VCO Calibration Aid
For those without access to precision voltmeters and digital frequency meters, calibrating VCO's to the exact 1V per octave ratio can be rather arbitrary. This circuit is an inexpensive way of making the job much easier.
TR1 supplies a constant current of about 0.9mA through the 8069 1.2V precision reference zener (available from Maplin YH39N). An exact 1V is obtained by the potentiometer R4/R5, which should be of best possible tolerance, and RV1 effectively slides this 1V difference between 0V and about 10V. Capacitors C1 and C2 simply suppress noise in the zeners. Voltage followers IC1a and IC1b ensure that the input loadings of the VCO's do not interfere with the rest of the circuit.
If only one VCO is available then the calibration must be done by ear, listening for an accurate octave jump; but the process is much easier with two VCO's. Set both S1 and S2 to the "low" position and tune the VCO's to the same frequency (by beat frequency or Lissajous techniques). Then set S1 to the "high" position and adjust VCO1's control voltage input gain preset to give a one octave difference — by beat or Lissajous — from VCO2's reference frequency. Go back to the "low" position, re-tune, and repeat the process until the octave is accurate. When using VCO's based on the CEM 3340 IC, RV1 can then be taken up to 5kHz or so to calibrate the upper end in a similar way.
The circuit can be made up on a small piece of Veroboard and will probably be able to use the synthesiser's own PSU; the ease with which one can then calibrate VCO's (especially in a polyphonic set-up) adequately repays the £5 or so it costs to build!
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