Theremin RF Engineers you might want to see this?

Posted: 1/10/2012 7:40:28 PM
RS Theremin

From: 60 mi. N of San Diego CA

Joined: 2/15/2005

The yes/no question: Am I generating a theremin voice with heterodyne control which develops a frequency modulated carrier?  Is this even possible? It’s news to me.

The wise FredM once said if you discover something unusual; present it to the theremin community without voicing your own opinion too quickly. This way others might take it a little more seriously.

I made a webpage and gathered the facts so far.  Maybe someone with RF experience could shed more light upon my observations and answer about the FM possibility.

This is so interesting to me that it is drawing to much attention away from the theremin work I should be doing. It was a full moon yesterday when I first made these observations.

  Visit webpage


Posted: 1/11/2012 2:15:51 AM

From: Colmar, France

Joined: 12/31/2007

The aliasing theory:

You do mix two oscillator signals of about 850kHz. So you'll get the AF at the mixer output, the oscillator signals themselves, and at least the sum frequency.

You should have thus your 260Hz, 850kHz, 849.740kHz and 1699.740kHz.

Then, your sound card samples this with 96kHz. That means that the 4 signals above are mixed with the multiples of 96kHz. 850kHz with (9 * 96kHz) gives for example a 24kHz aliasing. That's why I never trust sampled signals... At least not, if there isn't a matched anti-aliasing filter. Try using a 7kHz low pass filter with at least 12 dB/octave before feeding the signal into the sound card. How does the signal look now? How does it look on a true oscilloscope?

The self oscillating mixer theory:

Is the mixer active or passive? If it's active, it might be accidentally self oscillating and the demodulated signal might be PWM-ing it...

Posted: 1/11/2012 2:48:58 AM
RS Theremin

From: 60 mi. N of San Diego CA

Joined: 2/15/2005

Thank you Thierry for your response.

The 30 kHz frequency is measured at the theremin audio output with a frequency counter and true oscilloscope, the audio is embedded within this carrier. What is happening should not be. The early stage mixing is passive, two transistor oscillators, a single diode but the carrier just blows right through the diode and one TLO82 Op! I just tried it direct from the the theremin pitch board to a Moog theremin amp and the effect still exists, just quietly. Scope wise it is the same result. I have a FM demodulator chip on order to try. It is the other interesting behaviors that have me excited. It has only been two days since I first seen this effect. Self oscillation is most likely the influence in the way I wired the coils. I have little doubt about FM (PWM), whether this approach has any value I don't know.


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