Peter, That's is essentially the point - it is the Q of the resonator (assuming environmental effects are controlled) which ultimately determines the stability. Very high performance oscillators (the HP E1938A is an example) use a bridge circuit to enhance the loaded Q of the crystal. Other designs (the QAS and BVA) use a complex structure of crystal which places the electrodes outside the oscillating structure in order to enhance the loaded Q. 73, Murray ZL1BPU

Any decent crystal oscillator circuit will have a loaded Q that is a large percentage of the unloaded Q. The bridge circuit in the E1938A does not enhanced the loaded Q, compared to, for example, the 10811. It doesn't seriously decrease the loaded Q either, but in any event, Q is not the raison d'tre of the bridge circuit. What the bridge circuit does is make the oscillator circuit (as distinct from the crystal) very stable. If you don't have a bridge (ie any oscillator other than the E1938A), then the loaded Q of the crystal will affect how much the oscillator circuit can pull the crystal. In this sense it could affect stability. But if the circuit is sufficiently ovenized, and run on stable power supplies, etc., then the stability of the oscillator will be determined by the intrinsic stability of the crystal. There is no direct correlation between the Q of the crystal and its intrinsic stability. It is fairly easy to make a "junk" crystal with a Q of many 100,000's at 10 MHz. This Q is at least within an order of magnitude of the theoretical QF limit of quartz. Yet the stability is many orders of magnitude removed from a precision crystal. AFAIK, the loaded Q of exotic crystals like BVA is not significantly higher than other good crystals, since they are already close to the theoretical QF limit. There was an interesting paper given at FCS about 10 years ago about the quartz shortage during WWII. There were govt projects to search for a replacement material. It sounded like a good effort was made, but in the end nothing else even came close to quartz. Rick Karlquist N6RK Murray Greenman wrote: > Peter, > > That's is essentially the point - it is the Q of the resonator (assuming > environmental effects are controlled) which ultimately determines the > stability. Very high performance oscillators (the HP E1938A is an > example) use a bridge circuit to enhance the loaded Q of the crystal. > Other designs (the QAS and BVA) use a complex structure of crystal which > places the electrodes outside the oscillating structure in order to > enhance the loaded Q. > > 73, > Murray ZL1BPU > > > _______________________________________________ > 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. > >

From: "Rick Karlquist" <richard@karlquist.com> Subject: Re: [time-nuts] Better than Quartz? Date: Sun, 27 Apr 2008 17:21:58 -0700 (PDT) Message-ID: <15220.24.6.87.178.1209342118.squirrel@webmail.sonic.net> > Any decent crystal oscillator circuit will have a loaded Q that is > a large percentage of the unloaded Q. The bridge circuit in the > E1938A does not enhanced the loaded Q, compared to, for example, > the 10811. It doesn't seriously decrease the loaded Q either, but > in any event, Q is not the raison d'tre of the bridge circuit. > What the bridge circuit does is make the oscillator circuit (as > distinct from the crystal) very stable. If you don't have a bridge > (ie any oscillator other than the E1938A), then the loaded Q of > the crystal will affect how much the oscillator circuit can pull > the crystal. In this sense it could affect stability. But if > the circuit is sufficiently ovenized, and run on stable power > supplies, etc., then the stability of the oscillator will be determined by > the intrinsic stability of the crystal. > > There is no direct correlation between the Q of the crystal > and its intrinsic stability. It is fairly easy to make a > "junk" crystal with a Q of many 100,000's at 10 MHz. This > Q is at least within an order of magnitude of the theoretical > QF limit of quartz. Yet the stability is many orders of magnitude > removed from a precision crystal. I think your argument about stability needs to be clarified in terms of short term and long term stability. Correct me if I am wrong, but short term, i.e. jitter is fairly dependent on crystal Q value, among other things naturally, such as drive level and oscillator noise. Long term stability depends more on factors like environment etc. > AFAIK, the loaded Q of exotic crystals like BVA is not significantly > higher than other good crystals, since they are already close > to the theoretical QF limit. The BVA configuration adds a little bit of serial capacitance, as a side-effect as I remember it. I could dig up the Q value and other measures on my BVA (OSA 8600) if you like. > There was an interesting paper given at FCS about 10 years ago > about the quartz shortage during WWII. There were govt projects > to search for a replacement material. It sounded like a good > effort was made, but in the end nothing else even came close > to quartz. It is only more recent work in esoteric materials, configurations and temperatures that provided competing Qs, but then for most part in a totally different frequency range. Cheers, Magnus