DOEPFER A-178 Podręcznik - Strona 3

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3. Basic principles

The theremin acts as one plate of a capacitor and a

human body as the other plate.

towards and away from the antenna produces tiny

changes in capacitance (so tiny that they're measured

in the picoFarad range). The electronics within the

theremin measure this change in the following way:-

The signal from an oscillator (whose frequency can be

subtly adjusted with the offset control 1) is compared

with the signal in another oscillating circuit, whose

capacitor is made up of the antenna and an external

object like the human body. If the capacitance is

changed (by, e.g., moving your hand) the circuit's

resonant frequency changes. This is measured, and

converted into a control voltage.

Depending on the oscillator frequency, we may hit

either the rising or the falling edge of the oscillating

circuit's resonance - and thus produce respectively

either rising or falling voltage as the hand gets closer

to the antenna. The module is factory adjusted so that

as the hand gets closer to the antenna, the voltage

rises. This can be reversed if required, though.

System A - 100

Moving the hand

4. Controls

1 Offset

Control 1 is used to adjust the null point, so that the

control voltage at output " is at 0 V, when the hand is

some distance away from the antenna (more than

about 30 cm).

Because this module, like all theremins, is

very sensitive to fluctuations in humidity,

temperature changes, etc., it's necessary to

check adjustment each time you use it.

In addition to the offset control on the front

panel, there's also an internal trim inductor

for

coarse adjustment of this offset fre-

quency - see appendix on p. 8.

Using this trim pot for coarse adjustment may

be necessary if, for instance, you install

another antenna, and it's then no longer

possible to set the null point with the front

panel offset control; or you want to use the

falling edge of the circuit resonance, so that

the control voltage generated gets lower as

you approach the antenna.

A-178

Theremin