[time-nuts] Frequency of LC Tank

[Magnus Danielson]

On 12/04/14 21:23, 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"!

I propose to sample the voltage rather than just using a XOR-gate (and a 
limiter in front of it to square things up). Thing is, the voltage from 
your LC tank is very analogue, so make use of this fact.

I would put a PI-loop (one op-amp, two resistors and a cap) in there.

As for frequency, your actual drive-oscillator can run at 4 times the 
rate and then use a frequency divider to provide the phases. This can 
provide you with in-phase and quadrature signals. Look at Tayloe 
detector in a search engine near you. The driven frequency will remain 
in your range, but you now have a quadrature signal to compare with.

Cheers,
Magnus