[time-nuts] Re: tvb racal STDEV measurement anamoly
Dwayne
Dwayne.Esterline at krytronx.com
Fri Aug 5 23:04:33 UTC 2022
Tvb- what did you conclude caused the stdev measurement anamoly you
referenced at http://leapsecond.com/pages/racal/stdev.htm ?
Regards,
Dwayne Esterline
On Aug 5, 2022 5:59 PM, time-nuts-request at lists.febo.com wrote:
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Today's Topics:
1. Re: Testing frequency pulling on a DYI counter (Magnus
Danielson)
2. Re: Testing frequency pulling on a DYI counter (Tom Van Baak)
3. GPSDO/GNSSDO project: STM32G4 + u-blox ZED-F9T + TDC7200
(Carsten Andrich)
--------------------------------------------------------------------
--
Message: 1
Date: Fri, 5 Aug 2022 04:25:16 +0200
From: Magnus Danielson <magnus at rubidium.se>
Subject: [time-nuts] Re: Testing frequency pulling on a DYI counter
To: Erik Kaashoek <erik at kaashoek.com>, time-nuts at lists.febo.com
Message-ID: <bacf5887-4505-9777-ed39-09d550033c68 at rubidium.se>
Content-Type: text/plain; charset=UTF-8; format=flowed
Erik,
Good job there. I suspected that the actual frequency pulling, if
any,
was very low.
Yes, numerical issues in linear regression can be painful.
In the accelerated method I developed, you can engineer the values
to
avoid major numerical issues. Also, you are not the first to have
seen
such issues. This is done first-degree by making sure that things is
accumulated without making any roundings, and secondly by choosing
the
number of samples just right to make the unavoidable final division
not
too terrible to the end result.
It's well known that you get fractional issues too.
Cheers,
Magnus - tired after driving 600+ km
On 8/4/22 20:04, Erik Kaashoek wrote:
> Bob, Magnus,
>
> Using a second counter (my famous Picotest U6200A) locked to the
> reference output of the DIY counter and measuring the output of
the
> signal generator and also set to gate of 10 s it is confirmed that
the
> frequency pulling (if any) is below 1E-11 (not more digits on the
> display of the U6200A)
> Generator is set to 10.000,000,000,2 MHz and is measured as such
by
> the U6200A
> As there seems to be no frequency pulling I went back to the
> simulation of the linear regression algorithm and discovered that
when
> there is a integer divide/multiply relation between the internal
> reference and the measured frequency the regression looses some
accuracy.
> For sure if the reference is close to an integer multiple of the
> measured frequency (10 Mhz measured -> 200 MHz reference) the
> regression collapses completely in accuracy. I hoped that by
creating
> a fractional relation this collapse would not happen at 10 MHz but
is
> still there, although much smaller. For this test I'm using a "div
3
> times 64 e.g. 213.333,333,333,333... MHz" internal reference
frequency
> derived from the external 10MHz reference. Ton van Baak warned me
> against using fractional relations in a counter but otherwise it
is
> impossible to measure a 10 MHz input signal with any accuracy
without
> a HW time to digital as the interpolation no longer works. I can
> switch dynamically to 200 MHz or 245 MHz reference and these
produce
> much much worse results.
> I realize this test only measures if the TCXO used as reference in
the
> DIY counter does not show frequency pulling but it does not show
if
> the PLL used to convert the 10MHz to 213.333333333... MHz for the
> internal counters shows any frequency pulling.
> NOt
> Erik.
------------------------------
Message: 2
Date: Fri, 5 Aug 2022 05:55:16 -0700
From: Tom Van Baak <tvb at LeapSecond.com>
Subject: [time-nuts] Re: Testing frequency pulling on a DYI counter
To: time-nuts at lists.febo.com
Message-ID: <06dec117-7204-1f78-d802-59f87cb1f55b at LeapSecond.com>
Content-Type: multipart/mixed;
boundary="------------98CC5F907E674D28F1A164FE"
> the measured frequency the regression looses some accuracy.
Yes, the hp/Agilent/Keysight 53132A does that too. Here's the
footnote
from the user manual:
http://leapsecond.com/pages/53132/53132-reduced-resolution.gif
And it's not just at 10 MHz. Any fraction or multiple within 600 ppb
is
affected too. What impressed me is that the hp firmware engineers
specifically detected this condition and reduced the number of
digits
displayed accordingly. This avoids the user from getting a false
sense
of precision.
> Tom Van Baak warned me against using fractional relations in
> a counter but otherwise it is impossible to measure a 10 MHz
> input signal with any accuracy without a HW time to digital
> as the interpolation no longer works.
Right, which is probably why many high-end commercial counters use
interpolators. But you aren't, and that's ok because your design
spec is
on the order of 9 digits. Keep it simple. Later if you design a 10
or 11
or 12 digit counter you'll have to resort to using h/w interpolation
as
well. Even the Lars GPSDO uses a crude interpolator; it's not that
difficult. Many threads in the time-nuts archive on the topic.
----
Your slow sweeping experiments reminded me of a great example I once
ran
into. So I wrote it up with photos and plots:
http://leapsecond.com/pages/racal/
http://leapsecond.com/pages/racal/stdev.htm
Even if you aren't building a frequency counter, this strange event
is
quite interesting. One of the plots is attached; the rest are in the
URL
above.
/tvb
On 8/4/2022 11:04 AM, Erik Kaashoek via time-nuts wrote:
> Bob, Magnus,
>
> Using a second counter (my famous Picotest U6200A) locked to the
> reference output of the DIY counter and measuring the output of
the
> signal generator and also set to gate of 10 s it is confirmed that
the
> frequency pulling (if any) is below 1E-11 (not more digits on the
> display of the U6200A)
> Generator is set to 10.000,000,000,2 MHz and is measured as such
by
> the U6200A
> As there seems to be no frequency pulling I went back to the
> simulation of the linear regression algorithm and discovered that
when
> there is a integer divide/multiply relation between the internal
> reference and the measured frequency the regression looses some
accuracy.
> For sure if the reference is close to an integer multiple of the
> measured frequency (10 Mhz measured -> 200 MHz reference) the
> regression collapses completely in accuracy. I hoped that by
creating
> a fractional relation this collapse would not happen at 10 MHz but
is
> still there, although much smaller. For this test I'm using a "div
3
> times 64 e.g. 213.333,333,333,333... MHz" internal reference
frequency
> derived from the external 10MHz reference. Ton van Baak warned me
> against using fractional relations in a counter but otherwise it
is
> impossible to measure a 10 MHz input signal with any accuracy
without
> a HW time to digital as the interpolation no longer works. I can
> switch dynamically to 200 MHz or 245 MHz reference and these
produce
> much much worse results.
> I realize this test only measures if the TCXO used as reference in
the
> DIY counter does not show frequency pulling but it does not show
if
> the PLL used to convert the 10MHz to 213.333333333... MHz for the
> internal counters shows any frequency pulling.
> NOt
> Erik.
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