3 VCO
Card
3.1
Theory of Operation
The Voltage Controlled Oscillator Card (VCO CARD) consists of four
oscillator circuits based on the CEM 3340. The system uses four
identical cards, two for the LEFT side of the instrument and two for the
RIGHT side of the instrument. The VCOs on the card correspond to
VCO1 and VCO2 for two voices. Thus, the left and right side of the card
are identical. The cards may be interchanged to facilitate troubleshooting.
The functions on this board include:
•
Four VCO circuits each with Voltage Controlled Frequency,
Voltage Controlled Pulse Width and Voltage Controlled Tracking.
•
Waveform
mixers
for each oscillator
•
-5V reference circuit
•
VCO control voltage demultiplexers
3.2 VCO
Operation
The CEM 3340 is an integrated oscillator IC that generates sawtooth,
triangle and variable width pulse waves. The output of the sawtooth wave
is differentiated and sent to a CD4520 divider IC to generate a square
wave that is one octave below the oscillator frequency. The waveforms
are summed by an LF347 wide bandwidth op amp. Since this summer is
inverted, it is followed by an inverter to form positive waveform outputs
that are routed to the VCF card.
The triangle and sawtooth waveforms are routed through CD4053 analog
switches which are turned on or off by the DAC CARD. The pulse wave
is turned off by the CPU by adjusting the control voltage that varies the
pulse width to the 100% width value. This causes the pulse width to go
high, which is inverted by a transistor that feeds the mixer. The sub-
octave is turned off by the DAC when it toggles the rest line on the
CD4520 divider.
SYNC and LINEAR FM are also turned off/on by the DAC driving a
CD4053 analog switch.
Oscillator frequency is determined by the CPU tracking control voltage,
CPU initial pitch control voltage, modulation input control voltage (from
VCF) and pitch bend control voltage (from ANAMOD 2.) These voltages
are summed directly onto the CEM3340 via 1% scaling resistors. Note
that earlier model VCO cards use “blue” 1%, 100K resistor packs for this
