Bob,
to me your ADEV plot looks pretty exactly like a measurement system
noise floor plot with its typical 10/decade shape.
I would assume that you connected the same signal to both counter
inputs, or, both sources have been phase locked.
Adrian
Bob Stewart schrieb:
Hi Bob,
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.
Bob
________________________________
From: Bob Camp <[email protected]>
To: Bob Stewart <[email protected]>; Discussion of precise time and frequency
measurement <[email protected]>
Sent: Tuesday, September 2, 2014 6:38 AM
Subject: Re: [time-nuts] OCXO Phase Noise Measurement in Primitive Conditions
Hi
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…..
Bob
On Sep 1, 2014, at 9:31 PM, Bob Stewart <[email protected]> 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.
http://evoria.net/AE6RV/TIC/ADEV3.png
Bob
________________________________
From: Poul-Henning Kamp <[email protected]>
To: Bob Stewart <[email protected]>; Discussion of precise time and frequency
measurement <[email protected]>
Sent: Thursday, August 28, 2014 1:04 AM
Subject: Re: [time-nuts] OCXO Phase Noise Measurement in Primitive Conditions
--------
In message <[email protected]>, 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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