[time-nuts] GPSDO and holdover

Tom Van Baak tvb at LeapSecond.com
Tue Apr 22 21:57:50 EDT 2014


Hi Bob S,

I have noticed skipped 1PPS on the Adafruit GPS also. Some days are clean, other days miss a few samples. I have not explained it yet. I plan to run two GPS boards and two counters to narrow down the cause.

In any event, IMHO, a GPSDO should not go crazy if glitches like this occur. I don't think it should go into holdover for one missed sample, or even a few missed samples. But then you need to define what holdover is. I mean, by some definition a GPSDO is in "holdover" between every second.

I do not think there is any standard. Just conventions: some documented, some not. It's attention to a dozen little details like this that separate a quick hack GPSDO from a quality one. You'll also face a number of design issues at startup, and coming out of holdover. Lastly, you get to choose between it being an ideal time standard vs. an ideal frequency standard.

/tvb

p.s. Please fix your address book. The correct email for the list is time-nuts at febo.com

----- Original Message ----- 
From: Bob Stewart 
To: time-nuts-owner at febo.com 
Sent: Tuesday, April 22, 2014 6:03 PM
Subject: GPSDO and holdover


I hope I haven't asked this before, but is there a standard way of deciding to go into holdover mode?  I'm still wrapping up code for this Adafruit, and as I've posted before: every now and then it skips a PPS.  I'm trying to decide whether to allow a free pass (if not followed by another skip within some timeframe) or to immediately stop processing any further PPS pulses until I decide based on some criteria that they're reliable.

Bob - AE6RV



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