[time-nuts] Beginner question - unexpected possible jitter in 1 PPS output of Motorola ONCORE UT+ module

Hal Murray hmurray at megapathdsl.net
Mon May 26 21:58:05 EDT 2014


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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