Jim, Bob, we just had the pleasure of doing exactly this aligned-1PPS measurement two days ago. I had to measure the difference (noise) of two units that were locked to the same source. To jump ahead, the difference was 0ns +/- about 500ps noise range.
We used an HP 5335, no problem, it jumped back and forth by +/-1ns steps. If I had done very long averages, it may be useful. Next came an HP 5370A. A bit tedious to set up, but the noise floor of about 40ps was helpful, but the unit had about 200ps offset when in COM-A test mode so needs some adjustment. Then moved to a DTS-2070, once we found space for it and the correct attenuators to not damage the inputs it was quit funny to see single femtosecond resolution on a ~500ps pulse to pulse noise. Lastly we used the HP 53132A. This was the easiest to set up. It works fine as long as you stay within about -6ns, if you go earlier then the counter will measure an entire second, adding one second of error from its internal time base, and showing numbers like 0.999,999,997s. Since we were within a 1ns window, the numbers looked almost identical to the DTS-2070 so we know we have a good measurement. I took the output of the 53132A and ran it through Excel and got a standard deviation of 220ps. Not bad considering some of that was probably counter noise and the counter has 'only' 150ps resolution if I remember correctly. The 53132A it will be for future 1PPS to 1PPS measurements for me. Bye, Said Sent From iPhone On Feb 22, 2014, at 7:25, Bob Camp <li...@rtty.us> wrote: > Hi > > On some counters, if both inputs arrive at exactly the same time, they get > very confused. The normal approach is to offset one by a few hundred ns or > so. The exact offset is fairly non-critical. It’s real value depends entirely > on the amount of drift you expect to see over the time period you are > checking. > > If your oscillators are off by 1 ppm, they will slip by 1 us per second. If > you want to check them for 12 days or more you will need an offset of more > than one second. If they are off by 1 ppb, then your offset could be a bit > over one millisecond to handle a 12 day run. (12 days is roughly 1 million > seconds). > > Bob > > On Feb 22, 2014, at 8:17 AM, Jimmy Burrell <jimmydb...@gmail.com> wrote: > >> I need some help with a 'noob' question regarding some practical examples in >> some of the NIST literature. When attempting to compare two clocks, I'm a >> bit confused on the subject of exactly how to use my counter to compare a >> delayed clock relative to another. Or perhaps I should just say 'comparing >> two clocks'. Let's take some concrete examples. >> >> Let's say I want to characterize my Morion MV89 ocxo using my HP5335a. >> Obviously, I can tune the MV89's 10MHz by +/- 1Hz and feed it to the >> counter's input 'A'. Obviously, I can feed in a second, external reference >> clock at 10MHz into input 'B'. Suppose, however, I didn't have an external >> reference clock. Can I compare against the counter's internal time base by >> hooking a line from the rear jack time base output to channel 'B' input? Or >> am I making it too complicated? Do I simply plug into input 'A' and go? >> >> In a somewhat related question, in this article >> (http://www.wriley.com/Examples%20of%201%20PPS%20Clock%20Measuring%20Systems.pdf) >> where two clocks, both divided to 1PPS, were compared, W.Riley makes the >> following statement, "The two 1 PPS outputs were connected to a Racal Dana >> 1992 time internal counter having 1 nanosecond resolution, and the start and >> stop signals were separated sufficiently in time for the counter to function >> properly". I wonder what exactly is meant by "separated sufficiently in >> time for the counter to function properly" and how one would go about doing >> this? For example, is inverting one of the signals sufficient separation? If >> not, how is this typically done? Delay line? >> >> Thank you, >> >> Jim... >> N5SPE >> _______________________________________________ >> 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. > > _______________________________________________ > 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. _______________________________________________ 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.
