[time-nuts] Divide by five
albertson.chris at gmail.com
Sun Nov 9 11:18:24 EST 2014
On Sat, Nov 8, 2014 at 11:13 PM, Neil Schroeder <gigneil at gmail.com> wrote:
> At one point I was considering phase locking all of them together - but
> again that seemed less than straightforward. You can do it PLL back to
> back, but is there a way to have a loop that contains multiple clocks? I
> would think the "telephone game" would apply.
NTP does this but on a MUCH lower frequency and longer time scale. But I
think NTP's general method could apply. NTP will accept any number of
reference clocks. (Yes sone people run NTP using just one GPS receiver as
a reference but best practice is to use five references.) NTP compares the
set of ref. clocks with each other and first tries to find the subset of
clocks that track each other, assuming the outliers are "wrong". It
continuously checks this and maintains a set of "true tickers". From these
it computes a consensus time using a weighted average of the "true"
clocks. The weights are based on the jitter and other quality measuring
statistics. Using this method reference clocks can be taken on and off
line without need to re-start NTP.
You could do the same thing with a set of local oscillators. Divide each
down to 1PPS then every second you compare their phases. Keep statistics
on drift and standard deviation relative to your "consensus phase". You
then discipline an OCXO (or several OCXOs) to output that consensus.
Like NTP you might accept reference from any number of GPSes or Rb or other
standards. You controller might pick the best OCXO in real time as the
I figure the first generation where we are now, where we build a GPSDO
using just one GPS and one OCXO and next generation would use multiples of
There is not a lot of fancy electronics required. Just some phase
comparators that can run once per second. The once pre second data rate is
so slow that ANY $5 micro controller could keep up with even a dozen
reference clocks. I'd likely us one of the ARM based Arduinos because they
are easy enough to program that the barrier to entry is low enough others
might be able to contribute. (Think about this if you want to make your
design public. If the technology is exotic,no one but you will contribute
to further development.)
If you like those FPGA boards then you can usually synthesize a CPU and run
the controller code on that.
Redondo Beach, California
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