[time-nuts] PICTIC II questions

David davidwhess at gmail.com
Tue Feb 7 14:13:18 UTC 2012


On Tue, 7 Feb 2012 08:28:14 +0100, Attila Kinali <attila at kinali.ch>
wrote:

>On Sun, 5 Feb 2012 19:55:36 -0900 (AKST)
>"Richard H McCorkle" <mccorkle at ptialaska.net> wrote:
>
>>    While using a faster timebase or higher interpolator gain increases
>> the resolution that doesn?t imply the accuracy will also increase. The
>> PICTIC II uses CMOS logic with propagation delays that vary with
>> temperature much more than the ECL logic used in a commercial counter
>> like the SR620, severely affecting the accuracy below about 250ps. The
>> interpolator was modeled after the SR620 design but simplified to use
>> the least amount of hardware possible to reduce the size and cost. As
>> the timebase rate is increased a smaller cap is used so stray
>> capacitance and the capacitance of the switching devices have a larger
>> effect on the charge linearity. The PICTIC II uses software calibration
>> methods that are not as precise as those in a commercial counter so the
>> accuracy is not specified other than to say it works well for GPS
>> monitoring applications at 1ns resolution with a 10 MHz timebase once
>> set up properly. If you want to log GPS data over months at a time then
>> a $50 PICTIC II should be sufficient for purpose. But if you want lab
>> grade accuracy over long time intervals with 25ps resolution then by
>> all means use a lab grade commercial counter like the SR620 and not a
>> PICTIC II!
>
>The PICTIC II might not be lab grade, but, frankly, i don't see any
>big problems in the design itself. Ie if one would replace the slow
>CMOS logic by something faster, lets say an FPGA (not an expensive
>highspeed one, but one in the 20-30USD range, available at Digikey/Mouser/..)
>and increase the clock speed to 100 or even 200MHz, then one ought to
>get a resolution in the lower ps range. And i guess, that an accuracy
>of 20-50ps should be acheivable.

I was looking at this problem during the simple disciplined oscillator
thread.  I wanted something that would allow continuous evaluation of
the jitter in the PPS signal itself independent of the GPS receiver.
The discrete design for the clock delay timer in a Tektronix 2230 or
2232 oscilloscope is good to 2 GS/s or 50 pS and their older 7T11
sampling sweep plug-in does even better than that with no high
frequency clock signals required.



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