[time-nuts] Best Rubidium Frequency Standard
Charles Steinmetz
csteinmetz at yandex.com
Sun Mar 13 20:21:07 UTC 2016
Poul-Henning wrote:
>At least some of the firmware versions do not allow you to change the
>PPSIN offset second by second, so you cannot compensate for the
>"negative sawtooth", and the resulting "hanging bridges" means that
>you have to us a very unoptimal PLL time constant.
The cure for that is to use a GPS that has a clock synchronous with
PPS -- like a TBolt. (Why anyone does it any other way baffles me
completely, but they do.)
And the long time constant is not "unoptimal" -- it is optimized for
best stability. GPS stability (not including any uncorrected
sawtooth) is about 5x10e-9 at tau = one second, improving by 10x per
decade at longer averaging times. Most oscillators a time-nut would
be interested in start at around e-11 at one second, improving for 2
to 5 decades (OCXO and Rb, respectively -- generally improving as a
root per decade), then worsening at longer tau. At some point, these
stability curves cross. The whole point of GPS discipline is, at
each tau, to follow whichever curve is more stable by crossing over
from one to the other. This results in a TC of typically 100-1000
seconds for disciplined OCXOs, or 1000-100k seconds for disciplined Rbs.
So, the TC that is optimum for a disciplined OCXO is NOT optimum for
a disciplined Rb, assuming what one wants is the best possible
stability, at all tau, that the particular GPS and oscillator can
provide. The long TC needed to average out an uncorrected sawtooth
in the GPSDRb would be chosen as the optimum TC for stability anyway.
Best regards,
Charles
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