[time-nuts] Frequency of LC Tank.

J. Forster jfor at quikus.com
Fri Apr 11 16:54:20 EDT 2014

At resonance, an LC looks pure resistive.

For a parallel LC, sample the voltage across the LC and the drive current,
and tweek the frequency until they are in-phase.

For a series LC, sample the voltage across the L or C and tweek as above.

If you want to do it analog, dither the frequency a bit. With the
quardature of the sweep signal as reference for a lock-in. The output of
thye L-I is the tuning signal. (Roughly Pound Locking)




> Hi all,
> I'm thinking about an upcoming project, if this is off topic please
> disregard or contact me off list. :)
> I have a large LC tank, with a very lossy inductor. Being driven by a
> pulse width push pull driver, that is digitally controlled. The driver
> circuit will couple through a N:1 transformer. I need to be able to
> adjust the push/pull driver frequency to match the frequency of the tank
> circuit. (See frequency/time is involved :) ) The tank components can
> vary and are not adjustable, so the drive frequency needs to vary.
> I'm thinking some sort of a phase detector may be the way to go. I'm
> just not sure were to sample the V and I signals to look for phase
> differences, or where to get a good clean reference from.
> So the question is, when actively driving a tank circuit, how do you
> know you are driving it with the same frequency ad the same phase it
> naturally oscillates at.
> Any thoughts, suggestions, or readily available papers you guy could
> point me to?
> Thanks!
> Dan
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