On Tue, 7 Feb 2012 08:28:14 +0100, Attila Kinali <att...@kinali.ch> wrote:
>On Sun, 5 Feb 2012 19:55:36 -0900 (AKST) >"Richard H McCorkle" <mccor...@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. _______________________________________________ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.