[time-nuts] Oscillators and Ovens

jimlux jimlux at earthlink.net
Wed Nov 1 23:25:24 EDT 2017

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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