[ntp:questions] Re: Basic MSF clock setup

Chris Stenton jacs at gnome.co.uk
Wed Nov 12 13:04:46 UTC 2003


I've done exactly the same as you and recently built one of Jonathans radio
clocks for my FreeBSD box. I also have an Oncore GPS unit as well.

Using radiocld2 I find that the MSF unit stays generally within  +/- 0.2ms
of the GPS. However, you will not be able to improve on this accuracy via
PPS cos it seems to be a slow temperature/atmospheric? cyclic drift

Chris
.

"Chris Hastie" <chris at nospam.oak-wood.co.uk> wrote in message
news:SAsf5UDVffs$Ew31 at nospam.oak-wood.co.uk...
> In message <dbiqob.b78.ln at 192.168.42.254>, Jonathan Buzzard
> <jonathan at uk.me.buzzard> writes
> >In article <aUsd88HVdKs$EwnR at nospam.oak-wood.co.uk>,
> >       Chris Hastie <chris at nospam.oak-wood.co.uk> writes:
> >
> >> It strikes my that it shouldn't be too difficult to adapt the GENERIC
> >> driver for raw MSF, based on the existing raw DCF mode. Would there be
> >> any advantage to doing this over using radioclkd2 and the SHM driver?
> >
> >There would be little or possibly negative advantage in doing so.
>
> Fair enough
>
> >
> >> The other thing I'm wondering about is PPS.
> >
> >The best bet is to do some time pulse clock averaging in the SHM driver.
> >It improves jitter by an order of magnitude. Consider it a poor
> >man's PPS.
>
> I'm not clear what this means. I presume its something that should be
> done in radioclkd*, or is it a configuration option for the SHM
> reference clock in ntp?
>
> As far as I can make out, radioclkd2, which I'm using as I failed to get
> radioclkd to compile on FreeBSD yet, includes such averaging. Never the
> less, using the PPS driver as well does seem to improve clock stability,
> at least it reduces the mean offset and jitter relative to the SHM
> clock. I'm giving it a day or so in various configurations to try to
> work out which is best - though it would probably help if I properly
> understood all these numbers :) Are there features of radioclkd not
> present in radioclkd2 that mean I should be trying harder to figure out
> how to get it to compile on FreeBSD?
>
> > If you really have need for PPS then you should use
> >a GPS unit.
>
> I don't. I'm just trying to leverage the most I can out of £30 worth of
> bits blu-tacked to my window ledge and take advantage of living 30Km
> from the MSF transmitter.
>
> >Remember the philosophy behind my radioclock is to have
> >really simple physical hardware with smart software to deliver a
> >previously unobtainable simplicity/price/performance ratio. It
> >does not attempt to rival the accuracy of a GPS unit, though it
> >can make for a dirt cheap backup time source.
>
> Many thanks for putting the details up.
>
> -- 
> Chris Hastie





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