On Tue, 16 Dec 2003 17:06:27 +0000, ddwyer
wrote:


The butler type is a tuned amplifier were there is ideally zero phase
across the crystal and hence the crystal can be simulated by a resistor
of equal (in practise lower) resistance than the crystal loss.
Hence a butler tuned to 100MHz will oscillate a 50R resistor at that
frequency.
The 100MHZ crystal if 5th overtone will have a fundamental response at
approx. 20MHz and a 3rd overtone response at 60MHz .
The fundamental response (resistance) will be much lower than the 5th
resistance and in a zero phase amplifier with flat response the crystal
will preferentially oscillate at 20MHz.
Higher order overtones have the same C0 or stray capacitance as the
fundamental but this has a lower reactance at higher frequencies.
This is the reason and increase in resistance at higher overtones that
overtone oscillators get increasingly tricky.


main arguments are right, but I don't believe the frequency scheme is
right. If 20MHz is the fundamental, it doesn't make sense to tune the
2nd circuit to 100MHz, you could tune the 1st. You could also operate
the first tuned circuit at 60MHz and have o/p at 120 or 180MHz, with
sub-harmonics of n.order 60MHz.

It was a tendency 20 years ago to apply overtone xo's running at
150-200MHz, but they very soon dissappeared because of too much
instability problems

73
jm

http://home.online.no/~la8ak/c13.htm
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