[time-nuts] My Racal-Dana 1992
Charles P. Steinmetz
charles_steinmetz at lavabit.com
Tue Dec 20 21:08:20 EST 2011
>In that case, what and where is the reference for the heater servo?
It will be a voltage or current, suitably regulated and
conditioned. The temperature sensor's signal will be (or will be
converted to) a voltage or current that is compared to the reference
voltage or current. Zeroing and scaling are done in the servo
amplifier and/or the reference conditioning.
In principle, you could do the same thing with frequency -- feed a
voltage-controlled oscillator into a frequency-to-voltage converter
and servo the FVC output to a reference voltage. But the physical
signal we can generate directly most accurately is frequency (using
quartz crystals or atomic resonators). Think about it -- most time
nuts have one or more frequency standards that are not only stable,
but accurate to E-9 or better (sometimes quite a lot better), and can
scale that accuracy to a wide range of frequencies. The volt nut, on
the other hand, struggles to maintain repeatability -- much less
accuracy -- to parts in E-7 at just one voltage.
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