[time-nuts] Influence of Cycle Wraps on TInt-Measurements with53132A

[Magnus Danielson]

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.holzach at gmail.com>
> To: <time-nuts at 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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