for Pitch Linearity
by Mark Barton
I've designed and built a few theremins over the years and just ran across your webpage. It's very well done and I enjoyed browsing through it. I have a theory about the antenna linearity that you have achieved, and since you asked for feedback, here goes. I believe you have perfected what has already
been engineered into the RCA and Moog theremins. That is, a compound, stagger-tuned oscillator circuit that behaves the way we want. When more than one LC circuit are connected together with their
frequencies carefully adjusted (staggered), the slopes of the resulting frequency response can be tailored. When the frequency of just one of those circuits is changed, the frequency of the compound circuit changes at a different rate than a single resonant circuit (as in most theremin designs) would. Check the Radio Amateurs Handbook or The Active Filter Cookbook by Don Lancaster for more on stagger-tuned circuits. I have actually modeled some of these things in Mathcad.
The LC in the pitch oscillator and the LC in its antenna form the compound resonant circuit. The player changes the frequency of the antenna resonator much more than the oscillator resonator resulting in a net frequency change that is less hyperbolic than a single tuned circuit would yield, and evidently more exponential (musical) after heterodyning. When you adjust everything right, you can find a spot on the curve that gives the desired result. The RCA theremin had an extra little coil in the pitch antenna circuit and the Moog Etherwave also has an antenna coil in addition to the oscillator coil to form a compound resonator. The Moog has decent linearity when properly adjusted, but not perfect. I believe the Etherwave Pro is real good, but I have never
I'm going to be designing a new theremin soon with many new concepts and
features that I think is going to blow everyone's mind. I'll let you know when it's done and perhaps get your feedback.