[time-nuts] Re: Testing GPSDOs

[Andy Talbot]

Dunno if this will do what you want ...

Get hold of some PLL Fract-N synthesizers, like the ubiquitous ADF4351, so
you can multiply up from two sources. Mix the outputs and look at the
frequency deviation over time.
My tests using this technique can be seen at
http://g4jnt.com/10MHz_Reference_Source_Stability.pdf    and
http://g4jnt.com/ShortTermStabilityLeoBodnarGPSDO.pdf

I used synths I had to hand, but anything that will multiply up your test
and reference signals to a pair of frequencies separated by a few kHz, so
you can look at the beat note using soundcard software.


Andy
www.g4jnt.com



On Sun, 20 Feb 2022 at 17:10, Erik Kaashoek <erik at kaashoek.com> wrote:

> I'm trying to check the short term stability of a TCXO, for tau below 1
> second
> With my frequency counter and timelab the shortest tau usable is 0.02
> seconds but the decrease of accuracy of the counter with this small tau
> starts to limit the measurement. But with Vtune of the TCXO at min or
> max the frequency deviation is well below 0.5e-9. In between Vtune
> produce bigger frequency deviations, don't understand why but if there
> is digital stuff inside the TCXO anything is possible I recently learned.
> So I'm trying to use an SA/Modulation analyzer to look at the spectrum
> of overtones  with 1Hz resolution or watch the FM modulation analysis
> The FM modulation analysis does not give understandable information. At
> the fundamental of the 10MHz the frequency deviation is reported as
> 25Hz, for the harmonics of the 10MHz the frequency deviation increase
> with the harmonic so the 25Hz may be correct.
> But the measured modulation frequency is 10kHz so I tried to output the
> demodulated FM into a good PC audio card and use audio FFT SW so see if
> there is anything recognizable but I can not see any peak above the
> noise. Listening to the demodulated FM also does not reveal a lot, just
> a very faint wistle of some tones.
> The small span scans on the SA show 50 Hz sidebands, even when the whole
> TCXO is running on battery power so I'm confused.
> In short, this requires still a lot of time to learn how to measure and
> to create understanding.
> And investing in a timepod is currently not feasible.
>
> Is there any reference material to be found on how to do these short tau
> stability measurements?
> The method with the mixers Bob was referring to?
> Erik.
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