[time-nuts] BVA "warmup" drift and CNT-90 Allan measure

Magnus Danielson magnus at rubidium.dyndns.org
Mon Oct 29 19:48:59 EDT 2007

Hi fellow time-nuts!

I have been looking at the CNT-90 throughout the heatup of my BVA and one is
on frequency and one on the Allan measure. I chose tau of 100 s for the

The nice thing about the CNT-90 is that it has a plot of values which updates
with each measurement so you can put it on long accumulations and walk by and
have a look at the trend.

Frequency wise, the BVA started 200 Hz high and then dived down and then
overshot and slowly retarded back. This is still in progression, but the drift
slowly becomes less as expected. As I adjusted it to 5 MHz after a short period
of heatup, I am now up to 118 mHz up from that after almost 2 weeks.

The interesting thing is that the Allan deviation has gone from 13,0 uHz to
11,9 uHz in that period. I seriously doubt that the noise processes shifted
that much for the tau in question, but it shows how the CNT-90 Allan processing
does not compensate for drift. It should be noted that there is still
considerable drift, so the Allan deviation isn't meaningfull until the drift
have decayed out.


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