9720 VCO 25
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toward the positive rail by R82 and R74. These two resistors, along with R75 scale and
transpose the comparator output to a 5V p-p (-2.5V to +2.5V) pulse at J12. Higher Control
Voltages cause the switch point to happen later on the ramp and consequently determine the
Duty Factor (the percentage of the total period of the waveform that the pulse high).
While fundamentally identical, there are minor differences in the A and B oscillators and wave
shapers to accommodate different features. OscB's Pulse shaper (IC5:B) has the added panel
control (R206) and the voltage at it's wiper is summed with the PW CV input at J10 to deter-
mine Duty Factor. Also in OscB, the opamp IC3:A and Q1 together form a linear voltage
controlled current source for timing capacitor (C16). The linear frequency control input at J7 if
useful for producing complex FM Synthesis timbres.
In OscA C17 and R9 couple external sync signal to the "+" input of comparator IC6:D. Sync
inputs periodically boost the ramp input to this comparator slightly and if it comes at the right
time to exceed the reference voltage the comparator changes state and discharges the timing
capacitor earlier than would be normal. Depending on sync signal amplitude and frequency the
premature ending of the ramp may not happen on every cycle producing complex waveforms.
Voltage Summing/Exponential Current Source
Like most exponential converters the 9720 circuits exploit the exponential relationship between
the voltage applied to a semiconductor junction and the current that flows through the junction
because of that voltage. Taking OscA as typical, the voltage across the b-e junction of Q3
produces a collector current that is exponentially related to voltage increases. When properly
calibrated, collector current will double for each 18mV increase in b-e voltage.
The V/I characteristics of a transistor are very temperature sensitive, with T appearing as an
exponent twice in the equation describing the relationship. As is common in these converters,
Q2 is hand selected to be matched in characteristics with Q3 and is added to cancel out the
largest effect of temperature.
In most other expo converters the remaining temperature dependency is reduced by using a
Temperature Compensating (tempco) resistor to vary the gain of a simple opamp circuit that
sums several control voltages and scales the result to the 18mV/oct needed at Q2's b-e
junction. The 9720 expos take a slightly different approach here and instead use the two
stages of an LM13700 type Operational Transconductance Amplifier (IC4) to perform this
summing and scaling function. In this scheme a diode ordinarily used to linearize the OTA
transfer function is configured instead to reduce the gain of the summation as temperature
increases. This topology has several advantages over the traditional tempco approach: Cost is
less because the somewhat exotic and consequently expensive tempco resistor is eliminated.
From a purely technical standpoint the diode is a better approach because it exactly compen-
sates temperature variations over a range of temperatures while a tempco resistor provides
exact compensation at only a single temperature because while resistance varies as T, the
actual term being compensated is 1/T.
The glide processor consists of IC7:C & :D. Changes in CV at J1 are buffered by IC7:C wired
Содержание 9720
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