> I would like the data stream from my A/D to be long-term accurate. That is, > if the sampling rate is 60 Hz, I would like 60*86400*365 +/- 1 samples in a > [non leap] year.
> To that end, I'd like to discipline the TCXO using ntpd. I would like it to > *not* change the virtual clock frequency in software as it usually does, via > adjtime or whatever hooks are provided by the OS. Instead, I want the > frequency corrections to go through my own driver to adjust the hardware > oscillator directly. > Does the NTP software provide any hooks that could be used to accomplish > this? I thought of a crude approach, simply writing a program to examine > the drift file periodically, and using that information to adjust the TCXO. > But that (a) fails to stop ntpd from adjusting the virtual clock frequency > within the OS, and (b) isn't really a proper control loop. I guess it would > work, sort of, as a last resort. I don't think servoing out the drift will do what you want. Suppose your TCXO slows down a bit. After a while, ntpd notices, tweaks the drift, then you notice, and adjust it to go the right speed. But during that time, the clock was running a tiny bit slow so you lost a fraction of a cycle. I'm assuming your TCXO is driving your A/D. I think what you want to do is monitor the times when your D/A gives you data. Suppose you set things up so that each buffer holds 1 second of data. When the buffer fills, grab the time. Compare that with the expected time. ... -- These are my opinions. I hate spam. _______________________________________________ questions mailing list questions@lists.ntp.org http://lists.ntp.org/listinfo/questions
