[time-nuts] Beginner question - unexpected possible jitter in 1 PPS > output of Motorola ONCORE UT+ module (Hal Murray)
wb0gaz at yahoo.com
Tue May 27 08:33:32 EDT 2014
I'm using 74HC series for the divider/gate.
I appreciate all the troubleshooting help, however, I still don't know under what conditions the 1 PPS output should be relied on (with zero satellites acquired? one? four? all eight?, and what can be expected of the 1 PPS output interval if there are zero satellites, given that the signal is still being generated by the module?
As for the balance of the suggestions regarding circuit and counter behavior, these are all very good pieces of advice; I may just put together a stand-alone 1 PPS generator (just another oscillator and divider chain) so I can isolate whether this problem is possibly with the GPS module or (more likely) an impairment in my circuit, counter, measurement setup, or interfaces.
>wb0gaz at yahoo.com said:
>> I built a small CMOS divider connected to the UT+ 1 PPS output...
>What do you mean by "CMOS"? Old 4000 series parts or HC or ... ?
>> Here's the problem: my counts vary significantly (+/- 10000 or more) from
>> cycle to cycle, which is much higher than expected based on 50 nS period
>> uncertainty of the 1 PPS ...
>Do you have a scope?
>I'd check the triggering levels. Your counter may be setup to just barely trigger on your CMOS signal. Your CMOS divider may have troubles triggering on the output of the UT+.
>One experiment would be to connect the PPS directly to the gate input on your counter. That won't tell you your crystal frequency, but if things work, it should get a consistent count each pulse.
>If you have a digital scope, you can trigger on the PPS and look at the PPS in delay by a second mode. (That tells you the accuracy of the crystal in your scope.) If the PPS delayed by a second isn't stable, you know the UT+ is confused.
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