1.
2.
3.
1PPS input signals must have repeatable rise time with minimal ringing and must conform to the
following:
1PPS input is driven by a standard 3.3 V logic, 5 V CMOS, or 5 V TTL with normal operation at:
Input voltage logic high: 2.00 V minimum
Input voltage logic high: 5.50 V maximum
Input voltage logic low: 0.00 V minimum
Input voltage logic low: 0.80 V maximum
Maximum DC overshoot must be limited to 5.5 V or 10 mA, whichever is easiest to achieve.
Maximum DC undershoot must be limited to –0.5 V or 10 mA, whichever is easiest to achieve.
Minimum pulse width (or hold time) of 50 ns.
Input impedance >100 KΩ allows you to terminate the 1PPS at the input to the SA.22c with 50 Ω
at the SA.22c input pin or drive the SA.22c high impedance directly with a low impedance source
such as 50 Ω or any ACMOS gate as long as the input voltage level at the SA.22c pin is met as
described above.
The SA.22c rubidium oscillator subsystem must be locked to achieve synchronization.
The SA.22c must be set up with the proper time constant and damping factor.
SA.22c adapter kit is recommended to facilitate setup.
PC running Microsoft Windows with terminal emulation program. (57600 bps, 8 bits with no parity).
7.5 1PPS Algorithm Operation
There are two parameters that are modified by the user for 1PPS synchronization using the command y
– damping factor and tau.
Damping factor—determines the relative response time and ringing in response to each step. Values
must be between 0.25 and 4. Values less than 0.25 defaults to 0.25 while values over 4 defaults to 4.
Tau (or time constant)—expressed in seconds and determines the time constant of the PLL for following
a step in phase for the reference. The range of tau is 5 to 100,000 seconds. Values outside this range
cause both the damping factor and tau to change to the factory default settings.
7.5.1 Factory Default
The factory default requires no inputs to the rubidium oscillator. The default value for damping factor is
1 and the default value for tau is 400. These values are a good starting points and work well for most
GPS applications.
7.6 Setting the 1PPS Synchronization
The following are the steps for setting 1PPS signal synchronization (SA.22c with 1PPS enabled customer
version of firmware at revision 6.05c or higher installed.)
Connect the SA.22c to the adapter test board of the developer’s kit or to a correctly configured
equivalent system. (see )Appendix: Using the Developer’s Kit (see page 16)
Ensure that the terminal emulation program is configured to 57,600 BPS, 8 bits, no parity, no flow
control, and the keyboard caps lock is off. All inputs must be in lower case.
Power up the system. The header information from the SA.22c is displayed as follows, where the
firmware version needs to be confirmed.
SA22C by Symmetricom, Inc., Copyright 2006
SA22 Version 6.05C of 7/2006; Loader Version 3
Mode CN03 Flag 0000 [C91F]ok
Unit serial code is 0612SA3763-h, current tuning state is 6
Crystal: 60000000hz, ACMOS: 10000000.0hz, Sine: 10000000.0hz
097-16313-201 Designer Reference and User Guide Revision B 36
Ctl Reg: 0044, Res temp off: -1.5410, Lamp temp off: -1.9466
FC: enabled, Srvc: low
Enter Run Mode
FC mode is enabled
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