[sdiy] frequency to voltage? (theremin fun)

[Grant Richter]

"Electronics Today" (198?) had a design for a theremin based entirely on 2 x
CD4046 PLL chips. Run both oscillators at 160 Khz and hang an antenna on one
of the timing capacitor terminals. Use the internal XOR gate to get the
difference frequency.

It works much better if you put a 40 milliHenry inductor between the timing
cap terminal and antenna. This decouples the antenna and give you about a 2
foot control range. The antenna can be a transistor radio antenna. Parts
cost is under $10 for the whole thing.

You can also use another CD4046 to turn the difference frequency into an
output voltage. That can control a VCA for volume control.

If you build two circuits, set them about 50 Khz apart so they do not
interfere with each other.

> From: "Nils Pipenbrinck" <np at inverse-entertainment.de>
> Reply-To: owner-synth-diy at dropmix.xs4all.nl
> Date: Thu, 9 Aug 2001 16:33:59 +0200
> To: "Synth DIY" <synth-diy at dropmix.xs4all.nl>
> Subject: [sdiy] frequency to voltage? (theremin fun)
> 
> 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
> 
> 
> 
> 

.