[time-nuts] OCXO Phase Noise Measurement in Primitive Conditions
bob at evoria.net
Tue Sep 2 12:02:37 EDT 2014
I take it that the 3 segment ADEV pretty much defines a GPSDO. So, is my plot an indication that I'm riding the OCXO too hard to get the results I think I should be getting? What I think I should be getting is a flat phase line. But is a GPS receiver, even after applying quantization error corrections, accurate enough to give that? I see three things at play here: hardware, software, and my expectations. I'm trying to figure out which one (or several) of these needs to be addressed. I suppose I could hook the Rb back up and see the effect on the ADEV of the Rb vs OCXO as I loosen the gain values.
From: Bob Camp <kb8tq at n1k.org>
To: Bob Stewart <bob at evoria.net>; Discussion of precise time and frequency measurement <time-nuts at febo.com>
Sent: Tuesday, September 2, 2014 6:38 AM
Subject: Re: [time-nuts] OCXO Phase Noise Measurement in Primitive Conditions
Your ADEV is (mostly) a plot of 1-2 ns resolution out over your sample range. It’s not a “typical” three segment ADEV. It’s more likely that any departure from a straight line is random rather than systematic. Yes I’ve spent a lot of time looking at random plots that I was *sure* had significant information in them…..
On Sep 1, 2014, at 9:31 PM, Bob Stewart <bob at evoria.net> wrote:
> 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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