[time-nuts] Oscillators and Ovens
kb8tq at n1k.org
Wed Nov 1 21:20:43 EDT 2017
As mentioned in Rick’s post, it’s not really Q, it’s the motional capacitance that is the issue.
Even if you resonate out C0, you still have to deal with Cm. The only practical high Q designs
for crystals are very low Cm resonators. Yes, if you could do a design that had Rm of 0.1 ohms
it could be high Q without a high Cm. That’s not the way it works out.
Since the electrical equivalent circuit is related to the physics of the resonator, simply changing
this or that is not trivial. You might well have to change the basic material to impact some of this.
Simply as a side note, SC’s are “worse” (lower) for Cm than AT’s of similar frequency and overtone.
> On Nov 1, 2017, at 6:44 PM, Dana Whitlow <k8yumdoober at gmail.com> wrote:
> 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
> needed to pull it very far would be wholly impractical.
Depending on you definition of impractical … this is TimeNuts …. In the real world, yes indeed
> 2. Varactors themselves probably have pretty limited Q over much of their
They have a very finite Q. That turns into loss in the oscillator circuit. Loss goes up as the
pull increases. The varactor is a bigger part of the circuit as the pull increases. The Q of a real oscillator
is only partly from the crystal. Tossing more loss into the oscillator will impact the overall Q.
You might go from crystal / 2 to crystal / 5 (very arbitrary numbers and only for illustration … )
> 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:
>> 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
>> will come into the compensation circuit (even if it is noise free) and
>> degrade things.
>>> 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,
>>>> 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
>>> change the compensation very fast. Are good crystals high enough Q that
>>> would take too long?
>>> What's the time constant? I'd guess it's Q/freq, maybe with factors of
>>> or e or ???
>>> That seems small relative to how fast temperature changes. (but maybe
>>> relative to FCC smearing or things like that)
>>> These are my opinions. I hate spam.
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