[time-nuts] Influence of Cycle Wraps on TInt-Measurements with53132A
Tom Van Baak
tvb at LeapSecond.com
Tue Apr 29 17:19:59 EDT 2014
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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