>If you tune around on the tank, you should be able to cross the antenna's
>resonant point, this is characterized by a sudden jump, as the oscillator
>can't actually run at the point where they collide.

I think that you are talking about a Q rule side effect. In that case the
jump would be very abrupt.

The Q rule says that in an oscillator with several frequency-determining
elements (more or less equally coupled), the one with the higher Q factor
will determine the resonant frequency.

The sudden jump is well known from grid-dip-meter work where it is easy to
have the grid dip oscillator 'hijacked' by a high Q tank circuit (assuming
it is a direct oscillator type). This is exactly the Q rule at work. The
GDO will escape the capture from the tank when the bandpass effect of the
GDO and tank circuit will attenuate enough at the tank circuit's frequency
that the GDO tank can take over again.

For linear control as in thermin you probably want to avoid this jump. One
way would be to make sure that the antenna circuit has a larger Q than the
oscillator's tank (the opposite of what I was preaching until now ?), so
it would dominate tuning. This could be done by damping the oscillator
tank (in your scheme) with a small resistor. I'd try 30 ohms in series
with the tank tuning capacitor.

Peter

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