[time-nuts] I've designed a GPSDO, but how "good" is it?

Tim Shoppa tshoppa at gmail.com
Sun Aug 16 17:22:35 EDT 2015


Nick, professional GPSDO's achieve precision much less than one 10MHz
count, by doing sub-nanosecond level time interval measurements between GPS
PPS and the OCXO-divided-by-10M-PPS. During initial lock there is some
software smarts to reset the divide-by-10M chain at the right point so the
initial lock is a bit more like a frequency lock rather than a phase lock -
these smarts also kick in if there is a substantial disturbance to phase to
be resolved (that substantial difference probably doesn't require a real
EFC correction but instead is some kind of glitch ridden over by going to
frequency lock.)

Google things like "z3801a tic" (tic = time interval converter) to see past
discussions about the state of the art a few decades ago.

Tim N3QE

On Sun, Aug 16, 2015 at 2:47 PM, Nick Sayer via time-nuts <
time-nuts at febo.com> wrote:

> I’ve designed and make and sell a GPSDO on Tindie (
> https://hackaday.io/project/6872-gps-disciplined-tcxo). It’s brand new -
> I’ve sold a handful of them so far. So as to make this post not *entirely*
> self-serving, what I would like is some further guidance on how I can
> better characterize its performance.
>
> The GPS reference is a 1 pps signal (It’s the Adafruit Ultimate GPS module
> - a PA6H). The manufacturer claims an accuracy of ±10 ns, but that's
> accuracy relative to the true start of the GPS second. They don’t make any
> claim for stability.
>
> The oscillator itself (Connor Winfield DOT050V 10 MHz) has a short-term
> (though they don’t say how short that term is) stability of 1 ppb. The
> absolute accuracy of it is (I assume) irrelevant, because it’s a VCTCXO and
> the control voltage is steered by GPS feedback.
>
> The feedback loop takes samples over a 100 second period. That gives me an
> error sample with a granularity of 1 ppb. I keep a rolling sample window of
> 10 samples to get an error count over 1000 seconds. I've kept track of both
> of these values for extended periods (days) as well as logging the DAC
> value (the number that's proportional to the control voltage). The 1000
> second sample window error averages zero, and it almost never exceeds ±7
> (every once in a while if I physically move it, it will show a momentary
> error glitch, but that shows up in the short term feedback sampling too).
> The 100 second samples are almost all 0 or ±1, with an occasional ±2
> showing up. As I said before, if I bonk the oscillator, it may briefly show
> a ±6 or so for one sample.
>
> If I pit two of them against each other on a scope and take a time lapse
> video (http://www.youtube.com/watch?v=9HkeCI90i44), you can see that they
> stay mostly locked with occasional periods of drift. I sort of assume that
> that represents periods where the two GPS receivers disagree as they decide
> differently how to select among the available satellites.
>
> I've been saying out loud that the oscillator is ±1 ppb from GPS over the
> 1000 second window. I know of Allan variance, but I don't have anything
> else handy I can use for comparison. I also can't really afford to send one
> off for testing to a proper lab. In looking at
> http://tf.nist.gov/general/pdf/2297.pdf, it suggests that my results are
> relatively poor compared to what a GPSDO can achieve (more like 10^-12
> rather than 10^-9), but I assume that they’re able to use a higher
> frequency GPS reference than just 1 PPS (and they’re a lot pricier).
>
> What else can I do to try and characterize the performance? If mine is
> performing far more poorly than the same price ($175) can buy elsewhere,
> then what am I doing wrong?
>
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