[time-nuts] Interesting paper: Don't GPSD' your Rb...
randall.prentice at pscconsulting.com
Sun May 6 20:12:23 UTC 2012
The paper should really be titled "Another way to discipline your Rb to GPS".
Wouldn't it make more sense given the phase noise in the Rb to base your correction on a moving average diffence.
1) Reduce the Rb phase noise
2) Carefully make it a multiple of a day to cancel diurnal effects of the ionosphere.
From: time-nuts-bounces at febo.com [mailto:time-nuts-bounces at febo.com] On Behalf Of Bill Hawkins
Sent: Monday, 7 May 2012 7:02 a.m.
To: 'Discussion of precise time and frequency measurement'
Subject: Re: [time-nuts] Interesting paper: Don't GPSD' your Rb...
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.
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