I'm having a bit of a difficult time with analyzing this circut:
http://www.dvanhorn.org/TankCkt.gif

This is an oscillator tank, where C2 and L2 are parallel resonant.
The operating frequency is around 280kHz, with C2 at 3300pF and L2 at 98uH.
The interesting part is where I add L1 at about 40mH and Cg at 0-1pF with
Ctrim at some small but significant value. (ideally, series resonant at
about 290 kHz, but you have to take parasitics into account)

What I'm trying to do, is to determine the oscillating frequency as Cg
changes, and also to evaluate different values of L1 as the series resonance
of L1/Ctrim+Cg moves around the tank resonance. Obviously if the two
resonances collide, it's not nice.

I'm having a hard time finding how to express the change in Cg as a shift in
the resonant point of the oscillator tank.

The formulas that I have for impedance show only the magnitude of the
impedance, and probably aren't applicable to this situation. Where can I
find a better approach for analysis?

Once I have this down, I have to consider the parasitics and series
resistance, especially in L1 which are pretty significant, but I have a
handle on how to apply that.


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"Dave VanHorn" wrote in message
...

I'm having a bit of a difficult time with analyzing this circut:
http://www.dvanhorn.org/TankCkt.gif

This is an oscillator tank, where C2 and L2 are parallel resonant.
The operating frequency is around 280kHz, with C2 at 3300pF and L2 at
98uH.
The interesting part is where I add L1 at about 40mH and Cg at 0-1pF with
Ctrim at some small but significant value. (ideally, series resonant at
about 290 kHz, but you have to take parasitics into account)

What I'm trying to do, is to determine the oscillating frequency as Cg
changes, and also to evaluate different values of L1 as the series
resonance
of L1/Ctrim+Cg moves around the tank resonance. Obviously if the two
resonances collide, it's not nice.

I'm having a hard time finding how to express the change in Cg as a shift
in
the resonant point of the oscillator tank.

The formulas that I have for impedance show only the magnitude of the
impedance, and probably aren't applicable to this situation. Where can I
find a better approach for analysis?

Once I have this down, I have to consider the parasitics and series
resistance, especially in L1 which are pretty significant, but I have a
handle on how to apply that.


It might help if you told what you are ultimately trying to do.
Is this just a thought experiment designed to help you learn something?
Is it a circuit you have/see and are trying to understand it?
That antenna is an unknowns, as well, which I guess you are ignoring...?
From what I see/read, you are trying to do something which is very
awkward...that is, multiple coupled resonances.

Looks like you want to see what kind of "pulling" effect the Delta Cg is
going to have on your oscillator. Why inductive coupling?
The conditions which determine the frequency of oscillation are not confined
to the resonant frequency of your tank, but include phase and gain of the
active device as well.

I modeled a 1 MHz Colpitts in PSpice and it oscillated nicely. Adding other
stuff to muck about with it could be done...

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Steve N, K,9;d, c. i My email has no u's.