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

[Bob Camp]

Hi

Alll of the GPSDO’s that NIST is looking at are 1 PPS based. What’s different about them:

1) The OCXO’s (or Rb’s) are stable to 0,001 ppb at 1 second
2) The GPS is designed for timing (and spec’d by an outfit that understands timing — see below) (uBlox maybe)
3) The GPS puts out a sawtooth correction and the loop uses it.
4) The GPS PPS and OCXO are compared at the sub nanosecond level to feed the loop
5) The loop parameters are optimized such that the GPS does not feed though much at < 1,000 seconds
6) The OCXO is stable enough that it can chug along without help from the GPS for hundreds of seconds 

That’s the short list, there is probably a much more detailed list, but that gets into a lot of “that depends” 
sort of stuff. With $25 eBay “treasures” popping up from time to time the cost target is very fluid on these 
things.

One thing to take a look at: NIST has papers on L1 ionosphere correction (= lack of). Note from the papers how it must 
impact your module. After you dig a bit into that 10 to 50 ns per day variable, take a look again at the 
10 ns manufacturer’s accuracy claim in your module...

Bob

> On 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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