On 11/1/17 1:38 PM, Hal Murray wrote:
In general, OCXOs have crystals with high Q -> low phase noise, especially
compared to a TCXO, which *can't* have high Q, or the temperature
compensation circuit can't do it's work.
I don't understand that. Why can't I build a high Q TCXO? I don't need to
change the compensation very fast. Are good crystals high enough Q that it
would take too long?
Think of the oscillator as an amplifier and a high Q mechanical filter -
it gets the Q from being mechanically stiff.
In order to move the frequency, you have to electrically push it, which
is counter to the mechanical stiffness. You just don't have the tuning
range available.
We struggled on a project to build a high Q, but adjustable DRO, and
that was the fundamental problem.
What's the time constant? I'd guess it's Q/freq, maybe with factors of 2pi
or e or ???
That seems small relative to how fast temperature changes. (but maybe fast
relative to FCC smearing or things like that)
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