[time-nuts] Frequency of LC Tank

Richard (Rick) Karlquist richard at karlquist.com
Sat Apr 12 21:20:37 UTC 2014

It is very easy to make an impedance phase detector by
inserting a toroidal current transformer in series with
the load under test.  The center of the secondary is
connected to the load through a capacitor.  Each end of
the secondary goes to a diode detector.  When the
load is resistive, the DC outputs of the two detectors
are equal.  You can see this in the literature back
as far as at least the 1950's for autotuning antennas.
It is closely related to various FM discriminator circuit
like the ratio detector.


On 4/12/2014 12:23 PM, dan at irtelemetrics.com wrote:
> 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
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