VCO circuit

Posted on Feb 5, 2014

The basic principle of the theremin is theheterodyne oscillator. When I started developing 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 built them each on a small board and connected them to the micro controller to measure the performan

VCO circuit
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ce. There are huge differences. Some did not work at all for me, others were not stable. For the variable oscillator I decided for the voltage controlled RC oscillator circuit inside the 4046 phase-locked loop chip. This integrated oscillator revealed to be much more stable than the other circuits I tested. Then there is also a Z-Diode in the package that can be used to stabilize the supply voltage. To adjust the resonated the R1 input can be used. Controlling by VcoIn did not provide better results. I also tested around with the optimum frequency and component values of the oscillators. Clara Rockmores theremin used a coil with 1165uH and 170kHz, theTheremax goes up to 1Mhz while most theremins are in the range of500-700kHz. I did not find higher frequencies to be significantly better so I decided for 125kHz for the RC circuit and for 500kHz for the LC circuit. - Coupling of the resonators is a problem. It is almost impossible to put two oscillators in one circuit because the two oscillators try to pull one another to a common frequency and extinct the output signal. One IC for each oscillator with proper decoupling condenser is best. - The performance of a circuit is hard to predict. Calculations do not always confirm what is common senses among theremin sites. Measurements do not alway confirm calculations. - Too low sensitivity and high range is not necessarily more comfortable to play. Playing gets spongy. Truly linear...

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