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

Tom Van Baak (lab) tvb at leapsecond.com
Sat May 31 18:37:26 EDT 2014


Check rising/falling edge times, check trigger levels, check signal levels and load. Are you DC or AC coupled. How about power supply noise and grounding? Which edge are you actually triggering on? Is there crosstalk with the Tx line.

It is extremely unlikely that there's a problem with the UT+ itself -- this series of Motorola Oncore GPS boards has served the time & frequency community for two decades with no problems, or jitter, or off-by-100us errors.

It will be fun to meticulously track down the bug; don't just make it go away and move on. We will all learn from this interesting problem.

I'll trade you a perfect UT+ if you give up. At some point we time nuts stop getting excited about what works and delight in what doesn't, and why.

/tvb (i5s)

> On May 31, 2014, at 3:02 PM, David Feldman <wb0gaz at yahoo.com> wrote:
> 
> 
> Picked this project back up this afternoon after couple of weeks, narrowed the problem down but not solved.
> 
> Refresh: My ONCORE UT+ module appears to have very extreme jitter (on on order of 100 uSec, not nSec random variation between leading edges of successive pulses) in the 1 PPS output. I am using the 1 Hz signal, divided to 0.05 Hz at 50% duty cycle (10 seconds high, 10 seconds low), to gate my frequency counter's reference oscillator back into the same counter running in pulse-count mode so I can adjust the OCXO bias on the counter for calibration. 10 seconds count-up, followed by 10 seconds idle where I can record the measurement, reset the counter, and adjust the OCXO bias.
> 
> I've now built a simple 1 Hz pulse generator using a 32768 Hz crystal and CD4060 divider. Using that signal instead of the ONCORE UT+, my counter is behaving as expected (that is, each count-up session results in the same result, +/- a few counts as would be expected with a simple 1 PPS signal.) This tells me the gating circuit and interface to the counter's reference output and count input are probably OK.
> 
> When using the ONCORE UT+, the counts vary by 10000 or more from one 10-second capture to the next (each count-up period would total 130,000,000 counts if the ONCORE UT+ was working properly and the reference OCXO was properly adjusted.)
> 
> I am not yet able to determine why the UT+ 1 PPS signal has such severe jitter. TAC software tells me 7-8 satellites acquired/tracked once cold start has completed (10-15 minutes.) Is there something I'm missing about UT+ care and feeding? There are no RF or switching digital signals close to the UT+ module (the gating circuit is a couple of inches away, both mounted on the same perf board, with common ground and power supply connections.
> 
> Now that I've ruled out the counter/gating circuit interface, I begin to suspect the UT+ but don't really know what to expect in terms of normal/abnormal operation.
> 
> Any advice at this point?
> 
> Thanks,
> 
> Dave
> 
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