Sound and Oscillators
Sound and Performance of the Open.Theremin
The basic principle of the theremin is the heterodyne oscillator. For the Open.Theremin I knew that I needed a stable and reliable oscillator. So I searched the internet and found various schematics with different components. I tested many of them and there are huge differences. Finally I decided to use a combination of oscillating crystal for the fixed frequencies and LC oscillator for the variable oscillator. The variable oscillator is digitally tuned by a capacitor diode. I also tested around to find the optimum frequency and component values for the oscillators. Clara Rockmores theremin used a coil with 1165uH and 170kHz, the Theremax goes up to 1Mhz while most theremins are in the range of 500-700kHz. I decided for 500kHz for the Open.Theremin circuit.
There is an excellent abstract ON THEREMIN SENSITIVITY by Fred Nachbaur
- Coupling of the resonators can be a problem as the two oscillators try to pull one another to a common frequency and extinct the output signal. Use of separate IC for each oscillator with proper decoupling condenser is the solution.
- The hand-to-antenna capacitance is usually only a few picofarads
- With good resonator circuits, decent little theremins with high sensitivity can be built
- With the microcontroller the response curve can be corrected and can get a wide range up to one meter distance.
- Too low sensitivity and high range is not necessarily more comfortable to play. Playing gets spongy.
- Longer antennas means bigger range
Here some measurements of the digitalized and linearized antenna signal of the Open.Theremin:
The measured antenna signal (internal counter value)
Linearized output signal of the open.theremin
Comparison with two theremin measurements found on the web.