Hi,
This problem have since been addressed by Francois Vernotte as I saw in
a presentation last year. He addressed the issue somewhat different than
the traditional calculations to achieve the same problem.
Turns out that correlation eats your noise if I recall things right.
Cheers,
Magnus
On 04/14/2017 02:19 AM, Bob kb8tq wrote:
Hi
There are a number of papers from the 70’s and 80’s digging into three corner
hat data. The net result often
would turn out to be “less than zero” noise on one of the DUT’s. Since that’s
physically impossible the technique
got a bit of “attention”. The Cliff Notes version of the results is that
simultaneous measurements were the key
to getting decent results. The closer to “same time” (as in microseconds or
nanoseconds) the better. Even with very careful
data collection, odd things can still happen. Phase noise pops up at crazy low
levels or ADEV goes to bizarre
numbers. In many ways a TimePod (or other ADC based setup) is ideal for getting
the data synchronized. Running
all three devices on one is by far the best way I have seen to make the
technique work. It still can have problems,
but less so that other ways of doing it.
Bob
On Apr 13, 2017, at 7:16 PM, Bob Stewart <[email protected]> wrote:
Hi John,
I had a chance to think about this some more after I pressed the send key. The
ionospheric effects are certainly going to be different if the distance in time
between tests is large. And, of course, there is the fact that the KS has a
pretty old receiver compared the Ublox I use, so that even the reaction to the
ionosphere is likely to be different. So, I thought I'd experiment with some
runs with both GPSDOs in holdover to see if that would even the score, so to
speak. Of course then I have the temperature variable, so it's never going to
be perfect.
Anyway, thanks for the help. If I get anything that seems useful out of this,
I'll post links to the data.
Bob
From: John Miles <[email protected]>
To: 'Discussion of precise time and frequency measurement' <[email protected]>
Sent: Thursday, April 13, 2017 6:01 PM
Subject: Re: [time-nuts] Three-cornered hat on timelab?
Longer runs would be better to the extent that they give you smaller error bars
in your tau range of interest, certainly. But any effects that influence one
of your runs but not the others will render the 3-cornered hat solution
questionable, if not outright invalid. Only through many repeated runs can you
learn to tell the bogus data from the good stuff. So I'd make shorter runs at
first, until you're sure you know what you're looking at.
It doesn't matter which source is applied to the start versus stop channel, as
long as the assignments are consistent with the source labels you apply. I
would use frequency-count mode to simplify things, at least at first. This is
already a very challenging measurement for all the reasons mentioned.
-- john, KE5FX
Miles Design LLC
From: Bob Stewart [mailto:[email protected]]
Sent: Thursday, April 13, 2017 8:41 AM
To: Discussion of precise time and frequency measurement; John Miles
Subject: Re: [time-nuts] Three-cornered hat on timelab?
Hi John,
Thanks! With lesser equipment, such as the 5370A, would longer runs be better?
I used a set of 1 hr runs and the result wasn't quite what I had expected.
However, it may be that I had mislabeled the files, and thus got the sources
confused. Of course, it may be that the ionospheric effect was grossly
different between the three tests. So, with a 5370, Source A would be the
START input and Source B would be the STOP input, right? For my testing, the
sources are all 10MHz signals, and I'm driving the EXT input with 1PPS from a
GPSDO.
Bob
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