the FS752 is able to locate GNSS
satellites and lock to them. Until this happens, the timebase alternate display is shown
instead. The user may still view the number of satellites tracked and the average SNR.
Timebase Status
Under normal operation the state of the timebase can be assessed directly from the status
of the LEDs located in the green TIMEBASE section of the front panel.
Figure 12: Dedicated LEDs for indicating timebase status.
At startup all LEDs will be o. When the FS752 first locks to the GNSS 1 PPS, the
LOCKED LED will turn on. Thereafter, whenever lock is lost the LOCKED LED will
turn o and the red HOLDOVER LED will turn on. The STABLE LED will only turn
on when the timebase has been locked for some time, the 1 PPS is in good alignment
with UTC as generated by the GNSS receiver, and the loop time constant for the PLL is
at the optimum value for the timebase. This may take up to an hour or more.
USB Status
The FS752 can be controlled and queried over a virtual RS-232 communications
interface created by the USB drivers for the instrument. No communication is required
for normal operation. However, if communication is attempted, the front panel LEDs in
the USB section of the front panel may aid in diagnosing communication issues.
Figure 13: Dedicated LEDs for indicating USB communication status.
The RxD and TxD LEDs flash whenever the instrument receives or transmits data
respectively. If an illegal command is parsed or executed, the ERROR LED will turn on.
Instrument Operation 25
FS752 GNSS Time and Frequency Reference Stanford Research Systems
Error Reporting
Whenever an illegal command is parsed or executed, the specific cause of the error will
be shown on the front panel as an error number. See section Error Codes for 81 on page
details on the interpretation of error codes. Once an error is generated, the user must
dismiss the error by pressing the front panel display button before the normal front panel
display operation commences.
Gnss DO Application
In addition to the front panel interface, the FS752 provides a remote interface over
which full status information may be queried. The available command set is quite
extensive and it provides the user with detailed control over the operation of the FS752.
However, the user need not invest time in learning the commands that can be executed.
The GnssDO application, available from the SRS website, makes sending commands,
viewing status, and changing the conguration of the FS752 over the remote interface
easy.
The application queries the state of the FS752 over a virtual COM port created by the
USB device drivers. It then aggregates the information into groups and graphs that are
much easier to comprehend and interpret.
Installation Requirements and Setup
The -GnssDO application should run on any 64 bit Windows computer. No installation is
required. The application is actually written in Python, but it has been wrapped up into a
single executable with no library dependencies.
Before running the application, connect the USB cable from the FS752 to the host
computer . The host computer should be able to locate the FTDI device drivers
automatically via Windows Update. The device drivers will create a virtual RS-232
COM port for communicating with the FS752.
Once the device drivers are installed, run the GnssDO application. Connecting to the
FS752 via the application is easy. Merely select the appropriate communications port
and click Co The application will only display available COM ports, so it should nnect.
be relatively easy to identify the correct one.
Instrument Status
If all goes well, the Instrument Status page will be filled in with the current state of the
instrument similar to that shown in Figure 14. The application shows data for position,
tracked satellites, timebase status, events, warnings, and alarms. The data is continuously
updated as long as the connection is maintained.
Instrument Operation 26
FS752 GNSS Time and Frequency Reference Stanford Research Systems
Figure 14: Example instrument status page . for the GnssDO application
Configuration
The FS752 conguration options will be discussed in detail in the next chapter. While
the expectation is that most users will rarely need to change the default configuration,
the GnssDO application provides an easy method of doing so. The second tab, labeled
Conguration, is dedicated to controlling the FS752 conguration. Merely click on the
tab heading to activate the tab. A sample configuration page is shown in Figure 15
Figure 15: Example configuration page for the GnssDO application.
Instrument Operation 27
FS752 GNSS Time and Frequency Reference Stanford Research Systems
Console
Lastly, if the user wants to explore manually sending commands to the instrument, the
GnssDO application provides a console for doing that. The console is activated by
clicking on the tab heading labeled Console. A sample console page is shown in
Figure 16.
Figure application. 16: Example console page for the GnssDO
Commands are sent by typing in the command at the bottom and then completing the
command by pressing Enter on the keyboard. The command is sent to the FS752 and
recorded in the command history window in blue. The response to the command, if any,
will be displayed in black.
As commands are entered successively, they will be appended to the command history
window. It is possible to recall previously sent commands by using the up and down
arrows.
Configuration 29
FS752 GNSS Time and Frequency Reference Stanford Research Systems
Configuration
Aside from possibly correcting UTC for antenna cable delays, the expectation is that the
user should rarely need to change the conguration of the FS752. In most cases
installing the antenna in a location with a clear view of the sky, attaching the antenna to
the rear panel is all that is necessary, and apply power . In the rare case when the user
would like to change the conguration, it must be done through the remote interface via
USB. GnssDOThe application, available on the SRS website, greatly simplifies the
process, and is the recommended way for modifying the FS752 conguration.
Using the modify the conguration GnssDO to application is easy. The user connects to
the instrument on the appropriate COM port the Conguration tab, and click, selects s on
one of the Edit buttons. A dialog box of user options will be displayed. The user makes
the desired modications and clicks OK.
The conguration options are divided into four main categories: timebase, GNSS
receiver, alarm, and 1 PPS output. Each category will be discussed in detail below.
Timebase Conguration
In the application, rGnssDO efer to the Timebase pane of the Configuration page. This
pane enables the user to modify how the FS752 timebase locks to the GNSS satellites.
The user can modify the bandwidth of the phase lock loop as well as the criteria for
entering and leaving holdover.
Lock to GNSS
This option controls whether or not the FS752 will try to lock its timebase to the GNSS
satellites when they are being tracked.
Selecting Yes (the default) directs the to lock its timebase to FS752 the GNSS satellites
as soon as they are located and tracked.
Selecting No will force the FS752 to unlock from the GNSS satellites and enter manual
holdover. The frequency will be held at the last frequency control setting indenitely.
The FS752 will not attempt to re-lock to the GNSS satellites until this setting is changed
back to Yes.
Related Commands
TBASe:CONFig:LOCK
Loop Bandwidth
This option controls how the bandwidth of the phase lock loop is adjusted as the FS752
attempts to lock its timebase to the GNSS satellites.
Selecting Automatic (the default) directs the FS752 to automatically throttle the
bandwidth of the phase lock loop with which it locks to the G NSS satellites in order to
Configuration 30
FS752 GNSS Time and Frequency Reference Stanford Research Systems
maintain good phase alignment. When good phase alignment has been achieved and the
timebase frequency is stable, the will automatically reduce the bandwidth FS752 of the
loop to its optimum setting for the best overall frequency stability. Conversely, if good
phase alignment is lost, the bandwidth of the loop will be automatically increased until
good alignment is regained.
Selecting Manual directs the FS752 to use the specied manual loop time constant (see
below). The bandwidth of the PLL will be xed at that setting. Advanced users may
want to specify loop time constant with which the FS752 locks to GNSS to optimize the
FS752 frequency stability for their application.
Related Commands
TBASe:CONFig:BWIDth
Manual Loop TC
This option enables the user to set the manual time constant of the phase lock loop. The
manual loop time constant only applies when the Loop Bandwidth option discussed
above is set to Manual. The time constant controls how tightly the PLL locks the FS752
timebase to the GNSS satellites. The time constant may be set to values ranging from
3 seconds to 1 million seconds.
Short time constants enable the FS752 to corrections in the GNSS better follow any
timing. However, since the short-term stability of the FS752’s timebase is better than the
stability of the GNSS satellites, choosing a short time constant will generally degrade the
short-term stability of the FS752.
Conversely, choosing a time constant that is too long will degrade the long-term stability
of the FS752 by inhibiting its ability to correct its frequency wander and aging. This will
also lead to larger phase wander about UTC.
The optimum time constant for the FS752’s timebase when everything is warmed up and
settled is approximately 200 seconds (the default).
Related Commands
TBASe:TCONstant
Criteria to Enter Holdover
This option denes the threshold (1 µs by default) for categorizing a 1 PPS pulse from
the GNSS receiver as bad. Whenever the difference between the FS752’s estimate of
UTC and that of the GNSS receiver exceeds this threshold, the FS752 will reject the
pulse as ‘bad’ and refuse to continue locking to it. If the FS752 receives 10 ‘bad’ pulses
in a row, it will abandon lock and enter holdover.
Bear in mind that this option only applies to one possible cause for entering holdover.
The most common cause for entering holdover is that the receiver has lost track of all
satellites and is no longer generating any 1 PPS at all. When no 1 PPS is being
generated, the FS752 will enter holdover, regardless of the value of the threshold.
However, the threshold for bad timing can have an impact on how the FS752 leaves
holdover. For more details on this, see the next section.
Configuration 32
FS752 GNSS Time and Frequency Reference Stanford Research Systems
G S NS Receiver Conguration
In the GnssDO application, refer to the GNSS Receiver pane of the Configuration page.
This pane enables the user to modify how the GNSS receiver is configured and the
characteristics of the 1 PPS pulse it generates. The user can configure which
constellations of satellites are tracked, the alignment of the 1 PPS pulse, the position
survey, antenna delay corrections, and local time offsets.
Constellations Tracked
There are four major constellations of GNSS satellites in operation today: GPS,
GLONASS, BEIDOU, and GALILEO. The GPS and GLONASS systems are the most
mature and currently provide the most coverage. The others are also operational,
however, and will improve their coverage in the coming years. The FS752’s GNSS
receiver is capable of tracking any of the four types of satellites. In fact, it can track
more than one constellation of satellites, simultaneously.
This option identies which constellations of satellites should be tracked by the FS752’s
GNSS receiver. The default is to track both GPS and GLONASS satellites. The user may
choose the desired combination of satellites, but be aware that not all combinations are
supported. The selections are limited by available channels and tuning frequencies. If a
selected combination is rejected the receiver will merely remain at its previous setting.
If the combination of satellites is changed, the receiver will be forced to reset. This
means that all currently tracked satellites will be lost and the receiver will be forced to
search for them anew as if it were powering up for the first time.
Related Commands
GPS:CONFig:CONStellation
Timing Alignment
This option enables the user to congure the timing alignment of the 1 PPS output. By
default, time is aligned to Coordinated Universal Time (UTC). However, time may
alternatively be aligned to any one of the GNSS standards: GPS, GLONASS, BEIDOU,
or GALILEO. Although, all of the standards are based on network s of cesium atomic
standards, there are some important differences between them.
First, the UTC and GLONASS standards are synchronized to mean solar time.
Synchronization is achieved with the occasional insert ion s. of leap second When a leap
second is inserted, the last minute of that day will have 61 seconds instead of the normal
60 seconds. These corrections ensure that the time of sunrise and sunset do not shift over
time.
The other timing standards (GPS, BEIDOU, and GALILEO) are strictly atomic time
scales and are not corrected with the insertion of leap seconds. This means that the time
of day reported by GPS, for instance, differs from UTC by an integer number of
seconds. As of January 2017, GPS time was 18 seconds ahead of UTC. This difference
is called the UTC offset.
Furthermore, even when you correct for the UTC offset, the time reported by GPS will
differ slightly from UTC. This is because it is an independent clock and therefore its
Configuration 33
FS752 GNSS Time and Frequency Reference Stanford Research Systems
phase will deviate from that of UTC. This phase is continuously monitored by the
ground control segment of GPS, however, and they and docan ( ) steer GPS time to
ensure that the phase of GPS does not deviate by more than 1 μs from UTC (USNO). In
practice, the deviation is usually less than 10 he current UTC offset ns. Both t and the
current UTC to GPS phase deviation are broadcast by the GPS satellites as part of the
almanac. This is particularly relevant at startup, because it may take up to 12 minutes for
the receiver to download the almanac.
Similarly, each of the particular GNSS constellation also have their own version of
UTC. Thus, UTC as presented by GPS satellites will dier slightly from the UTC
presented by GLONASS satellites. When tracking more than one constellation, the
receiver will automatically choose an appropriate standard to follow. If it is important
that timing be aligned with a particular version of UTC, congure the receiver to only
follow that single constellation
The displayed time of day on the front panel and the physical phase of the FS752 1 PPS
will reect the selected timing alignment. When aligned to GPS, for example, the time
of day displayed by the front panel will be 18 seconds ahead of UTC.
When choosing to align time with a particular GNSS constellation, it follows that the
receiver must also be configured to track that constellation.
Related Commands
GPS:CONFig:ALIGnment
GPS:UTC:OFFSet?
Timing Quality
Normally, GNSS receivers must track at least 4 satellites before they can generate a
position and timing x. This stems from the fact that there are 4 unknowns to be solved:
latitude, longitude, altitude, and time. 4 equations are required to solve for 4 unknowns.
However, if the receiver is not moving, then it need not re-compute its position each
time. Rather, it can use the position computed from before, since it is not changing. In
this case only 1 satellite is needed to compute time.
In normal operation, the FS752 will automatically survey its position for 2000 seconds
to accurately determine its position. After that, the receiver will enter over determined
clock mode and all satellites will be dedicated to generating the best possible timing.
Only one satellite is required to generate a solution. However, the quality of such a
solution increases as more satellites are incorporated into it.
The re is a tradeoff to consider. Should the receiver generate potentially noisy timing
pulses when tracking just one satellite (the def ault)? Or should it wait until it is tracking
at least three satellites before generating timing pulses. The quality of the timing may be
significantly better for three satellites, but it also increases the likelihood that the FS752
will be forced into holdover. Noisier timing may be preferred to no timing at all.
Related Commands
GPS:CONFig:QUALity
Configuration 35
FS752 GNSS Time and Frequency Reference Stanford Research Systems
The user may correct for this delay, however, by entering a negative antenna cable delay
correction. The user would correct for a 30 ft cable by entering a correction of
−46.25 ns.
Related Commands
GPS:CONFig:ADELay
Local Time Oset
The user may prefer to have the local time of day displayed, rather than UTC. To display
local time, the user must enter the offset in hours between UTC and the local time. If the
oset is zero, UTC or GNSS time will be displayed according to the selected timing
alignment. When the oset is nonzero, the local version of UTC or GNSS time will be
displayed.
Related Commands
SYSTem:TIMe:LOFFset
Alarm
In the application, refer to the Alarm pane of the Conguration page. This pane GnssDO
enables the user to configure the conditions under which the system alarm is asserted.
The rear panel of the contains screw terminals to a SPDT switch actuated by the FS752
system alarm. The system alarm may be controlled manually, or it may be configured to
assert in response to anomalous conditions, such as detecting the timebase is in
holdover, or has drifted more than 1 n00 s from UTC.
Related Commands
SYSTem:ALARm?
SYSTem:ALARm:CONDition?
SYSTem:ALARm:EVENt?
SYSTem:ALARm:CLEar
SYSTem:ALARm:FORCe:STATe
Alarm Mode
The system alarm can operate in one of three modes: tracking, latching, or manual.
Tracking Current Condition
In tracking mode (the default) , the alarm is asserted when a configured condition is true,
and de-asserted when the configured condition is false. The user cannot forcibly clear
the alarm except by removing the condition. alarm
Latch Alarm Condition
In latching mode, the alarm is asserted when a congured condition is true. Unlike
tracking mode, however, it is not de asserted when the congured condition becomes -
false. Rather, it remains asserted until the user explicitly clears the alarm. The user can
clear the alarm from the front panel by viewing the alarm condition and pressing the
display button to clear it (see page 22). The user will not succeed in clearing the alarm if
the congured condition is still true because it will merely be reasserted. In this case the
alarm can only cleared by removing the condition.
Configuration 36
FS752 GNSS Time and Frequency Reference Stanford Research Systems
Manually Set State
In manual mode, the user explicitly sets the state of the alarm. The state can be toggled
by pressing the display button while view the alarm state (see page 22).
Related Commands
SYSTem:ALARm:MODe
SYSTem:ALARm:FORCe:STATe
Alarm Conditions
When the system alarm is in tracking or latching mode, the user must specify the
conditions which will trigger the alarm. The default is to assert the alarm when the
receiver is not locked to the GNSS satellites. The user also has the option to assert the
alarm if the FS752’s estimate of UTC differs from the GNSS satellites by more than the
given amount.
Related Commands
SYSTem:ALARm:ENABle
SYSTem:ALARm:GPS:TINTerval
SYSTem:ALARm:HOLDover:Duration
1 PPS Output
In the application, refer to the 1pps Output of the Conguration page. This GnssDO
pane enables the user to congure the output delay of the 1 PPS outputs relative to UTC.
Time Offset
The FS752 is calibrated so that the zero crossing or rising edge of the output at the BNC
corresponds to UTC. However, the user’s equipment may be located some distance away
from the FS752. Thus, it may be desirable to advance or delay the signal so that rising
edge of the output coincides with UTC at the input to the user’s equipment. Enter a
negative value to advance the signal. Enter a positive value to retard the signal. The
phase can be adjusted by up to 1 second.
The 1 PPS time offset adjustment has a similar as the antenna delay correction effect
discussed on . The dierence is that the antenna delay correction is applied page 34
directly to the receiver, where as this correction is applied to the output driver for the
1 PPS output. Corrections applied to the output directly can be implemented
immediately without disturbing the phase lock loop locking the FS752 to the GNSS
satellites. In contrast, when the correction is applied directly to the receiver, it will
appear to the FS752 as a step in phase of the reference 1 PPS that the phase lock loop
must follow. If the phase step is reasonably small the phase lock loop will gradually pull
the FS752 to the new phase reference by running o frequency. If the phase step is
large, the FS752 may lose lock and enter holdover before realigning with the new phase
reference.
Note that this time offset value applies to all distributed copies of the 1 PPS output as a
group. Individual adjustment of the phase of a single output independent of the other
outputs is not possible.
Configuration 37
FS752 GNSS Time and Frequency Reference Stanford Research Systems
Related Commands
SOURce:PHASe:SYNChronize:TDELay
Factory Default Settings
The factory default settings for the FS752 are shown in 5. Table
Table 5: Factory default settings
Parameter
Value
Set by *RST
Display
Time of day
Timebase GPS lock
Enabled
Timebase holdover mode
Jump to good 1pps timing
Timebase bandwidth mode
Auto bandwidth control
Timebase time interval limit
1 μs
Timebase manual time constant
200 s
G NSS timing alignment
UTC
G NSS timing quality
Require 1 satellite
G NSS survey mode
Redo at power-on survey
G NSS position xes in survey
2000
G NSS antenna cable delay
0 ns
Local time offset
0 hr
System alarm mode
Track timebase state
System alarm manual state
Off
System alarm holdover duration limit
0 s
System alarm timing error limit
100 ns
1 PPS output time delay
0 ns
Forcing Instrument Settings to Factory Defaults
Occasionally it may be useful to force ALL instrument settings to their factory default
state. This may be necessary, for example, when transferring a unit from a secure
location. Perform the following procedure to wipe the instrument of all user settings and
force all system settings to their factory default values:
1. Unplug the power cord to the . FS752
2. and hold the front panel display button. Press
3. While pressing the display button, -plug in the power cord to the . re FS752
4. Continue . pressing the display button until the display reads ‘reset’
5. Release the display button to initiate the reset.
Configuration 38
FS752 GNSS Time and Frequency Reference Stanford Research Systems
Related Commands
SYSTem:SECurity:IMMediate
Remote Programming 39
FS752 GNSS Time and Frequency Reference Stanford Research Systems
Remote Programming
Introduction
The FS752 may be programmed via remote interface.the USB Any host computer
interfaced to the instrument can easily control and monitor operation. its
USB
The FTDI drivers for the USB remote interface should be downloaded automatically by
Windows Update. The drivers will automatically create a virtual RS-232 COM port for
communicating with the FS752.
Virtual RS-232 COM Port
In order to communicate properly over the virtual RS-232 COM port, the instrument and
the host computer both must be congured to use the same settings. Use the following
setup when attempting to communicate: 115200 baud, 8 data bits, 1 stop bit, no parity,
and RTS/CTS hardware ow control.
Front-Panel Indicators
To assist in programming, there are three front panel indicators located under the USB
section of the front panel: RxD ERROR., TxD, and 17. See Figure
Figure 17: Communications indicator LEDs.
The RxD and TxD LEDs flash every time a character is received or transmitted,
respectively. This is useful when troubleshooting connections because it clearly
indicates when the successfully received andFS752 responded to a command.
The ERROR LED will be highlighted when a remote command fails to execute due to
illegal syntax or invalid parameters. Once highlighted, the LED will remain lit until the
error queue is cleared. Errors codes will be automatically displayed on the 6-digit LED
display whenever they occur. See section Error Codes 81on page , for details on
interpreting them. They can be cleared by pressing the front panel display button. When
all error codes have been cleared, the ERROR LED will turn off.
Remote Programming 40
FS752 GNSS Time and Frequency Reference Stanford Research Systems
SCPI Command Language
The FS752 uses the SCPI (Standard Commands for Programmable Instruments)
language for controlling the instrument over a remote interface. The SCPI language is an
ASCII based command language that organizes functions into a hierarchical tree of
commands with branches of the tree separated by colons.
SubSystems
The base or root of the tree represents a subsystem of the instrument. Each succeeding
branch of the tree subdivides the subsystem into related categories of functionality. The
final branch of the tree identifies a command related to the subsystem that can be
executed by the FS752. This structure facilitates understanding of the functions carried
out by commands. As an example, consider the subset of the STATus subsystem shown
below.
S : TATus
GPS:
CONDition?
ENABle
ENABle?
[: EVENt]?
OPERation:
CONDition?
ENABle
ENABle?
[: EVENt]?
STATus is a subsystem of the FS752 and it is at the root of the tree. At the next level
down, the STATus subsystem is divided into two branches: GPS and OPERation. Each
of these categories is then further subdivided into four virtually identical commands:
CONDition, ENABle, ENABle?, and [:EVENt]?. However, because of the hierarchical
structure of the language, we can infer that the commands listed under the GPS branch
refer to GPS receiver status, while those listed under OPERation carry out the same
functions but refer to OPERation status rather than GPS status. Thus, the hierarchical
structure of the commands aids the user in interpreting the operations carried out by the
individual commands.
Understanding Command Syntax
SCPI commands often take one or more parameters which modify or identify the
numerical value a variable should take. Some parameters are required. Others may be
optional. Furthermore, the data types for each parameter may dier. Thus, for brevity,
we need a set of conventions for dening commands which clearly identies all the
valid variations of the command without having the list each possibility separately.
These conventions are set forth here.
An example command is illustrated below:
SOURce:PHASe[:ADJust] {<phase>|MINimum|MAXimum|DEFault}
Remote Programming 41
FS752 GNSS Time and Frequency Reference Stanford Research Systems
Keyword Case
Keywords are case and lowerdefined -with a mixture of upper -case letters. The upper-
case letters indicate the short or abbreviated version of the keyword. This is usually the
first 3 or 4 letters of the keyword. The user may send either the short version or the
entire long version of the keyword in their programs. The case of the letters sent to the
FS752 does not matter. It is only used here to succinctly identify the two versions of the
keyword. Thus, SOUR, source, and Sour are all acceptable forms of the keyword. Oth er
forms, such as SOU, or SOURC, are not. Given the definition above, the following
commands are all identical:
SOUR:PHAS MIN
SOURCE:PHASE MINIMUM
SOUR:PHASE MIN
Punctuation Used in Definitions
The following punctuation is used to identify variations and options for the command:
• Braces ( { } ) enclose different parameter choices. The braces, themselves, are
not sent with command the
• A vertical bar ( | ) separates alternative parameter choices for the command. In
the example above, the choices are a <phase> or one of the keywords:
MINimum, MAXimum, or DEFault. The vertical bar is not sent with the
command.
• Triangle brackets ( < > ) indicate that you must specify a numerical value. In the
example above, < > would be specified as a number with optionalphase units.
Thus, one could set the using the following command: SOUR:phase PHAS
1.253 ns. The triangle brackets are not sent with the command.
• Square brackets identify optional keywords or parameters in the command.
Optional items may be omitted if desired. In such case a default value is
normally substituted for the parameter. In the example above, the keyword
ADJust is optional and may be omitted. Thus, the command
SOUR:PHAS:ADJ 125 ns is identical to the command that omits the keyword:
SOUR:PHAS 125 ns.
Examples
Putting it all together, all of the following commands are valid given the example
definition presented above.
SOUR:PHAS MIN
SOUR:PHAS DEFAULT
-9 SOURCE:PHASE 125e
SOURCE:PHASE 125 ns
SOURCE:PHAS MAXIMUM
Queries
Command queries are usually formed by appending a question mark ( ? ) to the
command. To query the current phase of the 1 outputPPS , we use the following
command: SOUR: ? PHAS
Remote Programming 43
FS752 GNSS Time and Frequency Reference Stanford Research Systems
Units
Some numeric values may be followed with a unit designation. The most common
engineering prefixes are also accepted. For example, set to the antenna delay may be
10 ns with the command . GPS:CONF:ADEL 10 ns
Discrete Parameters
Some parameters take one of a small list of allowed keywords. They often have a short
form and a long form, just like command keywords. In the command definition, the
uppercase letters indicate the short form. Either case may be used when sending the
short or long form of the value to the instrument. Queries will always return the short
form. Consider the following command definitions:
TBASe:CONFig:BWIDth [{ AUTo | MANual }]
TBASe:CONFig:BWIDth?
The user may specify auto bandwidth control by sending the command
TBAS:CONF:BWID AUTO AUT. The query will return , which is the short form of the value.
String Parameters
Quoted string parameters allow one to send almost any sequence of characters, including
characters that are normally reserved as separator characters, such as a comma,
semicolon, or colon. The string must begin and end with the same quote character: either
a single quote, or a double quote. The quote delimiter may itself be included in the string
if it is typed twice without any characters in between.
Command Termination
Commands should be terminated with a line feed <LF>. They may optionally be
terminated with a carriage return <CR> followed by a line feed <LF>. As previously
noted, multiple commands may be sent in a single line if they are separated by a
semicolon ( ; ). Commands are executed in the order received and execution commences
once the command separator or terminator is received.
Remote Programming 44
FS752 GNSS Time and Frequency Reference Stanford Research Systems
Status Reporting
Architecture
The FS752 reports on its status via a hierarchy of status registers. Instrument status is
stored in three 16-bit registers: the questionable status register, the operational status
register, and the GPS status register. Parsing and command execution status is receiver
reported via the standard event register (*ESR?). Summaries of all these registers are
reported as bits in the serial poll status byte for the instrument (*STB?). Although not
supported on the FS752, GPIB-like remote interfaces may be configured to generate a
request for service when a given bit in the serial poll status byte is set. This scheme
enables the user to be notified when events of interest occur, and to ignore events that
are not of interest. Detailed status is always available in the source registers. However,
with proper configuration, these registers need not be queried until an event of interest
actually occurs.
Each instrument status register has three associated status words: a condition register, an
event register and an enable register. The relationship of these three registers is depicted
in 18. Figure
Figure 18: Organization of status registers
Condition Register
The condition register reports on the current state of the instrument. At the far left of
Figure 18 are listed four items of state which feed into four different bit locations of the
condition register. When the instrument is in the given state, then the corresponding bit
in the condition register is set. When the instrument leaves the given state, the
corresponding bit is cleared. Bits in the condition register which are set indicate items of
state that are true at the time of the query. Bits in the condition register which are clear
indicate items of state that are false at the time of the query. Querying the condition
register does not alter the bits in the register. Only changes in the actual instrument state
alter the bits of the condition register.
When the condition register is queried, only one number is returned which is the binary
sum of all bits in the register which are set. The binary weight of each bit in the register
is shown in the enable register at the far right of F 18igure . The binary weight increases
Condition register
Event register
Enable register
EN
EV
C
0
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
State 1
—
State 2
—
State 3
—
State 4
—
0
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
<1>
<2>
<4>
<8>
<16>
<32>
<64>
<128>
<256>
<512>
<1024>
<2048>
<4096>
<8192>
<16384>
Not used
OR
Summary bit
Remote Programming 45
FS752 GNSS Time and Frequency Reference Stanford Research Systems
by a factor of two for each bit. Bit 0 has a weight of 1. Bit 1 has a weight of 2. Bit 2 has
a weight of 4, and so on up to bit 14 which has a weight of 16384. The number returned ,
by a query of the condition register is the sum of weights of the bits which are set.
For example, in Figure 18, State 1 feeds into bit 0 and State 2 feeds into bit 3. If both
State 1 and State 2 are true, then both bits 0 and 3 will be set. If all other states are false,
then the number returned by a query of the condition register will be the binary sum of
bit 0 and bit 3, which is 1 9. If only State 2 is true, then only bit 3 will be set and a + 8 =
query of the condition register will return 8. Similarly, if only State 1 is true, then only
bit 0 will be set and the query will return 1. Finally, if none of the states is true, the
query will return 0.
Event Register
Bits from the condition register feed into corresponding bits in the event register. The
event register differs from the condition register, however, in that the bits are sticky.
Once a bit is set, it remains set until explicitly cleared by a query of the event register.
The event register, therefore, enables the user to capture all events that have occurred
since the previous query of the register, even if the states in question are short lived and
not true at the moment of the query.
Like the condition register, a query of the event register returns a single number which is
the binary sum of all bits in the register which are set. (See the discussion of the
condition register above for a detailed explanation of this process.)
Unlike the condition register, a query of the event register clears any bits which were
previously set. Executing the *CLS command will also clear this register. Bear in mind
that the if corresponding bits in the condition register are still set, these bits will
immediately be set again after the clear from the query.
Enable Register
The enable register is a mask register that controls which bits from the event register will
set the overall summary bit. If a bit in the enable register is set, then the corresponding
bit in the event register will be combined with other enabled bits of the event register via
a logical OR operation to create an overall summary bit.
The user sets the enable register with a single number which is the binary sum of all bits
in the register which should be set. (See the discussion of the condition register above
for a detailed explanation of this process.) Continuing with the example from Figure 18,
If the enable register is set to 9 = 1 + 8, then the summary bit will be set if either bit 0 or
bit 3 of the event register is set. Therefore, if the user detects that the summary bit is set,
he can infer that either State 1 or State 2 or both were at least momentarily true since the
last query of the event register.
Enable bits are set via a command. They are not cleared by a query or the execution of
the *CLS command. To clear enabled bits, the user must send another set command with
those bits set to zero. Sending the number zero will clear all bits of the enable register
and prevent the summary bit from ever being set.
Remote Programming 46
FS752 GNSS Time and Frequency Reference Stanford Research Systems
FS752 Status
The FS752 status is reported through the standard event register and three instrument
status registers. The organization and hierarchy of these registers is depicted in
Figure 19. There are three instrument status registers: GPS receiver status, questionable
status, and operational status. These are all 16-bit registers which report on the status of
the instrument and its operation. The 8-bit standard event register reports on the status of
command parsing and execution. The summary bits from each of these registers feed
into a single, 8-bit condition register called the serial poll status byte (*STB). Summary
bits for the error queue and the output buffer also feed into this status byte. The serial
poll status byte, therefore, provides summary status for the entire instrument.
The serial poll enable register (*SRE) can be used to combine all the summary bits in the
serial poll status byte (*STB) into a single summary bit, also in the serial poll status byte
located at bit 6. In this way, summary status for the entire instrument may be condensed
down to a single bit. Although not supported on the virtual RS-232 remote interface
included with the FS752, this bit is often used to generate request-for-service interrupts
on GPIB-like remote interfaces.
Serial Poll Status Byte
The serial poll status byte provides summary status for the instrument as a whole. The
interpretation for bits in the serial poll status byte is shown in Table 6.
Table 6: Interpretation of serial poll status bits
Bit
Name
Meaning
0
1
GPS
An unmasked bit in the GPS receiver status has been set.
2
ERR
There is at least one error in the error queue. Query the error
with the command SYST:ERR?
3
QUES
An unmasked bit in the QUES status register has been set.
4
MAV
The interface output buffer has at least one character in it.
Perform a read of instrument to retrieve it.
5
ESR
An unmasked bit in the standard event status register (*ESR)
has been set.
6
MSS
Master summary bit. Indicates that the instrument is requesting
service because an unmasked bit in this register has been set.
7
OPER
An unmasked bit in the OPER status register has been set.
The serial poll status byte may be queried with the *STB? command. The service
request enable register (*SRE) may be used to control when the instrument asserts a
request-for-service on interfaces where that is supported.
Remote Programming 47
FS752 GNSS Time and Frequency Reference Stanford Research Systems
Figure 19: FS752 Status reporting
GPS Receiver Status
0
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
Time not set —
Antenna open
—
Antenna short
—
No satellites
—
UTC unknown
—
Survey in progress
—
No position stored
—
Leap second pending
—
Position questionable
—
Almanac incomplete
—
No timing pulses
—
0
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
<1>
<2>
<4>
<8>
<16>
<32>
<64>
<128>
<256>
<512>
<1024>
<2048>
<4096>
<8192>
<16384>
Not used
EN
EV
C
STAT:GPS:COND?
STAT:GPS?
STAT:GPS:ENAB
Questionable Status
EN
EV
C
0
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
Time of day
—
Warm up
—
Time unlock
—
Freq stability
—
EFC GPS
—
0
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
<1>
<2>
<4>
<8>
<16>
<32>
<64>
<128>
<256>
<512>
<1024>
<2048>
<4096>
<8192>
<16384>
Not used
STAT:QUES:COND?
STAT:QUES?
STAT:QUES:ENAB
Operational Status
STAT:OPER:COND?
STAT:OPER?
STAT:OPER:ENAB
EN
EV
C
0
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
Setting
—
0
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
<1>
<2>
<4>
<8>
<16>
<32>
<64>
<128>
<256>
<512>
<1024>
<2048>
<4096>
<8192>
<16384>
Not used
Standard Event Status
0
1
2
3
4
5
6
7
Operation complete
—
Query error
—
Device error
—
Execution error
—
Command error
—
Power on —
<1>
<2>
<4>
<8>
<16>
<32>
<64>
<128>
EN
EV
*ESR?
*ESE
Serial Poll Status
0
1
2
3
4
5
6
7
<1>
<2>
<4>
<8>
<16>
<32>
Not used
<128>
EN
C
*STB?
*SRE
Output
Buffer
Error
Queue
SYST:ERR?
RQS
MAV
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Standard Event Status Register
The standard event register provides status information on command parsing and
execution. The interpretation for bits in the standard event status register is shown in
Table 7.
Table 7 : Interpretation of standard event status bits
Bit
Name
Meaning
0
OPC
Operation complete. All previous commands have completed.
See command *OPC.
1
2
QYE
Query error occurred.
3
DDE
Device dependent error occurred.
4
EXE
Execution error. A command failed to execute correctly because
a parameter was invalid.
5
CME
Command error. The parser detected a syntax error.
6
7
PON
Power on. The unit has been power cycled.
The standard event status register may be queried with the *ESR? command. The
standard event status enable register (*ESE) may be used to control the setting of the
ESR summary bit in the serial poll status byte (*STB).
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Operation Status
Bits in the operational status register provide information on the operation of the
instrument. For example, bit 1 of the operational status register indicates that a hardware
setting in the FS752 has changed. Most settings change quickly, so this bit will normally
only be detected via the event register.
The interpretation for bits in the operational status register is shown in 9 Table
Table 9: Interpretation of operation status bits
Bit
Name
Meaning
0
1
Setting
Hardware instrument settings are changing
2
3
4
5
6
7
8
9
10
11
12
13
14
15
The condition, event, and enable registers for operational status are queried using the
following commands, respectively.
STAT:OPER:COND?
STAT:OPER?
STAT:OPER:ENAB?
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GPS Receiver Status
Bits in the GPS receiver status register provide information on the status of the GNSS
receiver and its ability to track satellites.
The interpretation for bits in the GPS receiver status register is shown in Table 10
Table 10: Interpretation of GPS receiver status bits
Bit
Name
Meaning
0
Time not set
Time of day information has not been received from
the satellites, yet.
1
Antenna open
The G S antenna does not appear to be connected NS
to the receiver input.
2
Antenna short
The receiver input appears to be shorted.
3
No satellites
No GNS satellites have been found
4
UTC unknown
The UTC offset from GPS is unknown. The offset is
recorded in the almanac which can take up to 15
minutes to download from the satellites.
5
Survey in progress
A position survey is in progress.
6
No position stored
No surveyed position has been stored in nonvolatile
memory.
7
Leap second pending
A leap second is pending. When pending, they are
normally scheduled for the end of the day on June
30
th
or December 31
st
.
8
9
Position questionable
The stored position does not appear to be correct
according to data now being collected. A new survey
may need to be collected.
10
11
Almanac incomplete
A complete almanac has not been downloaded from
the GPS satellites, yet. It can take up to 15 minutes of
continuous tracking of satellites to
download the
almanac.
12
No timing pulses
Timing pulses are not being generated by the
receiver. Timing pulses must be generated in order
for the FS752 to lock its timebase to the G S NS
satellites.
13
14
15
The condition, event, and enable registers for GPS receiver status are queried using the
following commands, respectively.
STAT:GPS:COND?
STAT:GPS?
STAT:GPS:ENAB?
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Common IEEE-488.2 Commands
* CLS Clear Status
Syntax
*CLS
Description
This command immediately clears all status registers as well as the SYST:ERR queue.
*ESE Standard Event Status Enable
Syntax
*ESE <value>
*ESE?
Description
Set the Standard Event Status Enable register to <value>. The value may range from 0 to 255. Bits
set in this register cause ESR (in *STB) to be set when the corresponding bit is set in the *ESR
register. The query returns the current value of the enable register. Definitions for the bits in the
standard event register are given on page 48.
Example
*ESE 1
Enable bit 0 so that an operation complete event will set the ESR bit in the serial poll status byte.
*ESR? Standard Event Status Register
Syntax
*ESR?
Description
Query the Standard Event Status Register. After the query, the returned bits of the *ESR register are
cleared. The bits in the ESR register have the following meaning:
Bit Meaning
0 – operation complete OPC
1 Reserved
2 – query error QYE
3 – device dependent error DDE
4 – execution error EXE
5 – command error CME
6 Reserved
7 – power-on PON
See page 48 for more detailed information on the event status register.
Example
*ESR?
A return of ‘176’ would indicate that PON, CME, and EXE are set.
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*IDN? Identification String
Syntax
*IDN?
Description
Query the instrument identification string.
Example
*IDN?
Returns a string similar to ‘Stanford Research Systems,FS752,s/n001025,ver1.00’
*OPC Operation Complete
Syntax
*OPC
*OPC?
Description
The set form sets the OPC flag in the ESR register when all prior commands have completed. The *
query form returns ‘1’ when all prior commands have completed, but does not affect the *ESR
register.
*OPT? Options
Syntax
*OPT?
Description
The query returns a comma separated list of the two possible installed options in the following order:
left rear panel board, and right oard rear panel b . They may take on the following values:
Option
Value
10 MHz distribution
A
1 PPS distribution
B
Not installed
X
Example
*OPT?
The query returns the current installed options. A return of “A,X” would indicate that one 10 MHz
distribution board is installed in the left slot. The right slot is empty.
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*PSC -Power on Status Clear
Syntax
*PSC <value>
*PSC?
Description
Set the Power-on Status Clear flag to -<value>. The Power on Status Clear flag is stored in
nonvolatile memory in the unit, and thus, maintains its value through power-cycle events.
If the value of the flag is 0, then the Service Request Enable and Standard Event Status Enable
Registers (*SRE, *ESE) are stored in non-volatile memory, and retain their values through power-
cycle events. If the value of the flag is 1, then these two registers are cleared upon power-cycle.
Example
Use the following commands to set power on status clear to 1 and then query the setting.
*PSC 1
*PSC?
*RCL Recall Instrument Settings
Syntax
*RCL <location>
Description
Recall instrument settings from <location> <location> may range from 0 to 9. Locations 1 to 9 . The
are for arbitrary use. Location 0 is reserved for the recall of default instrument settings. Note that this
command primarily affects the display, not the overall instrument configuration.
Example
*RCL 3
Recall instruments settings from location 3.
*RST Reset Instrument
Syntax
*RST
Description
Reset the instrument to default settings. This is equivalent to *RCL 0. See Factory Default Settings
on page 37 for a list of default settings.
*SAV Save Instrument Settings
Syntax
*SAV <location>
Description
Save instrument settings to <location> <location> may range from 0 to 9. However, location 0 . The
is reserved for current instrument settings. It will be overwritten after each front panel key press.
Note that this command primarily affects the display, not the overall instrument configuration.
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Example
*SAV 3
Save current instrument settings to location 3.
*SRE Service Request Enable
Syntax
*SRE <value>
*SRE?
Description
Set the Service Request Enable register . Bits set in this register cause the to <value> FS752 to set the
summary status bit when the corresponding bit is set in the serial poll status register, *STB.
Example
*SRE 16
Set bit 4 of the enable register. This will cause the master summary bit of the serial poll status
register to be set when the FS752 has bytes in its output buffer ready to be read. Definitions for the
bits in the serial poll status byte are given on page 46.
*STB? Status Byte
Syntax
*STB?
Description
Query the standard IEEE 488.2 serial poll status byte. The bits in the STB register have the following
meaning:
Bit Meaning
0 Reserved
1 GPS status summary bit
2 Error queue is not empty
3 Questionable status summary bit
4 Message available, MAV.
5 summary bit ESR status
6 – MSS master summary bit
7 Operational status summary bit
See page 46 for more detailed information on the serial poll status byte.
Example
*STB?
A return of ‘114’ would indicate that GPS, MAV, ESR, and MSS are set. GPS indicates that an
enabled bit in STAT:GPS is set. MAV indicates that a message is available in the output queue. ESR
indicates that an enabled bit in the *ESR is set. MSS reflects the fact that at least one of the summary
enable bits is set and the instrument is requesting service.
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*WAI Wait for Command Execution
Syntax
*WAI
Description
The instrument will not process further commands until all prior commands including this one have
completed.
Example
*WAI
Wait for all prior commands to execute before continuing.
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GPS Subsystem
Commands in the GPS Subsystem enable configuration of the GNSS receiver and report on its
operation.
GPS:CONFig:CONStellation GPS Configure Constellation
Syntax
GPS:CONFig:CONStellation <constellation mask>
GPS:CONFig:CONStellation?
Description
The first definition enables the user to set the combination of satellites tracked. The second definition
queries the current combination of satellites being tracked. The constellation mask is a single number
from 1 to 15 whose binary bits identify the combination of satellites that should be tracked as
identified in Table 11. The default constellation mask is 3, which has bits 0 and 1 set, meaning GPS
and GLONASS will be tracked. The user must execute the command GPS:CONF:SAVE to save the
current values to nonvolatile memory. Note that not all combinations are supported. If an
unsupported combination is requested, the command will be ignored without reporting an error.
Table 11: Constellation bit definitions
Bit
Constellation
0
GPS
1
GLONASS
2
BEIDOU
3
GALILEO
Example
GPS:CONF:CONS 3
Configure the GNSS receiver to track GPS and GLONASS.
GPS:CONFig:MODe GPS Configure Mode
Syntax
GPS:CONFig:MODe <anti-jamming>, <elevation mask>, <signal mask>
GPS:CONFig:MODe?
Description
The first definition enables the user to set anti-jamming mode, the elevation mask and the signal
mask. The second definition queries the current values for these parameters. <Anti-jamming> is a
Boolean value which enables or disables anti-jamming in the receiver. The factory default is enabled.
<Elevation mask> is the elevation angle in radians, below which satellites are ignored in over
determined clock mode. It can range from 0 to π/2. <Signal mask> is the minimum signal level in
dbHz, below which satellites are ignored in over determined clock mode. It can range from 0 to
55 dBHz. The default value for both masks is 0. The user must execute the command
GPS:CONF:SAVE to save the current values to nonvolatile memory.
Example
GPS:CONF:MODE 1,0.2618,10
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Enable anti-jamming. Set elevation mask to 15 degrees = 0.2618 radians. Set the minimum signal
level to 10 dbHz.
GPS:CONFig:SAVe GPS Configure Save
Syntax
GPS:CONFig:SAVe
Description
The user uses commands in the GPS:CONFIG subsystem to configure the operation of the receiver.
The configuration is lost, however, if the power is cycled unless this command is executed. It saves
the current GPS receiver configuration to nonvolatile memory, so that it may be automatically
recalled when the power is cycled.
Example
GPS:CONF:SAV
Save the current GPS receiver configuration to nonvolatile memory.
GPS:CONFig:SURVey:Mode GPS Configure Survey Mode
Syntax
GPS:CONFig:SURVey[:MODe] [{DISabled|REDo|REMember}]
GPS:CONFig:SURVey[:MODe]?
Description
The first definition sets the configuration. If mode is omitted, the GPS survey mode to the given
default mode is REDo. The second definition queries the current When DISabled GPS survey mode.
is selected no survey is carried out. This mode is appropriate in mobile environments. R o ED causes
the survey to be repeated each time the instrument is power cycled. REMember causes the results of
the survey to be saved in nonvolatile memory. When the instrument is power cycled, the surveyed
position is recalled from memory and the survey is not repeated. The user must execute the
command GPS:CONF:SAVE to save the current value to nonvolatile memory.
Example
GPS:CONF:SURV REM
Configure the GPS receiver to do a survey at startup the first time, but to remember the results of the
first survey from then on rather than repeating the survey.
GPS:CONFig:SURVey:FIXes Fixes GPS Configure Survey
Syntax
GPS:CONFig:SURVey:FIXes [<fixes>]
GPS:CONFig:SURVey:FIXes?
Description
The first definition sets the number of position fixes in the survey. If <fixes> is omitted, the default
number is 2000. The second definition queries the current number of position fixes in the position
survey. This is the number of position fixes that get averaged together to define the GPS antennas
location. Once the position is well known, the receiver can be put into over determined clock mode
and the receiver can provide improved timing by dedicating the signal from all satellites to timing.
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The user must execute the command GPS:CONF:SAVE to save the current value to nonvolatile
memory.
Example
GPS:CONF:SURV:FIX 3000
Configure the GPS receiver to include 3000 position fixes in the position survey.
GPS:CONFig:ALIGnment GPS Configure Timing Alignment
Syntax
GPS:CONFig[:TIMing]:ALIGnment [{UTC|GPS|GLONass|BEIDou|GALileo}]
GPS:CONFig[:TIMing]:ALIGnment?
Description
The first definition sets the 1pps alignment. If alignment is omitted, the default alignment is UTC.
The second definition queries the current 1pps alignment. When UTC is selected, all timing is
aligned to UTC. The other options align timing to the given navigation system. Any selection other
than UTC requires that that constellation be tracked as well. The user must execute the command
GPS:CONF:SAVE to save the current value to nonvolatile memory.
Example
GPS:CONF:ALIG UTC
Alignment of the 1pps output is to UTC.
GPS:CONFig:QUALity GPS Configure Timing Quality
Syntax
GPS:CONFig[:TIMing]:QUALity [{1SAT|3SAT}]
GPS:CONFig[:TIMing]:QUALity?
Description
The first definition sets the minimum number of satellites the receiver must track before outputting a
hardware 1pps pulse. If omitted, the default quality is 1 satellite. The second definition queries the
current timing quality. Timing quality generally increases as the number of satellites increases.
However, the user must also consider reliability and holdover performance. Degraded performance
may be preferred over no timing whatsoever. The user must execute the command
GPS:CONF:SAVE to save the current value to nonvolatile memory.
Example
GPS:CONF:QUAL 3SAT
Require the GPS receiver to track 3 satellites before outputting a hardware 1pps pulse for the
timebase to track.
GPS:CONFig:ADELay GPS Configure Timing Antenna Delay
Syntax
GPS:CONFig[:TIMing]:ADELay <delay>
GPS:CONFig[:TIMing]:ADELay?
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Description
Specify the surveyed position to be <latitude>, <longitude>, and <altitude>. The FS752 will take the
supplied values as the current surveyed position and enter over-determined clock mode. <Latitude>
and <longitude> should be specified in radians and <altitude> in meters. For <latitude>, use positive
values for north and negative values for south. For <longitude> use positive values for east and
negative values for The altitude refers to the height awest. bove the reference ellipsoid (WGS84).
Note that this is not the same as mean sea level.
Example
GPS:POS:SURV 0.652917714322,-2.1290993165,-26
Set the position for the position survey and go to over-determined clock mode.
GPS:POSition:SURVey:DELete GPS Position Survey Delete
Syntax
GPS:POSition:SURVey:DELete
Description
Delete the surveyed position stored in nonvolatile memory, if any.
Example
GPS:POS:SURV:DEL
Delete the current stored position in nonvolatile memory.
GPS:POSition:SURVey:PROGress? GPS Position Survey Progress
Syntax
GPS:POSition:SURVey:PROGress?
Description
Query how much of the position survey has completed. The will return an integer between 0 FS752
and 100 %.
Example
GPS:POS:SURV:PROG?
Query the progress of the position survey by the GNSS receiver.
GPS:POSition:SURVey:SAVe GPS Position Survey Save
Syntax
GPS:POSition:SURVey:SAVe
Description
Save the current position in nonvolatile memory. The receiver will subsequently use it to enter over-
determined clock mode where all satellites are used for maximum timing performance.
Example
GPS:POS:SURV:SAV
Save the current position in nonvolatile memory.
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GPS:POSition:SURVey:STARt GPS Position Survey Start
Syntax
GPS:POSition:SURVey:STARt
Description
Restart the GNSS receiver’s position survey. Note that previously saved results are not deleted by
this command.
Example
GPS:POS:SURV:STAR
Restart the GPS receiver’s position survey.
GPS:POSition:SURVey:STATe? GPS Position Survey State
Syntax
GPS:POSition:SURVey:STATe?
Description
Query whether a position survey is in progress or not. The query returns 1 if the survey is in
progress, otherwise 0.
Example
GPS:POS:SURV:STAT
Query whether a position survey is in progress.
GPS:SATellite:TRACking? GPS Satellite Tracking
Syntax
GPS:SATellite:TRACking?
Description
Query which GNSS satellites are being tracked by the receiver. The query returns the number of
satellites being tracked, followed by the IDs of the satellites as a comma ( , ) separated list.
Example
GPS:SAT:TRAC?
Query the number and IDs of the satellites being tracked by the receiver.
GPS:SATellite:TRACking:STATus? Status GPS Satellite Tracking
Syntax
GPS:SATellite:TRACking:STATus?
Description
The receiver has 20 channels for tracking satellites. This command returns the information shown in
Table for each channel12 , successively:
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Table 12: Satellite tracking information
Index
Parameter
0
Satellite ID number
1
Acquired
2
Ephemeris
3
Reserved
4
Signal level in dbHz
5
Elevation in degrees
6
Azimuth in degrees
7
Space vehicle type
If a channel is not tracking a satellite, it will return zero for all parameters. In all, 20 × 8 = 160
parameters are returned as a comma ( , ) separated list.
Example
GPS:SAT:TRAC:STAT?
Return tracking information on all satellites being followed.
GPS:UTC:OFFSet? GPS UTC Offset
Syntax
GPS:UTC:OFFSet?
Description
Query the current offset between UTC and GPS in seconds. As of January 1, 2017, UTC, which has
leap seconds inserted intermittently, is 18 seconds behind GPS which does not have leap seconds
inserted. Note that this command will return 0 until the time of day has been set properly.
Example
GPS:UTC:OFFS?
Query the current offset between UTC and GPS in seconds.
Source Subsystem
The Source Subsystem enables users to configure the phase of the 1 PPS output.
SOURce:PHASe Source Phase Adjust
Syntax
SOURce[{1|2|3}]:PHASe[:ADJust] {<phase>|MINimum|MAXimum|DEFault}
SOURce[{1|2|3}]:PHASe[:ADJust]? [{MINimum|MAXimum}]
Description
The first definition adjusts the phase of the selected output to <phase> or one of the keyword values.
The second definition queries the current phase for the selected output. se> <Pha is specified in
seconds. Positive values of <phase> cause the phase to lead the reference. Negative values of
<phase> cause the phase to lag the reference.
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Example
SOUR:PHAS:SYNC
SOUR:PHAS -10e-9
Synchronize the phase for the 1 output to UTC. Advance the phase by 10 ns. PPS
SOURce:PHASe:SYNChronize Source Phase Synchronize
Syntax
SOURce:PHASe:SYNChronize
Description
Adjust the phase for the 1 output to align with UTC, if possible. If UTC is not yet known, then PPS
phase is aligned to a common internal reference. The is aligned on the next second boundary of
UTC.
Example
SOUR:PHAS:SYNC
Synchronize the phase for the Sine output to UTC.
SOURce:PHASe:SYNChronize:TDELay Source Phase Synchronize Time Delay
Syntax
SOURce[{1|2|3}]:PHASe:SYNChronize:TDELay {<delay>|MINiumum|MAXimum|DEFault}
SOURce[{1|2|3}]:PHASe:SYNChronize:TDELay?
Description
This command enables the user to control alignment of the signal of an output relative to UTC. The
first definition sets the time delay to <delay> or the given limit. The second definition queries the
current value of the time delay. The <delay> can range from −1.0 to +1.0 seconds. Negative delays
advance the phase of the signal. Positive delays retard the phase of the signal. This command is
useful for correcting insertion delays of cables used to get signals from the FS752 to application
equipment. The factory default is 0.0 seconds. This is a system setting which is unaffected by a
*RST command or a recall of default settings. The setting is stored in nonvolatile memory and
automatically restored at power on.
Example
SOUR:PHAS:SYNC:TDEL -100 ns
Advance the phase of the ns. This will correct for1 tput by 100 PPS ou several feet of cable delay in
getting the signal to application equipment.
Status Subsystem
Commands in the Status Subsystem report on instrument status. Each element of status has 3
registers associated with it: a condition register, an event register, and an enable register. For a
detailed discussion of these registers and how they are related see Status Reporting on page 44.
STATus:GPS:CONDition? Status GPS Condition
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Syntax
STATus:GPS:CONDition?
Description
Query the current condition of the GPS receiver. See section GPS Receiver Status on page 51 for
detailed information on the interpretation of GPS receiver status. See Status Reporting on page 44
for more information on dition con registers.
Example
STAT:GPS:COND?
Query the current condition of the GPS receiver status register.
STATus:GPS:ENABle Status GPS Enable
Syntax
STATus:GPS:ENABle <mask>
STATus:GPS:ENABle?
Description
The first definition sets the mask for combining GPS receiver status bits into the summary bit located
in the serial poll status byte. The second definition queries the current mask. See section GPS
Receiver Status 51 on page for detailed information on the interpretation of GPS receiver status. See
section Status Reporting on page 44 for more information about enable registers.
Example
STAT:GPS:ENAB 1
Set the summary bit in the serial poll status byte if the time of day of the instrument has not been set
by the GPS receiver.
STATus:GPS:EVENt? Status GPS Event
Syntax
STATus:GPS[:EVENt]?
Description
Query the GPS receiver status event register. See section GPS Receiver Status 51 on page for
detailed information on the interpretation of GPS receiver status. See Status Reporting on page 44
for more information on event registe rs.
Example
STAT:GPS?
Query the event register for GPS receiver status. This returns all bits that have been set since the
previous query. The query then clears all bits.
STATus:OPERation:CONDition? Status Operation Condition
Syntax
STATus:OPERation:CONDition?
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Description
Query the current condition of operational status for the . See section Operation Status on FS752
page 50 for detailed information on the interpretation of the operation status bits. See Status
Reporting on page 44 for more information on condition registers.
Example
STAT:OPER:COND?
Query the current condition of operational status for the . FS752
STATus:OPERation:ENABle Status Operation Enable
Syntax
STATus:OPERation:ENABle <mask>
STATus:OPERation:ENABle?
Description
The first definition sets the mask for combining operational status bits into the summary bit located
in the serial poll status byte. The second definition returns the current mask. See section Operation
Status 50 for detailed information on the interpretation of operational on page status bits. See section
Status Reporting on page for more information about enable registers. 44
Example
STAT:OPER:ENAB 2
Set the summary bit in the serial poll status byte if instrument settings changed since the previous
query of the event register.
STATus: Status Operation EventOPERation:EVENt?
Syntax
STATus:OPERation[:EVENt]?
Description
Query the event register of operational status for the FS752 Operation Status 50 . See section on page
for detailed information on the interpretation of the operation status bits. See Status Reporting on
page 44 for more information on event registers.
Example
STAT:OPER?
Query the event register for operational status. This returns all bits that have been set since the
previous query. The query then clears all bits.
STATus:QUEStionable n? Status Questionable Condition:CONDitio
Syntax
STATus:QUEStionable:CONDition?
Description
Query the current condition of questionable status for the FS752. See section Questionable Status on
page 49 for detailed information on the interpretation of questionable the status bits. See Status
Reporting on page 44 for more information on condition registers.
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Example
SYST:ALAR?
Query the current state of the system alarm.
SYSTem:ALARm:CLEar System Alarm Clear
Syntax
SYSTem:ALARm:CLEar
Description
Clear the event register for the system alarm. When the current mode for command
SYST:ALARm:MODe is LATCh, this will clear the alarm assuming current limits are not being
exceeded.
Example
SYST:ALAR:CLE
Clear the event register for the system alarm.
SYSTem:ALARm:CONDition? System Alarm Condition
Syntax
SYSTem:ALARm:CONDition?
Description
Query the condition register for the system alarm. This register indicates which of the possible alarm
conditions are currently true. The system alarm will only be asserted if a condition is true and it is
enabled in the enable register.
Example
SYST:ALAR:COND?
Query the condition register for the system alarm.
SYSTem:ALARm:ENABle System Alarm Enable
Syntax
SYSTem:ALARm:ENABle <mask>
SYSTem:ALARm:ENABle?
Description
Mask possible alarm conditions so that only those that are enabled here can cause the system alarm
to be asserted. When the current mode for command SYST:ALARm:MODe is TRACk, this register
masks the condition register for the system alarm, SYST:ALARm:CONDition. When the current
mode for command SYST:ALARm:MODe is LATCh, this register masks the event register for the
system alarm, SYST:ALARm:EVENt.
Example
SYST:ALAR:ENAB 1
Enable alarm if the time of day has not been set by GPS.
Remote Programming 69
FS752 GNSS Time and Frequency Reference Stanford Research Systems
SYSTem:ALARm:EVENt? System Alarm Event
Syntax
SYSTem:ALARm:EVENt?
Description
Query the event register for the system alarm. This register indicates which of the possible alarm
conditions that have been latched since the last time the event register was cleared. When the current
mode for command SYST:ALARm:MODe is LATCh the system alarm will be asserted if an event
condition is true AND it is enabled in the enable register. Note that unlike the event registers in the
Status Subsystem, reading this register does not clear it. It must be explicitly cleared with the
SYSTem:ALARm:CLEar command.
Example
SYST:ALAR:EVENt?
Query the condition register for the system alarm.
SYSTem:ALARm:FORCe:STATe System Alarm Force State
Syntax
SYSTem:ALARm:FORCe[:STATe] {1|ON|0|OFF}
SYSTem:ALARm:FORCe[:STATe]?
Description
The first definition sets the forced state of the alarm. The second definition queries the current forced
state of the alarm. This value only has significance if the current mode for command
SYST:ALARm:MODe is FORCe.
Example
SYST:ALAR:FORC ON
Assert the alarm if the alarm mode is FORCe.
SYSTem:ALARm:MODe System Alarm Mode
Syntax
SYSTem:ALARm:MODe {TRACk|LATCh|FORCe}
SYSTem:ALARm:MODe?
Description
The first definition sets the alarm mode to one of three options: track, latch, or force. The second
definition q the current . ueries alarm mode
Tracking mode causes the alarm to follow current conditions. The alarm is asserted when current
limits are exceeded. The alarm is de-asserted when current limits are no longer exceeded.
Latching mode causes the alarm to be asserted when current limits are exceeded. However, the alarm
will not be de-asserted until explicitly requested to do so and the limit is no longer exceeded.
In force mode, the user manually sets the state of the alarm.
Example
SYST:ALAR:MODe
Remote Programming 70
FS752 GNSS Time and Frequency Reference Stanford Research Systems
Query the current mode for the system alarm.
SYSTem:ALARm:GPS:TINTerval System Alarm GPS Time Interval
Syntax
SYSTem:ALARm[:GPS]:TINTerval {<time error>|MINimum|MAXimum|DEFault}
SYSTem:ALARm[:GPS]:TINTerval?
Description
The first definition sets the time interval between GPS and the internal timebase that must be
exceeded before the alarm condition for a timing error is asserted. The <time error> may range from
50 ns. The second definition queries the current value for the time ns to 1 s. The default is 100
interval.
Example
SYST:ALAR:TINT 1 us
Set the timing error limit to 1 This means that the alarm condition for μs. timing errors will not be set
unless the measured time interval between GPS and the internal timebase exceeds 1 μs. Note that the
system alarm will not actually be asserted unless this condition is also enabled with the
SYST:ALARm:ENABle command.
SYSTem:ALARm:HOLDover:Duration System Alarm Holdover Duration
Syntax
SYSTem:ALARm[:HOLDover]:DURation {<duration>|MINimum|MAXimum|DEFault}
SYSTem:ALARm[:HOLDover]:DURation?
Description
The first definition sets the amount of time in seconds that the must be in holdover before the FS752
alarm condition for holdover is asserted. The <duration> may be any 32 bit unsigned integer. The
default is 0. The second definition queries the current value for the duration.
Example
SYST:ALAR:DUR 100
Set the holdover duration to 100 seconds. This means that the alarm condition for holdover will not
be set unless the FS752 is in holdover for more than 100 seconds. Note that the alarm will not
actually be asserted unless this condition is also enabled with the SYST:ALARm:ENABle command.
SYSTem:COMMunicate:SERial:BAUD System Communicate Serial Baudrate
Syntax
SYSTem:COMMunicate:SERial:BAUD {4800|9600|19200|38400|57600|115200}
SYSTem:COMMunicate:SERial:BAUD?
Description
The first definition configures the RS-232 port to operate at the selected baud rate. The second
definition queries the current baud rate. Note that the new configuration does not take effect until the
port is reset via a SYSTem:COMMunicate:SERial:RESet command or the power is cycled.
Example
SYST:COMM:SER:BAUD 115200

Produktspecifikationer

Varumärke: SRS
Kategori: Inte kategoriserad
Modell: FS752

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