VCO calibrator

Bob Moog left many inventions, not the least of which is the 1 volt-per-octave standard. This is a convenient way to represent musical pitches with electrical voltage, and is used almost universally in professional analog gear.

When calibrating an analog Voltage Controlled Oscillator, there are usually two trim adjustments that must be adjusted accurately before the VCO will perform in tune. This can be a painstaking chore as the two trims are interactive and require iterative adjustment.

I designed a simple circuit that allows VCOs to be tuned by artisanal methods. The circuit puts out a stepped voltage in precise 1.00V increments. Musically, this corresponds to intervals of one octave. The circuit can be set for a 4- or 8-octave range.

Thus the VCO can be tweaked by ear without need for instrumentation. Simply listen to the output and adjust for the truest octave relationship between all the notes in the sequence.

VCO calibrator

Note that in order for the CD4051 to work properly with analog signals ranging from 0 to +12V, pin 7 must be connected to -5V.  pin 8 goes to GND.

In the video, the perfboard circuit is the VCO calibration aid.  The green PCB is a CMOS 4069 VCO, modified slightly and built as a dual version using one 4069.

As you can hear, this dual VCO exhibits decent tracking over almost an 8 octave range.  Not excellent, maybe not even good by some standards.  However, most people enjoy the sound of two slightly detuned oscillators.