[sdiy] frequency to voltage? (theremin fun)

[Nils Pipenbrinck]

Hi.

Yesterday I built a little theremin based on a circuit I found in the
internet somewhere.. It's not the classic circuit.

Oscillator is built around a cmos hex-inverter, and a cmos pll ic controlles
it's frequency. It's really fun to play around with that toy. (especially if
you run it through a dsp based guitar processor) :)

Now I want to add amplitude control.

I thought about just building a second one, and use some kind of frequency
to voltage converter, add a vca and have amplitude control (or filter cutoff
controll or whatever comes into my sick mind..)

The circuit outputs a nice 9V squarewave (50% duty cycle).. any ideas how to
get that into a frequency dependent voltage cheap?

The only idea I have is to use a moderate lowpass behind the output, add a
full wave rectifier and then plug that into a high q lowpass (5hz, 18db or
so).

That's just a naive idea.. would that work?

Thanks,
    Nils Pipenbrinck