Voltage Controlled LFO

Posted on Feb 6, 2014

This is a nice LFO for all of your modulation needs. The frequency can be voltage controlled to add a new dimension to your sound palette. The pulse width of the rectangle wave can be varied between approximately 10% and 90% again lending variability to the modulation capabilities of this LFO. Voltage control can be used to vary the pulse width as

Voltage Controlled LFO
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well. If you power your circuits with +/-15 volts then observe the recommended changes that are noted on the schematic. I recommend a complete review of this page before buying a board. U1 and associated components comprise a linear voltage to log current converter. The current flowing into the collector of Q2 controls the current flowing from the bias input of U3-A transconductance amp to ground via Q3. The transconductance of U3-A is used to control the frequency of the oscillator. U2-A and associated components comprise an integrator. When current flows into U3-A`s output the integrator ramps up and when current flows out of U3-A`s output the integrator ramps down. When the integrator`s output goes above the threshold of comparator U4-A and associated components it`s output goes high (to near +V). The output of U4-A is fed to the non-inverting input of U3-A transconductance amp. While U4-A`s output is high current flows out of U3-A`s output and the integrator ramps down until the voltage at the input of U4-A goes low enough to overcome the hysteresis provided by R18 and it`s output goes low. When this happens the comparator starts to ramp up again and thus we have a triangle wave at the output of U2-A. The amp bias of U4-A is controlled by the current generated by the linear voltage to log current convertor. This controls the current that flows in and out of U4-A and thus the frequency of the oscillator. The...

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