In message , William
Sommerwerck writes
You would find it rather difficult to FM a crystal oscillator.
Possible, but difficult.

Not really. It's done all the time in temperature-compensated oscillators,
simply by varying the series or load capacitance.


OK then - 'Possible, but much more difficult than with a free-running
L-C circuit'.

When an oscillator is crystal controlled, to get a lot of FM, it usually
takes a bit more than 'slipping a variap' into the circuit. You usually
have to use some form of VXO circuit. Even then, the amount of FM you
get can depend a lot on the characteristics of the crystal. And it may
not be very linear. But it all depends on what you need.

Some early crystal-controlled FM equipment avoids the problems of FMing
the crystal itself. Instead, in a later stage they phase-modulate the
signal (producing a predictable amount of PM) and, of course, multiply
up to the final frequency.

Ian.
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