> Many thanks for these pointers, very interesting and useful work. This > does show that for a long sampling time, quite a high degree of > accuracy could be obtained. The downsides are that the sampling period > would have to be very long and could easily be affected by power cuts > causing failure to my proposed testing method. So theory proved but > impractical. Looks like I'll have to bite the bullet and go with a GPS > disciplined ocxo to make a good frequency standard. > > Cheers, > Steve
Ah, if you are a time-nut of course you will bite the bullet and get a GPSDO. But you should also not give up on the PC idea. If it works it will be a great gift to many people (OK, maybe not the pico- and nanosecond crowd, but to regular millisecond kind of folks). And, I no longer think long sample times are required. Consider the following. Assume you can get NTP to give you ms or sub-millisecond accurate time-stamps. Also assume you divide down your UUT to something like 1 kHz and feed that into one channel of your sound card (and as Bruce pointed out, maybe the slower the rise time the better in this case). Now, collect 16-bits of waveform data at 44.1 kHz. First, note that it's not exactly 44.100000 kHz -- but over time, as your circular input buffers fill up, you can use NTP time-stamps to calculate what the sampling rate precisely was/is. Then, looking at your highly oversampled waveform data, you can calculate the phase of your UUT frequency relative to the now precisely known sound card sampling rate. Over time you will see the phase drift, which then directly gives you the UUT frequency error. So what you end up doing is using the sound card like a high resolution vernier between NTP timekeeping on the inside and your UUT on the outside. I bet you a Thunderbolt that you can measure to 1 ppm within ten seconds. /tvb http://www.LeapSecond.com p.s. For extra credit, tee your UUT into both channels, do twice the math, and see if you can measure both differential phase, and differential phase drift between them. _______________________________________________ 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.
