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 <kb...@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 <hmur...@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. > > > > > > > > _______________________________________________ > > time-nuts mailing list -- time-nuts@febo.com > > To unsubscribe, go to https://www.febo.com/cgi-bin/ > mailman/listinfo/time-nuts > > and follow the instructions there. > > _______________________________________________ > time-nuts mailing list -- time-nuts@febo.com > To unsubscribe, go to https://www.febo.com/cgi-bin/ > mailman/listinfo/time-nuts > and follow the instructions there. > _______________________________________________ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.