[time-nuts] OCXO Phase Noise Measurement in Primitive Conditions

Bob Stewart bob at evoria.net
Mon Sep 1 21:31:13 EDT 2014

Hi Poul,

I've made a few hardware changes and some software changes, and I was wondering if you (and/or others) would take a look at this plot.  The green is phase over 70,000+ seconds with ~100ps resolution, while the blue is the ADEV.  (The sharp departures in phase are from large temperature changes and not quite perfect temperature compensation software.)  I don't see your horizontal line down at the bottom, but there are some worrying horizontal lines between about 3tau and 150tau.  Should I take that to mean that I have some noise/oscillations from about 1/3Hz to 1/150Hz?  Those up at about 150tau I think I can see by eye on another plot of different data.  I've got a lot of data collected from this run (still running) including pTerm, iTerm, dTerm, and tTerm (temperature) updates to the DAC.  I hope to try to tease something out of that later, though the pTerm and iTerm are mostly +1,0,-1 stuff.  FWIW, in real terms, the dGain is at about 4.2.  The phase
 starts to get ugly if I bring it much below that.  iGain is .01 and pGain is .05.  There are misc damping and limiting factors applied, as well.  

Just about ready to do a second board with updates from what I've learned, so any help is appreciated.  The plan is to release the source code at some (not too) future date and make boards available, if there's interest.  But I've still got a lot to do before that.



 From: Poul-Henning Kamp <phk at phk.freebsd.dk>
To: Bob Stewart <bob at evoria.net>; Discussion of precise time and frequency measurement <time-nuts at febo.com> 
Sent: Thursday, August 28, 2014 1:04 AM
Subject: Re: [time-nuts] OCXO Phase Noise Measurement in Primitive Conditions

In message <1409175707.2080.YahooMailNeo at web142706.mail.bf1.yahoo.com>, Bob Ste
wart writes:

This looks like a step in the right direction.

The "correct" allan plot will have a clearly visible horizontal
segment somewhere in the 100-10000 second range (depending on OCXO quality).

This is where the GPS long-term stability "takes over" from the OCXO's
better short-term stability.

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