[time-nuts] GPSDO and holdover

Tom Van Baak tvb at LeapSecond.com
Wed Apr 23 01:57:50 UTC 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.


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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