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

Graham / KE9H ke9h.graham at gmail.com
Sun Aug 16 16:53:26 EDT 2015


>From your description, it sounds like you have a frequency locked loop with
a maximum frequency sensitivity of 1 ppb, as opposed to a phase locked loop.

At the end of the 100 second count period, do you start over with a counter
set to zero, or do you carry forward any error from the previous period?

--- Graham


On Sun, Aug 16, 2015 at 2:31 PM, Tom Van Baak <tvb at leapsecond.com> wrote:

> Hi Nick,
> Nice project. Thanks for sharing.
> I was hoping someone would someday use a cheap Sparkfun / Parallax /
> Adafruit GPS to make a low-cost GPSDO. Mostly what people on this list do
> is go to the extreme of using serious GPS timing receivers (such as Oncore
> M12+T, or ublox-5T or 6T or 8T) but those require significant amounts of
> configuration, tuning, survey, etc. to meet ultimate performance levels.
> The good thing about using a cheap hobbyist-grade 3D GPS/1PPS receiver is
> that they work anywhere, without fiddling or survey, within seconds of
> power-on. Ok, you lose a few ns of precision compared to serious receivers
> -- but for a TCXO that doesn't matter.
> Anyway, to answer your question -- to measure its true performance you
> only need two things. 1) a phase meter (or time interval counter) that's
> good to 1 ns or better, and 2) a local reference standard that's maybe 10x
> better than the TCXO and the Adafruit GPS. Usually that means a cesium
> standard, or supremely qualified GPSDO, or equivalent.
> A number of us here on the time-nuts list have such equipment at home. And
> unlike professional labs, we will do it for free/fun if you loan the GPSDO
> for a week.
> If you want to play with raw timing data from an Adafruit GPS board see
> file gps-mtk3339.txt.gz under http://www.leapsecond.com/pages/gpsdo-sim/
> and then use TimeLab for phase, frequency, and Allan deviation analysis (
> http://www.ke5fx.com/timelab/readme.htm).
> /tvb
> ----- Original Message -----
> From: "Nick Sayer via time-nuts" <time-nuts at febo.com>
> To: <time-nuts at febo.com>
> Sent: Sunday, August 16, 2015 11:47 AM
> Subject: [time-nuts] I've designed a GPSDO, but how "good" is it?
> 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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