[time-nuts] Frequency of LC Tank.

Magnus Danielson magnus at rubidium.dyndns.org
Sat Apr 12 06:48:46 EDT 2014

On 11/04/14 22:15, Dan Kemppainen wrote:
> 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?

As you drive it with a pulse, you induce energy to it. If you sample the 
voltage (or current) 90 degrees of from your drive-pulse, that 
quadrature will indicate if you are early, late or prompt. As your 
sampling point is also a sign of your current rate, and the pulse forced 
the LC tank and your oscillator into sync, the frequency error will 
cause the phase difference and hence voltage difference to be 
observeable. As you are fairly close in frequency, so will the phase 
error and you can assume the phase to voltage to be almost linear and 
away you go with a phase detector.


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