[time-nuts] 10811 EFC load question
Charles P. Steinmetz
charles_steinmetz at lavabit.com
Fri Oct 29 01:41:28 UTC 2010
>What is the current requirement for the 10811 EFC +/- 5Vdc ?
>Is there any advantage to having it served by batteries?
Only the leakage current in the varactor, which is why Rick suggested
operating with Vefc at the high end of the range. But even a mercury
cell is not stable enough to do this job without electonic
regulation, so you pretty much need to suffer with regulator noise
even if you use a battery. In any case, there is a limit to how
quiet it pays to make the EFC supply, because, as Rick also pointed
out, the other end of the varactor is connected to an internal
reference that is not the quietest voltage source in the world,
filtered with a corner frequency of around 3 Hz. By referencing to
this voltage rather than ground, as Rick suggested, you could reduce
noise modulation (but it requires bringing the reference voltage out,
which HP did not do).
I'm not sure how much practical difference this would make, compared
to the inherent phase noise of the crystal. The 10811 normally runs
with the EFC pin grounded (6.4 V across the varactor, referenced to
ground at the anode). I suppose the best thing to do is remove the
varactor entirely and replace the trim cap with one that has greater
mechanical resolution, though I imagine finding a part that has at
least 10x better mechanical resolution and is no worse than the
existing trimmer in terms of mechanical and thermal drift may not be
a trivial task.
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