24 9720 VCO
010321
Oscillators
- The two VCOs in the 9720 have essentially identical core elements. Taking OscA
as typical, IC7:A, IC6:D and Q10 are the active elements in a relaxation oscillator topology.
C11 is the timing capacitor which is charged by a current source to be described shortly. As
C11 charges a linear voltage ramp is produced across the capacitor. JFET input opamp IC7:A
is configured as a voltage follower that buffers the voltage on the capacitor from the imped-
ances of other processing elements.
One of these processing elements is a Schmitt trigger comprising comparator IC6:D and
associated components. When the Voltage ramp from buffer, coupled by R19, passes a
threshold of about -4V the Schmitt trigger changes state and it's output is pulled from the
negative supply rail up to ground by R70. The resulting current flow through R69 turns on Q10
which quickly discharges C11 through R88. When the capacitor has discharged to the point
that the voltage at the output of the buffer is less than -8.5V, IC6:D's output is again pulled to
the negative supply voltage and Q10 turns off. With Q10 no longer conducting C11 begins
charging again and the cycle repeats. Diodes D9 and D10 limit base-emitter junction satura-
tion and capacitor C10 helps in discharging the parasitic capacitor at the b-e junction, both in
the interest of speeding up the switching time Q10. C11 discharges in less than a µSec.
Opamp stage IC7:B and associated resistors comprise a buffer that both scales and trans-
poses the relaxation oscillator's ramp to a 5V p-p (-2.5V to +2.5V) output available at J11.
The ramp at the output if IC7:B also drives the Triangle wave shaper that has opamp IC8:B as
it's active element. As the ramp increases toward ground from its minimum of -6.5V, diodes
D15 and D16 are both reverse biased and the opamp acts as a unity gain inverter with it's
output decreasing from +2.5V. The output continues decreasing until the voltage of the ramp
reaches the voltage at the opamp "+" input (Set by trimmer R87) then both diodes begin to
conduct and the opamp stops being an inverter and turns into a voltage follower. It's output
stops decreasing and begins to increase. The output continues to increase until the ramp
reaches it's peak (+3V) and resets back to minimum, then the diodes once again reverse bias
and the opamp switches back to an inverter with it's output high. Setting Sym trimmer R87 sets
the inflection point and is adjusted so that the triangle shaper output voltage is the same at the
end of the ramp as it was at the beginning. Capacitors C18 and C21 add a high frequency roll-
off to the triangle shaper that minimizes the "glitch" in the triangle waveform that is a natural
consequence of the ramp resetting from high back to low.
The triangle output of IC8:B is shaped to approximate a sine wave by opamp IC8:A and diodes
D11-D14, D17 and D18. As the amplitude of the triangle rises and falls the diodes forward and
reverse bias to produce lower gains at higher input voltages causing the triangle peaks to bend
over into a sine wave.
The ramp from IC7:B also drives the Pulse Width modulator consisting of comparator IC6:C
and the parts around it. After being scaled and transposed by R71 and R72 the ramp is
compared to a CV from J3 that is scaled and transposed by R76 and R77. When the represen-
tation of the ramp voltage at the comparator's "+" input is greater than the control voltage at the
"-" input the comparator changes state and it's output is pulled from the negative supply rail
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