When you tune

Can one of our beloved electronics mavens explain just what happens when we turn the knob to change the sensitivity of the field around the pitch antenna? Are we altering the frequency of the fixed-pitch circuit so that the variable circuit has a different effect? Or are we somehow (How?) changing the effective values of the variable capacitance we're creating between hand and pitch rod?

It ain't a voltage thing, right? Because a tank circuit like these has a frequency unrelated to applied voltage--isn't that what resonant circuits are all about?

Thanks, folks!

That depends on the theremin design. Having studied lots of schematics, I discovered that there were several different "pitch adjustment systems":

a) A variable capacitor which acts either on the variable or on the fixed oscillator. Mostly found in historical designs.

b) Very rare: A varicap diode which acts like a variable capacitor under the influence of a variable DC voltage coming from a potentiometer. Same effect as under a)

c) A potentiometer which gives a variable DC voltage in order to influence the working point of one of the oscillators. Found in the Moog Etherwave Standard, the Burns B3 and in lots of cheap CMOS designs.

From an academic point of vue, it doesn't matter on which of both oscillators you act, since it is only their frequency difference which comes into account after the mixer. You also never adjust the behaviour of the field around the pitch rod, but only the audible consequence of influencing the field by your hand capacity. One might get the impression that "tuning" would also have an effect on linearity, but this is only a nice side effect which is due to the superposition of hyperbolic and logarithmic functions.

I'll stop here before this becomes a dissertation on function theory...

"You also never adjust the behaviour of the field around the pitch rod, but only the audible consequence of influencing the field by your hand capacity."

Ah, comes the dawn! So that pitch tuning dial works not on the field or on its generative circuit but on the signal emerging from the circuit associated with it.

Thanks to M. Thierry, "qui comprend sans effort le langage des fleurs et des choses muettes," not to mention the insides of theremins!

Bests,

Eliot

. . . i.e. the potentiometer varies the intensity of the signal that comes from the hand-influenced pitch "antenna" circuit. It comes AFTER your hand has done its work, and BEFORE the two signals (fixed and variable) are summed (or, more accurately, _differenced_) to yield the signal that eventually goes to the amp. Have I got that right?

[i]. . . i.e. the potentiometer varies the intensity of the signal that comes from the hand-influenced pitch "antenna" circuit. It comes AFTER your hand has done its work, and BEFORE the two signals (fixed and variable) are summed (or, more accurately, _differenced_) to yield the signal that eventually goes to the amp. Have I got that right?[/i]

Mais non, pas forcément!

The potentiometer just changes the off-load frequency of one of both oscillators. Lets see an example:

The potentiometer acts on the fixed oscillator and allows to vary its frequency from 170 to 172kHz. The variable oscillator runs on 171kHz and goes down to 168kHz when you approach your hand.

With the potentiometer on the 170kHz side you will hear 171-170=1kHz when your hand is far away. Your hand comes nearer and the pitch goes down to 0Hz (Zero beat) when your hand forces the variable osc. down to 170kHz. Your hand comes still nearer until the variable osc. is at 168kHz. The pitch will then go up from 0 to 2kHz. The playing range is reduced since the zero beat zone is inside the useful playing distance and you can only go up to 2kHz

With the potentiometer on the 172kHz side you will hear 172-171=1kHz when your hand is far away. Your hand comes nearer and the pitch goes steadily up to 172-168=4kHz when your hand comes nearer. This is the other extreme. Zero beat is far away and you will not be able to play frequencies below 1kHz.

"Tuning" would mean to adjust the potentiometer so that both oscillators frequencies were close beneath so that zero beat marks the beginning of the useful playing distance which gives you the maximum range (0 to 3kHz).

The pitch adjust is initially not intended to directly influence the playing range, but to compensate other effects: The environment (grounding, humidity etc.) influences also the variable oscillator by lowering its off-load frequency because it adds more or less capacity to the pitch antenna. Out of that, oscillators vary their frequency with time and temperature. The goal of adjusting (by whatever circuitry) is to come back to the ideal situation described in the paragraph before.

I hope that this is understandable - it is very difficult for me to explain complex stuff in English - a language which I do not really master (compared to French or German). I must use a dictionary in order to write such postings.

Thank you, Thierry, for all your effort on behalf of my my humble Anglophone self!

I'll study your last response. I think I'm beginning to understand . . .