Oscillator design - fpga-theremin/theremin GitHub Wiki

Choosing type of sensor

Thereminists control pitch by moving right hand and volume by moving of left hand.

Digital theremin must have some type of sensor to feel hand movements

Antenna capacity based sensors - like in classic theremins
Optical sensors (luminance or laser distance meter)
Ultrasonic distance sensors - I've tried to use it in Arduino based design - too low sensitivity
Camera based sensor - high resolution high speed camera and complex video processing is required to get acceptable sensitivity and low latency
More?

Capacitive sensor

Based on measuring of antenna capacity which is being changed when hand is moved near antenna.

Antenna capacity varies by ~10-15% for hand moves between far and near positions.

Capacity of antenna may be measured using RC or LC circuit.

RC type has too low sensitivity on far hand distances, and parameters too much depend on temperature. Useful for toys only.

LC tank based measuring provides much better stability and sensitivity.

For better results, LC tank should have high Q. C is the smaller the better. L (inductor) - air core is preferred.

LC resonant frequency measure

Following methods may be used for measuring of LC resonant frequency:

LC oscillator - LC resonant frequency is output frequency of oscillator - classic way
Pass fixed reference frequency to LC input, detect phase shift between signals before and after LC - phase shift 90 degrees will be when reference frequency equals to LC frequency. XOR logic may produce PWM signal depending of phase shift.
Use PLL to control input frequency for LC, track phase shift by PLL to keep frequency equal to LC resonance (dewster's method).

LC oscillator

LC oscillator is classic way to measure antenna capacity.

It has good sensitivity on far hand distances from antenna.

The only problem here is that it's hard to measure frequency of oscillator with high precision.

LC oscillator frequency varies in range 3-4% (or less, depending on oscillator type) reacting on hand movement.

Direct measuring of this frequency is usually hard, at least for MCUs.

Usually some heterodyning technique is used to extend frequency change rate, or even to convert it to output pitch range - this method is used in most of analog theremins and some digital theremins.

Direct measure of LC oscillator frequency using FPGA is possible. Using hardware deserialisation components and delay lines for oversampling may allow to get more bits of measured frequency value.

Averaging of measured values allows to increase effective number of bits.

Averaging adds better precision but adds latency. Latency (time between hand movement and sound changing) must be as low as possible. Latencies more than 10 ms are easy to feel. The bigger latency is, the harder is to play proper note (e.g. move hand based on audio out to keep desired pitch).

I believe, for good digital theremin, latency should be less than 1ms.

Bad example of digital theremin with high latency is Moog Theremini (it's latency is up to 0.2 seconds).

With FPGA it's possible to create design with latency of a few samples (e.g, 0.05ms or 0.1ms).