[time-nuts] Measurement of Peak Frequency Using a Perseus SDR

Mark Goldberg marklgoldberg at gmail.com
Mon Jan 21 03:49:14 UTC 2019


On Sun, Jan 20, 2019 at 5:10 PM Magnus Danielson <magnus at rubidium.se> wrote:

> Hi Mark,
>
> I have not seen anyone comment on this post yet.
>
> It's an interesting approach and in general, this is one of the ways we
> can expect that frequency/stability measurements to be done these days,
> by sampling the RF and analyze it.
>
> It would be interesting to figure out why you have a "bump" there, so I
> wonder what part does that.
>

The bump in the later plot, with the Bodnar measured against the
Trueposition is almost certainly due to a PLL. There is the Bodnar and also
the Wenzel and I don't know exactly what each is doing and what all the
time constants are.

I have no clue why there is a bump on the first wraparound plots at about
Tau=1000s.


>
> For one thing, if the frequency estimation algorithm you depend on does
> averaging/least-square style of algorithms, the lower taus will be
> significantly lower than expected. TvB has some pretty good plots from
> experiments illustrating this.
>
>
In my various experiments I could see when it was averaging and the lower
Taus did decrease. I chose such that this did not happen. I reviewed TvB's
paper at http://www.leapsecond.com/pages/adev-avg/ many times and saw most
of the effects described there, resolution, noise and averaging.


> Another thing, one reason to get a "bump" is due to high Q in a PLL
> circuit.
>
> However, I'd be careful to judge it to be any of these without more
> careful look on the data.
>
> It can be useful to alternate views to figure things out. For instance,
> swapping between MDEV and TDEV could give you some hints. Similarly
> swapping between frequency and phase does the same in raw-data view.
>
> For phase/frequency plots, it may be worthwhile to average the data
> using the +/- keys in TimeLab, as the filtering away high-frequency
> noise may make it easier to see lower-frequency variations.
>

I've done this ad nauseam, eventually finding some anomaly that happened at
various times of the day.


>
> In general, I recommend you to have a third reference to play around
> with and measure. As one fools around with different combinations one
> learns to see which artefacts follows which device.
>

I'm pretty sure the Bodnar is the least stable device I have, as it is
based on a TCXO and it does have some issues that I know Leo has improved,
but I can't spare the time to send it back to him right now.

I don't have another device to use that will give me the 80 MHz I need for
the SDR clock that also gives me good phase noise. Both the Trueposition
and Bodnar/Wenzel are pretty good, at least better than what I am measuring.


>
> Do keep up the investigations. Try different approaches and learn.
>
>
 At this time, I am going to use what I have. I think it is the best
approach I could come up with, using the available equipment.

Regards,

Mark



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