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

[Tom Van Baak]

Deirdre,

That's a nice piece of work. Thanks for posting it. And I hope we continue to see results as you refine the decoding process.

Some comments:

1) Knowing the UTC tick (via GPS) is, of course, a benefit. If it were an isolated MSF receiver you wouldn't have this information, but for your purposes of exploring algorithms it is useful.

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. For example, you know all the bits for the year and month and day of week shouldn't change over an entire day and so this increases your decoding confidence. Day of week is correlated to date. Hours and minutes will increment and you can anticipate this. A large number of month, day, hour, and minute values will never be transmitted. For any BCD value, 6 of 16 states are invalid. There are parity bits. So all of these constraints can guide your optimal weighted decoding process. You essentially decode against a known template and this is more robust than having to decode blind.

3) It's clever to use a 20 kHz synchronous logic analyzer for this analysis. As you observe, the 50 us granularity is sufficient for your needs. 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. 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.

Attached are some frequency and ADEV plots for your oscillator. Note the initial frequency drift during the first hour of the run; probably a thermal effect. Ok, these plots have nothing to do with decoding MSF, but it does show how one can use a logic analyzer as a cheap "counter" for long-term frequency accuracy and stability measurements. It's essentially what the picPET, or any other time-stamping counter, does.

/tvb

----- Original Message ----- 
From: "Deirdre O'Byrne" <deirdre.dub at gmail.com>
To: "Discussion of precise time and frequency measurement" <time-nuts at febo.com>
Sent: Tuesday, February 06, 2018 10:40 AM
Subject: [time-nuts] Receiving the MSF time signal on cheap radio modules


> OK so it's not the microsecond or nanosecond stuff that much of this list
> is about, but I've been running an experiment for the past few days
> gathering data on how well (or otherwise) a pair of cheap EM2S radio
> receiver modules receive the MSF radio signal. I've been trying to see if I
> could design a decoding algorithm that would be more noise-tolerant than
> the algorithms I've seen out in the wild.
> 
> Details are on my github, and the results are presented in a paper -
> 
> https://github.com/deirdreobyrne/MSF-EM2S/blob/master/paper.pdf
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