On 05/06/2012 09:01 PM, Bill Hawkins wrote:
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.
Using analog integrator for 1 PPS stuff is hardish and your performance
will most probably suffer. You want your integrator to be digital as it
is clearly a better memory over time.
However, this is not to be confused with the time-constant, which should
be much higher than 1000 s to use the stability of the rubidium where
the GPS noise is worse. See PRS-10 manual for good illustration.
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.
Why wait with the updates of the DAC? By incrementally average for each
second I think you get a smoother transitions.
You sure want time-constants in excess of 1000 s, but you can achieve
that by using 1 s updates. Again, read the PRS-10 manual for a fairly
good description.
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.
I fail to see the benefit, and I have many times learned the hard way to
see the downside of too low update rate.
Comments accepted with enthusiasm.
:)
Cheers,
Magnus
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