EM Theremin debugging (Lab project)

Posted: 11/18/2011 10:00:23 PM

Joined: 11/18/2011

My lab partner and I are building the EM Theremin for an electronics lab, and today we finished putting everything together. We are following the article from EM pretty closely.

So far, we know for certain that the pitch oscillators work, the pitch tuner works, the detector works and the volume oscillator works. I'm uncertain about the volume tuner, and I'm not sure how to check this without a fully functional circuit.

The problems we are having are the following:

- The antennas. We know that our pitch antenna is not functioning properly. We aren't getting any changes in the variable pitch oscillator's frequency.

- The VCA & VCA processor are not working.

For the VCA/processor I think it will just be a matter of sitting down and checking all of the connections again.

But I am very concerned about the antennas. We should be seeing at least some change in the signal from the oscillators with the pitch antenna.

Right now, we have the inductors for the pitch antenna on a smaller breadboard, separate from the main part of the circuit, and we were using copper rod about the size of a coat hanger for an antenna. This breadboard is connected by a small lead to the variable oscillator on the main board.

I was hoping someone here could give me some advice or suggestions.
Posted: 12/4/2011 11:21:00 PM

From: Colmar, France

Joined: 12/31/2007

The pitch antenna has to be a ~40cm (16") long and 1cm (3/8") thick rod. Only then it has the correct static capacitance to provoque a frequency change in series with the 4 x 10mH inductors.

The 4 x 10mH inductors may not have too much static winding capacitance themselves. That's why you must use the recommended 3-pi-wound RF inductors with a SRF > 700kHz and you can't mount them on a breadboard since its copper dots or stripes would add too much capacitance and thus lowering the SRF. 

If you do still not get any frequency change, one of the coils may be defective (i.e. due to overheating it when soldering). Since each of these coils has a winding resistance of ~30 Ohms, the total DC resistance of your four coil assembly should be ~120 Ohms.

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