Theremin Music Instrument effect. The typical theremin has two r.f. oscillators, one having a fixed, the other a variable, frequency, with their output signals combined in a mixer/amplifier stage. At “tune-up,” the oscillators are preset to “zero beat” at the same frequency. The frequency of the variable oscillator is controlled by an external tuning capacity–the “antenna”—which is a “whip” or simple metallic plate.
As the musician”s hand is moved near this antenna, the variable oscillator shifts frequency and a beat note is set up between the two oscillators. The pitch is proportional to the difference in frequency between the two oscillators. This beat note, amplified, is the theremin”s output signal. The more advanced theremin designs—such as the version presented here—use a third oscillator to control output volume and two antennas. This theremin also uses a unique FET volume, and a FET output stage.
Go to that page to read the explanation about above circuit design.