Osc A Octave Switch - Sequential Pro One Technical Manual

Figure 2-3
OSC A OCTAVE SWITCH
Pitch a c c u r a c y amid
varying temperatures is
the
main challenge of VCO design. As i t
hears
and
cools, t h e exponential control current generator charges t h e timing
capacitor more or
less quickly, altering t h e oscillation period (pitch). The outstanding
stability of t h e Pro-One's oscillators results from t h e inclusion in
the 3340 of
a
temperature compensation CTEMPCO) circuit
which has
t h e same temperature
charac-
teristics as t h e exponential
generator, but
which adjusts t h e effective CV to
counteract
drift in
the
exponential generator. This adjustment takes place in t h e
precision multiplier.
R128 adjusts
OSC
A
SCALE by determining t h e current gain of
t h e
precision multiplier
through t h e temperature compensation ( K O )
circuitry. R122 and C103 compensate
t h e
precision multiplier. R134 (at pin 14) converts t h e current output of t h e precision
multiplier into a drive voltage for t h e 3340's exponential current generator. R137 (at
pin
13)
sets
t h e
reference current for
the
generator. d 1 3 6 / ~ ~ 0 6 compensate t h e
generator.
C105
(pin 11) is t h e timing capacitor which iS' t h e main determininant of
oscillator frequency range. I t is a polysterene type for low leakage and best stability.
R 132 feeds-back a high-frequency tracking correction voltage output from pin 7 which
is trimmed by
R135
OSC A HI TRIM to
sharpen
t h e oscillator in compensation for a n
inherent high-end flatness resulting from error i n t h e exponential generator.