Theremin

Electricity and Magnetism > AC Circuits

DCS# 5L20.xx

THEREMIN

wavetek function generator	107-01-A
large coil	101-07-D
3 ft coaxial cable	015
decade amplifier	202-08-A
mixer / low-pass filter circuit	202-20-B3
amplifier and speaker	202-09
oscilloscope with camera	202
aluminum plate	202-18-B

DESCRIPTION

An LC circuit whose output is amplified and fed back to the circuit serves as a high frequency oscillator. A function generator is set to a frequency just above the resonance frequency of the LC circuit and the two signals fed to a diode, which acts as a mixer. Because the diode is non-linear, its output includes sum and difference frequencies. A low pass filter blocks the original and sum frequencies. The signal is amplified and sent to a speaker. The function generator should be adjusted so that the difference frequency is in the audio range.

The antenna + your hand form a variable capacitor. Vary their separation to change the LC resonance frequency and thus the pitch.

Provided f_fg > f_LC, increasing the capacitance should decrease f_LC and increase the difference frequency and the pitch, consistent with conventional theremins. Note that this is opposite the suggested setup for the metal detector, for which f_fg is set < f_LC so the pitch rises and falls with the resonance frequency.
NOTES: The coil inductance is 6 mH, and the effective capacitance of the cable and connections is roughly 700 pF, so the resonance frequency is around 75 kHz; The setup is identical to the metal detector, except for the aluminum plate antenna connected to the coil.

Make sure the metal plate is isolated from ground and fairly distant from the rest of the circuit.

Set up the scope to display the LC oscillator signal and output of the audio amplifier.

REFERENCE