Sequential Pro One - OSC a Octave Switch

Figure 2-3
OSC A OCTAVE SWITCH
Pitch accuracy amid varying temperatures is the main challenge of VCO design. As it
hears and cools, the exponential control current generator charges the timing
capacitor more or less quickly, altering the oscillation period (pitch). The outstanding
stability
of the Pro-One's oscillators results from the inclusion in the 3340 of a
temperature compensation CTEMPCO) circuit which has the same temperature charac-
teristics as the exponential generator, but which adjusts the effective CV to
counteract drift in the exponential generator. This adjustment takes place in the
precision multiplier.
R128 adjusts OSC A SCALE by determining the current gain of the precision multiplier
through the temperature compensation (KO) circuitry. R122 and C103 compensate
the precision multiplier. R134 (at pin 14) converts the current output of the precision
multiplier into a drive voltage for the 3340's exponential current generator. R137 (at
pin 13) sets the reference current for the generator. d136/~~06 compensate the
generator.
C105 (pin 11) is the timing capacitor which iS' the main determininant of
oscillator frequency range. It 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 the oscillator in compensation for an
inherent high-end flatness resulting from error in the exponential generator.