7 Appendix: One Pulse Per Second Source Connection
This appendix describes how to connect a 1PPS source, such as a commercial GPS receiver, to an SA.22c
to achieve long term accuracy and excellent holdover, or flywheeling performance.
7.1 Connection Requirements
The following connections are required for 1PPS setup:
Power
GPS antenna/receiver or other 1PPS source reference.
1PPS cabling from the source to the SA.22c
No serial port communication is required for initial setup unless the changes are made from the factory
default settings. Information on setup, operation, and integration is provided.
7.2 Background
GPS technology has made time and frequency synchronization possible (available) worldwide.
Connecting the 1PPS output from a commercial (civilian) GPS receiver to an SA.22c provides a cost
effective system that maintains highly accurate time and frequency even when GPS signals are
unavailable, for example, during jamming and antenna maintenance.
The GPS system provides 1PPS signals worldwide with good long term stability (that is, <1×10
–12
averaged over 24 hours). However, the short term stability of this signal is often compromised by
various noise sources, for example, man-made, atmospheric conditions, crosstalk, RF multi-path or inter
symbol interference, and GPS receiver oscillator limitations.
Microsemi has pioneered the use of rubidium oscillators in telecommunication applications.
Telecommunications applications require long term and short term stability beyond the range of free
running quartz oscillators. For example, cellular CDMA systems require 1PPS signals to be synchronized
within 2 μs over long periods of time even when GPS signals are not available. To achieve this
performance, system designers must combine the benefits of short term stability (from a rubidium or
low noise OCXO) with long term stability (from GPS, Loran-C, Glonass, or Cesium). Microsemi is the
leader in system products with microprocessor driven circuitry that uses the GPS 1PPS system to steer
various oscillators (Cesium, Rubidium, and Quartz). These products make it possible to combine the
short term with long term stability. With SA.22c, the solution is more cost effective. The performance
level of SA.22c used with a GPS receiver approaches the performance of Cesium oscillators used in
telecommunication systems.
7.3 1PPS Functions
The SA.22c can be configured to:
Generate a rubidium controlled 1PPS signal
Measure the difference between an incoming 1PPS signal and the SA.22c 1PPS
Synchronize the SA.22c’s frequency and 1PPS output to the incoming 1PPS and provide long
holdover times
The following figure shows the SA.22c 1PPS disciplining block diagram.
When an externally generated 1PPS signal is applied to the 1PPS input pin of a properly configured SA.
22c, the unit provides the time interval error difference between the 1PPS input and the 1PPS generated
inside the SA.22c (see table ). The difference is 1PPS States Returned with the j Command (see page 33)
read using the RS232 communications j command. The j command displays the difference between the
1PPS input and the 1PPS generated internally by the SA.22c. The j command generates a number
representing the number of TICS in a delta register. If the SA.22c has a 60 MHz crystal, each TIC is 16.7
ns (1.67×10 ). This number is in hex format.
–8
The test bench setup configuration allows the SA.22c to be disciplined by the incoming 1PPS signal. The
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