Hi Nick,
In fact, if no-one else has volunteered, contact me offline and I'll run a test
for you on my equipment, which is an HP5370A and a Symmetricom PRS-45A Cesium
standard.
bob at evoria dot net
From: Tom Van Baak <[email protected]>
To: Discussion of precise time and frequency measurement <[email protected]>
Sent: Sunday, August 16, 2015 2:31 PM
Subject: Re: [time-nuts] I've designed a GPSDO, but how "good" is it?
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" <[email protected]>
To: <[email protected]>
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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