The operation of detector in theremin

 Hello,

I want to build a theremin,so i must first understand the way it works.

While i was searching for some relevant information and papers i found and i read this one:  http://www.cs.nmsu.edu/~rth/EMTheremin.pdf

 The question that has been created regards the piece of pitch arrangement.

 According to the general schematics i saw in wikipedia and in other papers,the process of pitch arrangement is the following :

 The frequency of variable pitch oscillator (vpo) is controlled by varying the distance between the hand and the antenna. Also,with a potentiometer in front panel we control the frequency of fixed pitch oscillator (fpo).These two frequencies are mixed in a mixer and then go through a low-pass filter . Finally,only the difference between the two frequencies “survives” and is get amplified.

Now,in the project i posted,at page 2-second paragraph it says:

"The difference, or beat, frequency is extracted by the detector and appears..." 

But in the schematic (page 2 and 3) i can't see a mixer.

So, instead of a mixer and a low-pass filter it uses a detector.

As i read, the detector recovers the information contained in a modulated signal as a demodulator does.So, if i want to relate this with the theremin's operation,i can say that the modulated signal that detector extract (as the text mentions) is created by a carrier signal (the signal from the one pitch oscillator) and a message signal (the signal from the other pitch oscillator).As the modulated signal pass through detector is demodulated and rectified (because of the diode).In the end we have the envelope function of the original signal.

So,do you know if these two ways: (1) using mixer and low pass filter,or (2) detector are similar and give us the same result ; 

The envelope function of signal that extracted from detector is equal to the frequency that arising after the mixer and the filter ;

Thank you.

Detector, mixer and demodulator are synonyms in that case. Without a detector, mixer or demodulator, the envelope is symmetric on the positive and negative halves of the carrier signal and thus adds to zero. A simple detector in form of a diode cuts away one half, so that the envelope will be asymmetric and thus be detectable. A more refined active demodulator or mixer would do basically the same but with less energy loss which plays no role in a theremin but is important when it comes to detecting very weak radio signals. The low pass filter eliminates the RF part, so that only the AF part goes to the next circuit stage. 

It is a very simple principle like in the crystal detector radios 100 years ago, no mystery and no magic. The only difference is that the the carrier and modulation frequencies are almost identically high, so that the envelope is not built by a modulating signal but by the difference frequency of both which can be understood with a quick look on the addition and multiplication theorems for cosine functions (8th grade stuff).

BTW: It makes no sense building the EM-Theremin circuit which is 20 years old. Based on that, Moog has brought the Etherwave theremin to market and has constantly improved the circuit over the years so that today's instruments are more stable, have a wider pitch range, better linearity, and a better volume response than the old "original" circuit. The Etherwave circuit board fully operational, pre-adjusted, and tested costs only 100$ when you order it as a spare part. You'll never be able to build it in the same quality for a lower price.

' The Etherwave circuit board fully operational, pre-adjusted, and tested costs only 100$ when you order it as a spare part. You'll never be able to build it in the same quality for a lower price.' - Thierry

Ocht Thierry, you party pooper!
Taking the fun out of buying the components, making the circuit board from whatever you have, like veroboard or copper plating, soldering, testing, invariably getting things not quite right, de-bugging, re-testing and maybe eventually getting the board to work.
...and there you go taking the fun out of the process!

You're 99.9% right of course, better to build the board and put it in a box with antennae - more likely to work and a lot less costly!

(BTW the 0.1% remainder is for all who want to go through the agnst)

Thierry said: "both which can be understood with a quick look on the addition and multiplication theorems for cosine functions (8th grade stuff)."

Your explanation is good but the above statement tells me when they were picking teams for football in 8th grade you were left sitting on the bench. Oops

Here is the Link to that bargain $96 EWS board.

Christopher

I never waisted my time on football or other sports. The sickening smell of the changing rooms was already repugnant enough.

The calories which you transform by using your brain stink much less than those used by muscles!