[time-nuts] OCXO noise as they retrace

Attila Kinali attila at kinali.ch
Wed Jun 29 11:49:39 EDT 2016


On Mon, 27 Jun 2016 21:55:55 +0000 (UTC)
Bob Stewart <bob at evoria.net> wrote:

> I've had a specific GPSDO running for some time now, and I notice that
> the noise at tau 1s has gotten worse as the retrace flattens out.
>  In this case, the  ADEV was about 3.6E-11 a month or so ago, and has now
> gone up to about 8.5E-11.  (Measurements performed by a 5370A against a
> PRS-45A Cs standard.)  Is this normal as the startup drift settles out?
>  It's been on the same power supply module during this time, but I have been
> using it to test new code, so the DAC has been cycled from midpoint to lock
> numerous times.  Another unit that's been running for some time has done
> essentially the same thing.
>
> Weather?  Environment?  GPS demons?

As the change is just about a factor of 2, my first guess would be that
you see numerical effects due to the finite tuning resolution of the GPSDO.
You can see it as a similar effect as in delta-sigma modulators. As long
as the output value is changing, the noise power (which can be assumed
to be constant for a first order approximation) will be spread over a
wide frequency range. But, if you output a constant value, the noise power
will get concentrated in a couple of spurs, which will then stick out quite
a bit.

A simple test for this hypothesis would be to send the GPSDO into hold-over
mode and measure the ADEV of the OCXO again with constant EFC voltage.
Another indication would be, if the ADEV increased only in a narrow range,
while slightly decreasing overall (though, this is a much weaker argument
as it can be confunded by other phenomena).


			Attila Kinali
-- 
It is upon moral qualities that a society is ultimately founded. All 
the prosperity and technological sophistication in the world is of no 
use without that foundation.
                 -- Miss Matheson, The Diamond Age, Neil Stephenson


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