At 07-02-10, John Ackermann wrote: >Hi Raj -- > >You've already gotten some good answers. If all you want to do is >measure the frequency offset rather than characterize the stability, a >simple approach is to first get as close as you can by adjust for >minimum march of the 10 MHz signals across the oscilloscope, then use >either the Racal counter or the digital o'scope to measure the delay >between the two signals and how it changes over time, preferably >measuring at 1 PPS rather than 10 MHz. > >In other words, measure the time difference between the leading edge of >the PPS signals, averaging for a while (depending on how close the two >already are) to improve resolution and reduce the noise. Write down the >delay figure, note the wall clock time, wait a while, then come back and >measure the delay again. The change in delay over the elapsed time will >tell you the frequency offset, e.g., if you see 1 microsecond per day, >that's 1.16x10e-11. > >Adjust and repeat. As others have mentioned, being a time-nut requires >patience. :-) > >It's best to do this at a lower frequency than 10 MHz, and ideally at 1 >PPS, as there's only 100 nanoseconds between cycle slips at 10 MHz, and >that limits how long you'll be able to measure before you've drifted a >complete cycle. > >73, >John > Ok, the past few days I have been working on exactly the same thing. That is, adjust an LPRO to my homemade GPSDO. Good to know I followed a proven procedure. Initially I wanted to build two PPSDIV/TADD circuits but did not have the right PICs. Instead I used two PIC12F devices and put them on the same veroboard. The software for only a single 1Hz output was not too complex.
There was also a Racal 1992 I could use to measure and log the phase shift. If that would not have been the case I probably would have tried a XOR port to both (synched) outputs and use an R + C to measure the DC voltage. That would make a nice little gadget for people not having a high-res counter. Also the TADD 74AC04 drivers would not be needed this way. Anyway, I finally dared to adjust the LPRO. Can somebody tell if the following seems normal? * The LPRO measured 1.77 E-10 high (before any adjustment) * Lamp voltage is about 5.7V * The GPSDO 1Hz pulse seems to move (noise like) + or - 15ns around its linear regression line * The LPRO takes more than 8 hours (perhaps even 24) to reach the 1E-11 level? The last point is guessing since my GPS signal is super stable. I thought I managed to adjust it to that level after the LPRO had been powered on for more than 24 hours. But powering it on two days later shows a higher drift after 5 hours than where left before. Also I would like to know if I have to repeat this procedure once it is built into its final enclosure. Right now it is open on the bench and clamped to a heatsink. Temperature might be different inside a box. Cheers, Joop _______________________________________________ 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.
