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Joop[_2_]

On 16 Mar 2007 06:12:27 -0700, wrote:

On Mar 16, 5:49 am, "W3JDR" wrote:

The series resonance is, for practical purposes, invariant. The motional
parameters (L and C) of the series resonance are such high reactances (small
capacitance; high inductance) that external components have only a tiny
influence on the series resonance.

Yes....this is the point of a VCXO...to allow an almost
infinitessimally small, but still useful, variation about the crystal
frequency while maintaining most of the crystal's stability.

The series resonant frequency is the lower of the two crystal 'resonances'.
The parallel resonance is above it. When you make a VCXO with any
substantial tuneability, you're probably operating the crystal at its
parallel resonance. This leads to the common observation that you can 'pull'
a crystal up in frequency more than you you can pull it down.

Nearly all VCXO's I've run across work the other way. You can pull
the frequency down substantially while maintaining good stability
(typically on the order of 0.1%), but not up. This certainly applies
to the circuit for which the original poster provided a link.

Do you have any examples of practical circuit schematics which use
parallel resonance and which can be pulled substantially up in
frequency ? I assume it should be possible to do with a parallel
inductor, for example in a Franklin oscillator circuit, but as was
pointed out the inductor values can be inconveniently large.

Steve
I have to agree with Joe. Basically there is no such thing as an
crystal oscillator in "parallel resonance". However there are
oscillators that use the crystal in the feedback path to add
substantial phase shift. Such as in the Pierce oscillator where it
behaves inductive. The phase shift changes so rapidly that it can
still make a low-drift oscillator.

In the book by Matthys where he compares various oscillators there is
one in chapter 10.6 where the deviation from the (series) resonant
point is the highest. It is a circuit where the crystal sees a very
high impedance as opposed to regular circuits where highest Q is
obtained with very low impedance. This in effect makes the crystal
load to be around C0 of the crystal with some output and input
capacitance. And therefore it is probably the smallest effective
physical series capacitance obtainable and thus the highest frequency.

Looking up the Franklin oscillator you mentioned, I notice this also
is providing a high impedance to the resonant elements. So yes, it
seems a valid way of implementing an alternative to Matthys' example.

Now also cancelling the effect of C0 of the crystal by adding parallel
inductance might push it a bit further. Right now I would not be able
to predict the effect on loaded Q of the crystal. Lowering Q is
normally not done, but in this case we are primarily in quest for wide
pulling range right?

In a low impedance Butler (overtone) oscillator I have seen C0
cancellation by using parallel inductance as well. There sometimes is
an L plus series R used to lower the Q of the L/C0 combination. This
seems not appropriate for a high impedance oscillator circuit. I would
expect best effect if the Q of the inductor is high (low Rs).

Sorry this is still theory. I have no examples of VCXO in this
context.

Cheers,

Joop