[time-nuts] Frequency of LC Tank

[dan at irtelemetrics.com]

Magnus,
  You are very much on the track that I was thinking. I belive you are 
absolutly correct
in that a 90 degree phase shift would be ideal.   

I did a bit more digging last night, and it turns out that an XOR phase 
comparator
  looking at the tank voltage and drive voltage may be ideal, as you 
have suggested here. 
My main concern was that I plan to adjust the pulse width of the 
push-pull the drive circuit to
  adjust the power into the tank circuit. (Actually the drive will be 
full bridge, transformer
coupled to the tank). That change in pulse width is where I was stuck, 
mentall. However since
  I'm in the 10Khz to 100Khz range and am generating the push-pull PWM 
digitally, I can
  just generate a second output at the same frequency and phase (or 
even different phase)
  than the drive signal to compare to the tank voltage.  As you say 
"away you go with a phase
detector"! 

Didier,
I guess the thing that's different here than in most situations, is 
that normally you try not to load
  the tank circuit more than necessary. Here I'm loading the tank 
circuit considerablly, knowing that it
  will change frequency with the change in Q. This change in frequency 
is what I need to find, track, and follow. 

The tank will be very lossy (Maybe consuming 20Kw to 30Kw of power if 
all goes well). I'm also certian
Q will move all over the place. I just want to stay near the peak of 
the bell, even if it's a short fat bell shaped
curve. Since the frequecny is low, I was thinking that even a modern 
optocoupler should get me
  phase information well. At these power levels a little loading 
souldn't be a big deal! :) 

Dan

> 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. 
> Cheers,
> Magnus
> ------------------------------
> Keep in mind that anything you connect across your tank circuit will 
> affect its resonant frequency and Q (signal source and measuring 
> device). You need to make sure your equipment is very loosely coupled 
> to the UUT through small value capacitors for instance. 

>  Didier KO4BB