Cross-talk between channels and also from time-base is to be expected.
Non-linearity in the interpolator is another possible source. Not having
it calibrated will definitly cause a cyclic pattern over the period of
phase-difference vs. coarse clock. This pattern may look like time-base
cross-talk and well, it does not really care if it is non-linearity or
interpolator linearity, it has the same additive effect and thus needs
to be compensated for that interpolator.
Cross-talk would be between the start and stop channels, so the
time-difference needs to be corrected after the individual interpolators
have been corrected for.
Another resolution detail exist in interpolators, consider that you have
an analog interpolator which is not adjusted, so that it has a range of
893 ADC steps over the 1000 fractional steps of the coarse counter, this
will means that there is 107 missing codes, so nearby steps will have a
higher likelihood. This causes some additional noise to measurements but
should not cause a bias in results for most cases, well except for noise
measurements like ADEV and MDEV with friends.
Cheers,
Magnus
On 04/29/2014 11:19 PM, Tom Van Baak wrote:
Hi Hans,
See if your plots look like approximately like these:
http://leapsecond.com/pages/53132/2324.gif
http://leapsecond.com/pages/53132/4099.gif
I did this as part of a week-long 51132A TIC resolution and linearity test.
I believe this is evidence of interpolator non-linearity within the 53132
counter. It happens on each 53132 counter I tested although each has its own
unique pattern. See, for example:
http://leapsecond.com/pages/53132/all7-phase.gif
http://leapsecond.com/pages/53132/all7-tdev.gif
There may be input signal conditioning, cross-talk, and DUT pulling effects
too. I haven't sorted it all out yet.
Note the counters all meet spec. But under the spec is this very interesting
world of interpolator non-linearity. It is exposed any time you very slowly
ramp through the interpolator range, or if you apply pure noise and look at the
distribution of all the bin's (histogram). So these subtle, periodic effects
are expected in any interpolator design, but it is cool to actually see and
measure it.
If they are consistent for a particular counter you can convert these
"calibration" measurements into a correction table and thus improve the
resolution of all subsequent time interval readings. The SR620 does this with an EEPROM
table.
In my test I compared two 5 MHz oscillators that were about 5e-11 apart in
frequency. That way it took about 4000 seconds to complete one 200 ns cycle
wrap. Collect data for a day and you have a nice series of waveforms. I see
both 100 ns periods (due to the 10 MHz 53132 clock) and 200 ns periods (due to
the 5 MHz DUT).
Avoiding cycle wraps with dividers doesn't really solve the problem. Also, it's
not always practical to continuously sit in a small fraction of the full
interpolator cycle. One solution is applying interpolator calibration, as
mentioned above. But the solution I use is exactly opposite of your intuition
-- for best resolution I welcome as many cycle wraps as possible. This is
especially effective if you compute phase slope (frequency offset) with a least
squares fit, instead of point-to-point.
/tvb
----- Original Message -----
From: "Hans Holzach" <hans.holz...@gmail.com>
To: <time-nuts@febo.com>
Sent: Tuesday, April 29, 2014 11:28 AM
Subject: [time-nuts] Influence of Cycle Wraps on TInt-Measurements with53132A
i use an Agilent 53132A as a TIC and Uli's "Plotter" to analyze the time
interval data of two oscillators. after removing the cycle wraps and the
drift there often remains a repeating pattern that i have not been able
to explain, e.g. TI increases, then drops a little bit and starts to
increase again, etc.
autocorrelating the data reveals clear and nice peaks. today i noticed
that the distance between two peaks is equal to the time from one cycle
wrap to the next.
it is obvious that using frequency dividers and avoiding cycle wraps
would eliminate or at least reduce the problem. but of course i'd like
to understand why this problem arises. any hint will be very much
appreciated!
thank you,
hans
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