[time-nuts] Question on crystal jumps

[Steve Rooke]

Bruce,

2008/10/24 Bruce Griffiths <bruce.griffiths at xtra.co.nz>:
> You've missed a vital point:
> There is no RF output from the rubidium cell itself at all.
> It doesn't oscillate as does an active hydrogen maser.

Thanks for putting me right on this, I obviously need to study this
more. So as I see it, a CPU measures the output of the rubidium lamp
and tunes a signal source which is used to excite the lamp to give the
highest output. The output of the signal source is used to discipline
a 10MHz ocxo and the output of this becomes our reference standard.
Well, we are at the mercy of the xtal oscillator, if it jumps, so does
the output, straight away, and it does not matter how good our high Q
rb standard is as there will be a jump which should be corrected by
the feedback circuit, IE. the output of the ru lamp photo detector
will go down and the cpu will have to correct it causing the oxco to
be corrected again. How long this takes is down to the time constand
of the feedback mechanism.

OK, even given this setup, would it not be possible to remove the ocxo
from the loop and just use the cpu to control a non-xtal'd signal
source rb exciter. The output of this signal source could then be
divided down to 10MHz and filtered. I appreciate that using a rb
exciter that is not disciplined would not be as stable but with good
design this can be mitigated and, once the signal generator is on
frequency it should be possible to keep it that way with the optical
feedback to the cpu without a lot of noise from the cpu hunting for
the photo detector peak. After all, in the ocxo design, this is also
being varied by the cpu and can be kept stable so why not the rb
exciter oscillator as well.

73 - Steve
-- 
Steve Rooke - ZL3TUV & G8KVD
Omnium finis imminet