[ntp:questions] Making ntpd tweak a TCXO

Hal Murray hmurray at megapathdsl.net
Thu Jan 28 13:00:28 UTC 2016


> 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.  
...


-- 
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