[time-nuts] PICTIC II questions
azelio.boriani at screen.it
Tue Feb 7 08:39:53 UTC 2012
Yes, I was thinking of trying a PICTIC II partial redesign with a Xilinx
CPLD, using other type of fast turn off diodes and so on.
On Tue, Feb 7, 2012 at 8:28 AM, 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
> 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.
> Or am i missing something?
> BTW: does anyone know how these days low cost FPGAs perform in terms
> of jitter? (the data sheets are kind of scarce in that regard). And
> how do they compare to state of the art ECL logic?
> Also, does one have a schematics of a current SR620?
> Didier's site has a schematics, but it's from 1989 and i'd like
> to see how things are done with currently available components.
> Attila Kinali
> Why does it take years to find the answers to
> the questions one should have asked long ago?
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