[time-nuts] Allan Deviation question

[Magnus Danielson]

Hi Bill,

On 07/04/2012 10:23 PM, Bill Dailey wrote:
> I am measuring a 10MHz OCXO.  I am wondering about my methods and their
> effect of Allan Deviation.
>
> SETUP:
>
> I am running the signal into my radio (gps disciplined)  and taking the
> resulting audio and running it to a sound card (oscillator gps disciplined)
> then looking at the result in Spectrum lab.  I am using a sample rate of
> 48kHz and an FFT  length of 32,768 and decimating the signal by 48 to give
> me decent resolution.  The resulting FFT window is 32 seconds.

At this point you should have 1 kHz sampling rate, and the FFT window 
will be 32,768 s long. So far my only question is what your radio is 
dialled into, as it will give the beat frequency on the audio side and 
also the scaling factor for the phase-deviations.

> Then looking at the peak frequency or phase every second (can measure both
> instantaneously).

How do you do that? If you have a 32,7 s long FFT window, you only get 
results at that rate. Are they interleaved? 32 interleaved will give you 
measures every 1,024 s.

> QUESTION:
>
> Does that window time affect my Adev at short time intervals of say 1s to
> 30s or can these numbers be trusted?

If you taking the readings out of the top bin on your spectrum plot, 
then the FFT window time will scale the "instruments" hardware 
bandwidth, as to be expected from the Nyquist theorem. This has proven 
not to be all the truth, as there will be a droop (bias error) for the 
short-tau ADEV measures, which wears off and is essentially gone after a 
decade or so. Hence, using long FFT windows will reduce the rate of 
samples and cause short tau measure of interest to be biased and untrusted.

The recommended practice is to use a higher sampling rate, and in your 
case shorter FFT window, maybe skip the decimation, such that you can 
afford to remove low tau0 multiples such that what you see is what you 
can trust.

Please have a look at the Allan Deviation wikipedia page, I think you 
will find some useful stuff there. I'd happy to adjust it if needed.

> Thanks for your time.  Please save esoteric and playful discussions, just
> interested in the ability to trust my measurements.

It's a fair question to ask. It's also a good question to ask.

It would be good if you could calibrate your setup by using known 
sources with known phase-noises. While I like the tool I have, I will 
sure that I do my calibrations one way or another.

Cheers,
Magnus