WP-20 MODULE DESCRIPTIONS
VCO (See figure 1)
R42 acts as a voltage level control and is
fed from S14 which selects either the 9 volt
supply or the LFO output. R40 performs a similar
function but with voltage selected from either
the A.D. generator or LFO. These voltages are
mixed via R17 and R18 and fed into the noninver-
ting input of buffer amp 1C3A.
IC3B, IC3C and associated components com-
prise the actual oscillator. 1C3B is used to
integrate the DC input control voltage and IC3C
is connected as a Schmitt Trigger. The output
of the Schmitt Trigger controls clamping tran-
sistor Q4.
When Q4 is not conducting, current flows
through R15 into the noninverting input of IC3B.
This causes the output voltage to ramp up until
the trip voltage for IC3C is reached. The out-
put of IC3C now goes high turning on transis-
tor Q4 which clamps the noninverting input of
IC3B to ground. The output of IC3B now ramps
down until the lower trip point of IC3C is
reached. This turns Q4 off and the cycle is
ready to repeat. The frequency of this wave-
form depends on the output voltage of IC3A. The
higher this voltage, the higher the resulting
current flow is through Rll and R15 causing the
cycles to occur faster.
The combined action of these components
produces a triangle waveform at the output of
IC3B and a squarewave of the same frequency at
the output of IC3C. Voltage divider R8 and R9
reduce the amplitude of the squarewave to match
that of the triangle wave.
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