[time-nuts] Phase noise from Allan Deviation ?

[Anders Wallin]

>From PN to ADEV you integrate under the PN-curve with a certain
weighting-function. This paper shows the weighting function:
https://www.researchgate.net/publication/6308454_Considerations_on_the_measurement_of_the_stability_of_oscillators_with_frequency_counters

to go the other way around you would indeed have to assume the shape of the
PN-curve, such as the Leeson model or similar. This is quite an assumption
and probably best avoided.

What you want to do is rather than mix down to 1Hz instead mix down to
maybe 1 kHz...200 kHz or so - stream all those phase-samples to the
computer, and compute the PN as the PSD of the phase data.
As an aside I've found that FFT (e.g. Stable32 uses this) sometimes gives
funny PSD results, and the Welch-method (periodogram) is more robust.

Anders


On Mon, Dec 14, 2015 at 5:15 AM, Tom McDermott <tom.n5eg at gmail.com> wrote:

> I've constructed a homebrew setup to measure time intervals using a
> software defined radio.  Basically a single-channel downconversion to
> about one hertz, then count samples from the SDR clock to time stamp
> the zero crossings.  This is done in gnuradio and saved to a file for
> post processing. The resolution is theoretically good, but the accuracy
> is unknown.
>
> The result produces ADEV vs. Tau charts with reasonably sane looking
> results.
> The 'unknown' is a Rubidium oscillator locked to CDMA pilot (TS2700),
> and the internal crystal oscillator in the SDR radio as the reference.
> Thus, ADEV probably is mostly measuring that internal crystal rather than
> the TS2700.  Later on a GPSDO will be tried as the reference clock to
> see if the Adev results are better.
>
> It brings up a question:  Is it possible to estimate the phase noise of
> that
> internal crystal from the ADEV measurements?  There are a bunch of
> papers that go the other way:  from Phase Noise to Adev.   Searching
> brings up only one paper that goes from ADEV to Phase Noise but it's text
> does not seem to be readily available.  It apparently models the oscillator
> as a couple of well known error models.
>
> -- Tom, N5EG
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