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In message , Joop
writes On Fri, 16 Mar 2007 22:51:34 GMT, "colin" wrote: The reason for the parallel inductance in the overtone mode is the low impedance of the crystal self capacitance at high frequencies, this on its own can be quite low, and the series resistance at overtone can be quite high, so this can allow the tank circuit to dominate rather than the crystal unless it is cancelled. I know why the inductor is usually necessary. The thing is that for the purpose of the Butler the effect of C0 can be neutralized. The question is whether it can be made to have a similar effect in the "pulling arena" as well. Of course it should not have to much side effects in normal operation of the oscillator. In the overtone butler there is also another LC resonance circuit present that determines the possible operating frequency (read desired overtone). Such a thing might be necessary in most circuits where an inductor is placed in parallel with a crystal. Joop I've eventually found some good plots of the frequency responses of crystals (see below). http://g4oep.atspace.com/crystal_fil...rystal_filters ..htm#4)%20Single-Crystal%20Ladder. This info from G4OEP is about filters, rather than VXOs, but it graphically illustrates how a crystal can be neutralised in a bridge circuit, using a small capacitor rather than large inductor. The capacitor will be the same value as the parallel capacity of the crystal. The plots in Figs 9, 10 and 12 also show how the throughput peak moves around to some extent as the neutralization is adjusted. The circuit is essentially the same as I used when I was testing a load of crystals to see if they met spec wrt frequency accuracy and ESR. You should be able to use the circuit of the single crystal filter (in-to-out, without the resistive padding etc) as the resonant part in a VXO, and adjust the neutralization to pull the frequency of oscillation. It's worth a try. Ian. -- |
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