[time-nuts] Phase noise from Allan Deviation ?

Bob Camp kb8tq at n1k.org
Mon Dec 14 07:53:02 EST 2015


Hi

ADEV to phase noise can be full of gotcha’s. What you have out of your 
system is a series of phase samples. You don’t *have* to convert those 
samples to ADEV. There are a whole bunch different variances you can 
calculate. The best thing to do (by far) is to save the phase samples. That
way you can post process them with whatever variance you happen to be
interested in. 

As an example, you may find that modified ADEV and Hadamard help you
sort out various noise processes. There also is the option of doing an FFT on
the phase samples.  

Bob


> On Dec 13, 2015, at 10:15 PM, 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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