[time-nuts] Oscillators and Ovens

Dana Whitlow k8yumdoober at gmail.com
Wed Nov 1 18:44:52 EDT 2017


Bob,

This discussion is getting really interesting.  In thinking about the
crystal Q versus
tuning range conundrum, two (presumably-overlapping) concerns come to mind:

1. The motional parameters of a high-Q crystal are such that the external
network
    needed to pull it very far would be wholly impractical.

2. Varactors themselves probably have pretty limited Q over much of their
range.

Is my thinking on the right track at all?

Dana  K8YUM

On Wed, Nov 1, 2017 at 5:13 PM, Bob kb8tq <kb8tq at n1k.org> wrote:

> Hi
>
> A high Q crystal by design is very difficult to tune. Putting it in a
> circuit that will
> swing it far enough to compensate it degrades the Q. In addition, thermal
> noise
> will come into the compensation circuit (even if it is noise free) and
> degrade things.
>
> Bob
>
> > On Nov 1, 2017, at 4:38 PM, Hal Murray <hmurray at megapathdsl.net> 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?
> >
> > 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)
> >
> >
> > --
> > These are my opinions.  I hate spam.
> >
> >
> >
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