[time-nuts] Interesting paper: Don't GPSD' your Rb...

Bill Hawkins bill at iaxs.net
Sun May 6 19:01:31 UTC 2012


Here is a different tactic for disciplining Rb from GPS/TXCO -

Consider the relatively (relative to a second) long stability of
an Rb oscillator and the not-so-good stability of GPS. Perhaps
using 1 PPS for a sampling period for stabilizing Rb is way too
short. Maybe 1000 seconds is better. That's way too long for an
analog integrator to do, so a microcomputer is required.

Count the Rb and GPS 1 PPS signals for 1000 counts of Rb 1 PPS.
You'll need to interpolate between 1 PPS GPS intervals to the
level of accuracy required, so maybe we count cycles of 10 MHz
from the GPS, using as many registers (cascaded integer counters)
as required for 1 E10 (or more) counts (2540BE400 hex). At the
end of 1000 seconds, use the difference between the lowest counter
reading and 0xEB400 (times an appropriate gain) to tweak the value
for the DAC doing the fine correction to the control voltage or
current. Use the upper counters for a sanity check on the reading.

As may be evident, I have never tried to discipline an Rb, but I
am well aware of the effect of sample time on the control of a
long time constant loop. True, the effect is strongest when dead
time dominates the time constant, but that is not the case here.
Still, I think there is value in using a long sampling time for
the control action.

Comments accepted with enthusiasm.

Bill Hawkins




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