At 02:53 PM 2/14/01 -0000, Andy wrote:
> in view. The 1PPS output is specified to be within 50ns of "true
time" (GPS time
> or UTC, selectable) with SA on. Now that SA is gone, the accuracy is
even better
> (there is always a ~30ns sawtooth error due to finite resolution in
the software/hardware).
>
Surely, if the 1us jitter is a function of clock timing synchronisation
then by averaging over many 1s periods, as the Brooks Shera design does,
errors will be averaged out and high accuracy frequency sources may be
properly locked to this 1PPS signal. No GPS disciplined frequency
source worth its name would, or should, ever try to correct output
frequency absolutely on a pulse by pulse basis.
When removing jitter from a signal that's accurate over the long term,
we can do better than just averaging. By letting a counter run (e.g.
from the oscillator we're trying to discipline) and latching it with the
1 PPS signal, we get the number of oscillations that have occurred from
one second to the next. One approach would be to simply average up a
bunch of these and say that's the best answer. But if we don't clear
the counter after each 1PPS latching signal - if we just let the counter
continue running upwards, then we can use least squares to solve for the
*slope* of the line. Over any given time period that least squares slope
solution will give a significantly better result than averaging alone.
Try it if you don't believe it.
From memory, I think that design only claims about 10^-9 accuracy anyway
locking with a time constant of many tens of minutes using a digital
phase locked loop, still far less than GPS 'ought' to be capable of,
although this was in the days when Selective Availability was still on.
But accurate enough for most LF experiments - at the moment. For bit
timing purposes, I would have thought 1ms was good enough for 100ms or
slower BPSK.
Absolutely. What we have now is plenty good enough for BPSK. There is
a problem which will limit us to MS100 or slower anyway. It's that we
have considerable difficulty compensating for the number of *milliseconds*
delay through our transmitters and our receivers. At MS100 or slower,
the existing GPS 1PPS outputs (even from "cheapie" GPS receivers) are
very well suited to our purposes.
Bill VE2IQ
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