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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