From: "Rick Karlquist" <[EMAIL PROTECTED]> Subject: Re: [time-nuts] Better than Quartz? Date: Sun, 27 Apr 2008 17:21:58 -0700 (PDT) Message-ID: <[EMAIL PROTECTED]>
> 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 _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
