# Simple Design Question

Posted: 3/28/2007 4:07:27 PM

Joined: 3/28/2007

I'm new to theremins, and I want to design my own, and I only want it to be able to bend pitches, IE no volume. I was wondering, how does the antenna portion work because I already have an oscillator and I just want to attatch an antenna to it to bend the output signal. Is this possible, and if so, is there a simple schematic I could use?
Posted: 3/28/2007 4:30:12 PM

From: Jax, FL

Joined: 2/14/2005

The pitch circuit works by comparing two coils, one fixed and one variable. The Difference between the two is interpreted as the pitch.

There are many single antenna theremins out there and the schematics should be easy to find.

You could set one up with a CV output that could be used to control the oscillator.
Posted: 3/28/2007 5:15:24 PM

From: Croxley Green, Hertfordshire, UK

Joined: 10/5/2005

Hi flashbandit. Welcome to Theremin World.

A quick cut 'n' paste from wikipedia (http://en.wikipedia.org/wiki/Theremin#Operating_principles).

[i]The theremin uses the heterodyne principle to generate an audio signal. The instrument's circuitry includes two radio frequency oscillators. One oscillator operates at a fixed frequency. The other is a variable frequency oscillator, the frequency of which is controlled by performer's distance from the frequency control antenna. The performer's hand acts as the grounded plate (the performer's body being the connection to ground) of a variable capacitor in an L-C (inductance-capacitance) circuit. The difference between the frequencies of the two oscillators at each moment generates a beat frequency in the audio frequency range, resulting in audio signals that are amplified and sent to a loudspeaker.[/i]

I'm not an electronics expert but I imagine the reason for using two RF oscillators rather than bending an audio frequency oscillator directly is related to the size of the pitch field - my guess is that using an antenna as a variable capacitor in an audio frequency oscillator would give a playable field of a millimetre or so.
Posted: 3/28/2007 6:34:24 PM

From: Escondido, CA

Joined: 3/25/2007

Yeah, the rods or plates on a theremin are actually not antennas. Their purpose is not to convert EM waves into electrical voltages or vice versa. They are one plate in a capacitor, and your hand is the other plate going to ground (earth). It's great for teaching people about capacitance because you're actually a part of the circuit. There are two oscillators that operate in the supersonic range--millions of hertz. One oscillator's pitch is changed by your hand so that it creates a beat frequency in the audible range with the fixed oscillator. (The higher added frequency is filtered out.) A theremin can create a control voltage that can be used as a controller for a voltage controlled oscillator (instead of using a keyboard or something), which DD already said.

Parsa
Posted: 3/28/2007 9:53:46 PM

Joined: 3/28/2007

So, if I were to use the simple theremin diagram (http://www.thereminworld.com/pics/schematics/simple.jpg), could I make it work with just one oscillator? Could I divide the signal and compare one side to the antenna side to allow pitch bending?

Also, just curious, what is the capacitance range of your hand and the antenna? so I can do some math for making simple modifications.
Posted: 3/29/2007 5:36:01 AM

From: Croxley Green, Hertfordshire, UK

Joined: 10/5/2005

Could you make it work with one oscillator? No.

Your posting suggests to me that you have a slight misunderstanding about how oscillators work. The question would be sensible if the oscillator generated a fixed frequency wave and [i]then[/i] the capacitor (i.e. the pitch "antenna" and your hand) changed the frequency of the signal made by the oscillator. It doesn't work like that.

I found this nice little Flash animation that shows how an LC oscillator (one made of a coil L and a capacitor C) works.

LC Oscillator Demo (http://www.greenandwhite.net/~chbut/lc_oscillator.htm) (scroll down to "LC Oscillations")

The capacitor is an integral part of the circuit, not something that comes after it.

Now for Beat Frequencies.

Here's a Java applet that illustrates beat frequencies.

Beats Demo (http://www.mta.ca/faculty/science/physics/suren/Beats/Beats.html)

Let's say the red wave is the fixed frequency oscillator, and the green one is the variable frequency oscillator which includes the pitch "antenna" and your hand as the capacitor. Moving the slider for the green wave is the same as moving your hand back and forth.

The yellow wave at the bottom is the result of combining the red and green waves.

See how the amplitude of the yellow wave varies when the red and green waves are different frequencies? That's your beat frequency - the bit you can hear - in the world of communications this is called the message. The yellow wave itself is called the carrier wave and is way to high to hear. (This is how AM - amplitude modulated - Radio works)

Here's a shockwave demo of Amplitude Modulation.

Once it's loaded click on "Interactive Playspace," set the carrier frequency to its highest setting and select a sine-wave message. Looks just like the beat frequencies, doesn't it.

After the theremin has combined ("heterodyned") the variable and fixed Radio Frequency waves it then demodulates the signal to remove the carrier and leave the message, just as an AM radio does. The circuitry that does this is called a filter.

There you go. I hope that I got it right and that it helps. :-)
Posted: 3/29/2007 10:13:03 AM

Joined: 3/28/2007

Gordon, thanks so much, that was a better explanation than I could have asked for.
Posted: 3/29/2007 10:47:09 PM

Joined: 3/29/2007

You need two oscillators, one with a fixed and the other with a variable frequency. lookie here: http://pickyguide.com/musical_instruments/theremins_guide.html
Posted: 4/9/2007 10:39:12 AM

From: France

Joined: 4/8/2007

And you need it to be shielded from each other, to avoid EM feedback and locking between oscillators...

But this problem is a matter of inductor i think, and i don't know if it happens with logical electronics..