[time-nuts] Receiving the MSF time signal on cheap radio modules

Bob kb8tq kb8tq at n1k.org
Tue Feb 6 20:16:37 EST 2018


> On Feb 6, 2018, at 3:48 PM, Deirdre O'Byrne <deirdre.dub at gmail.com> wrote:
> Tom,
> Thanks for the feedback!
> On 6 February 2018 at 20:29, Tom Van Baak <tvb at leapsecond.com> wrote:
>> 2) Not all decoding errors are equal. Since this is a time code instead of
>> arbitrary binary data you can use the internal structure of the data to
>> your benefit.
> As I said to Poul-Henning, that is the next level of error detection, which
> also has application in error correcting some of the "almost-right" signals.
>> 3) A side-effect of your data set is that you can track performance of the
>> oscillator inside the logic analyzer: convert the 700k GPS timestamps into
>> interval, find and replace the 4 glitch lines with 2 lines of 1.000150, and
>> then use Stable32 or TimeLab to plot. I used a 10 minute running average to
>> reduce the 50 us quantization noise. Note the mean frequency of your
>> timebase is 152 ppm low.
> I made it out to be 152.2ppm, which is kinda disappointing. But the signal
> analyser cost very little, and you get what you pay for.
> I have not yet wrapped my head around how to create ADEV plots, so thanks
> for your work on that - it's interesting to see that (presumed) initial
> thermal effect.
>> Over 8 days this results in a cumulative sampling error of 105 seconds. If
>> your decoding algorithms are relative instead of absolute this won't be a
>> problem. OTOH, you may be able to use your decoding process to detect this
>> drift and then compensate for it in software. You have the beginnings of a
>> MSF-Disciplined-Oscillator project.
> MSF disciplined oscillator?! I don't trust these receivers to any better
> than about the 20ms mark, so such a disciplined oscillator would have quite
> a long integration time!

Once upon a time, that *was* how people did disciplined oscillators. Part of the
answer to “how?” is that their target accuracies were not as tight as what we now
think of as normal. 


> Thanks again.
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