Fellow Time Nuts, The PICTIC II is a spin-off of a low cost front end developed for a GPSDO to compare the oscillator to GPS 1PPS over very short (< 20us) time intervals. In that application all that was required was 1ns TIC resolution to match the resolution of the GPS sawtooth correction to get the best accuracy possible from the GPS receiver. While the interpolator can provide gains in the 400-800 range the data span with a 10 MHz timebase is typically reduced to 100 counts so variations up to +/- 7 counts in the interpolation hardware over temperature have minimal effect on the 1ns data accuracy. The interpolator hardware was much better than required for that application so the PICTIC II was developed as a learning platform to get others playing with interpolation and improving the design, not as a piece of precision test equipment. If you want to measure the long-term performance of a GPSDO against GPS 1PPS and not tie up your SR620 for months doing so then the PICTIC II is just the ticket. If you think you can measure long intervals with an XO timebase or achieve the 25ps accuracy of a $5000 SR620 using a $50 PICTIC II then you are being totally unrealistic. Other factors besides resolution need to be considered when deciding how to populate the PICTIC II depending on how it will be used. An XO timebase may be fine measuring intervals of a few microseconds as originally intended for a GPS monitor, but as the measurement interval increases so do the errors from the timebase. A high stability timebase is required if you want to measure longer intervals accurately and most of us have a high stability 10 MHz source available. The timebase rate will determine the digital portion of the count and reducing the interpolation required for a given resolution will improve the accuracy. There is a point where the system cost increases faster as the timebase rate increases and using a high stability 1 GHz timebase for a 1ns resolution time interval counter is not low cost solution. That is why most commercial time interval counters use some form of interpolation. The PICTIC II demonstrates how to achieve 1ns resolution with a 10 MHz timebase (XO, OCXO, Rb, Cs) while keeping the size and cost to a minimum. While using a faster timebase or higher interpolator gain increases the resolution that doesnt 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!
Richard >> I know the resolution can be enhanced by swapping out some parts, but it's >> been too long to recall the details. > > On the schematic is a list of alternate XOs. For 50 MHz, the 1000 pf > cap is changed to 180 pf. This is stated to give 25 ps maximum > resolution. If this simple change is all that is required, I can > easily substitute the parts to obtain better resolution. Has anyone > made these changes and used a PICTIC II with a GPSDO and rubidium as > inputs? Is it good enough? > > Joe Gray > W5JG > > _______________________________________________ > time-nuts mailing list -- [email protected] > To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > and follow the instructions there. > _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
