[time-nuts] OCXO noise as they retrace

John Miles john at miles.io
Mon Jun 27 20:50:10 EDT 2016


> 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?
> 

Are you sure it's not the PRS-45A?  Cs standards get noisy as the tube ages, typically becoming much noisier before failing entirely.  This is usually pretty easy to diagnose by watching the beam current fluctuate.

You're also bumping up against the limits of what your counter can do.  Readings in the 4E-11 to 8E-11 range at t=1s are common with 5370s, depending on any number of factors.  

-- john, KE5FX
Miles Design LLC




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