[time-nuts] GPSDO using 100Hz
didier at cox.net
Tue Nov 25 01:53:44 UTC 2008
This is what I believe, feel free to set me straight :-)
The 100Hz is done by synthesis from the GPS receiver's crystal oscillator,
which is usually not on a harmonic of 100Hz, at least not precisely (cheap
crystal) so the receiver will usually generate each pulse to the best of
it's ability to line up to 100Hz (so these will have somewhat deterministic
jitter due to the difference between 100Hz and the divided crystal), and
only once per second will try to align the average pulse train to GPS. There
is pure jitter 99 times out of a hundred, and actual correction the 1/100
time. The advantage is that the once-per-second correction is burried under
(spread by would be a better term) a fair amount of 100Hz noise, so it's
probably easier to filter, allowing the use of a faster filter than a 1 PPS
output for the same level of 1 PPS attenuation. You can't use the scope to
determine if the jitter is pure jitter or a GPS correction, but I bet a TIC
feeding a PC would.
The point is that older receivers in particular simply don't have the
horseower to update the timing solutions 100 times/second, or 10,000 times
From: time-nuts-bounces at febo.com [mailto:time-nuts-bounces at febo.com] On
Behalf Of WarrenS
Sent: Monday, November 24, 2008 6:48 PM
To: Discussion of precise time and frequency measurement
Subject: [time-nuts] GPSDO using 100Hz
My Oncore's phase is definitely different on each 100Hz cycle. A digital
storage scope shows it very well.
Some things that come to mine:
1) Not all Oncores are the same, mine is an old 8 channel one. (Don't
remember the number, it is the one with the sawtooth capability)
2) The actual error amount is updated each 1 sec and the processors still
dithers that to get the 100 Hz to be close to the correct value
3) Other TBD
Mike Monett wrote:
> Ok, it seems they are calculating the best time for each 100Hz pulse
> individually. That makes life a bit easier for a PLL.
Not according to the datasheets which imply the phase of the 100Hz (or
10kHz) burst is adjusted once per second.
This should be very easy to verify if the leading edges of individual pulses
are time stamped with sufficient resolution.
> My interest is I have a totally new way of locking to the 1PPS pulse
> that should improve the performance dramatically. The question is
> would it also work with a 100Hz signal.
> The answer is yes. It would also work with a 10KHz signal, but this
> would require a bit more horsepower. I was planning on using a
> simple inexpensive microprocessor for everything but I don't know if
> it would be fast enough to do 10KHz.
> But there's not many of those around anymore, are there?
> Best Regards,
> Mike Monett
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