Casio CZ-1 - Page 114

21-4.
VCO
(Voltage
Controlled Oscillator)
A-
IN
*
A-OUT
BIN
BOUT
The VCO
is an oscillator
whose oscillation
frequency
varies in accordance
with
the input voltage
level.
In
the left
figure, the voltage levels
of
the A-OUT
and
the
B-OUT
are
opposite.
n
i
When A-OUT
is
HIGH,
B-OUT
drops
to LOW.
(2)
From
A-OUT.
electric current
flows into
B
IN
via
a
differentiation
circuit.
As
a result,
the voltage of
B-IN
drops
gradurily
while
the A-IN voltage
gradually rises.
AIN
A-OUT
JUTTL
""LTLTLT
Threshold
"itvtl
CA
AOUT>H
h
7*
Thrtshold
Itvtl
CB
B
OUT>H
|-
A-IN
(3)
When B-IN
becomes
lower
than
the threshold
level,
B-OUT
rises
to
HIGH.
When A-IN
becomes
higher
than
the
threshold
level,
A-OUT
drops
to
LOW.
(4)
The circuit oscillates repeating
the above
operations.
The following shows the actual
circuit
of VCO.
When
control
terminal (a)
is
GND (zero
volt),
it
takes
approximately
1 5
microseconds
for
the
differentiation
circuit
to
reach the
threshold
voltage.