Calibrating and tuning a VCO

Let’s look at how to set up a VCO to track at 1 volt per octave (1 V/oct).

This will apply to many VCO designs but I’m thinking particularly of 3340 based designs such as the LMNC 1222 Performance Oscillator or my Hero and Sidekick VCOs.

The idea is to get the pitch to change by 1 octave when you change the CV by 1 volt — or to change by 2 octaves for a 2 volt change, et cetera. A 1 octave change is a doubling of the frequency, so you want the frequency to go up a factor of 2 for a 1 volt change, 4 for a 2 volt change, and so on.

You’ll need a way to send accurate voltages into the V/oct input. By “accurate” I mean preferably voltages that differ by, say, 2.000 V plus or minus about 0.002 V. 1.99 or 2.01 isn’t good enough. One easy way is to use a CV keyboard, or a MIDI keyboard with a MIDI to CV converter. It should give you voltages accurately 2.000 V apart when you play notes two octaves apart on the keyboard. Another easy way is if (as in the 1222 or Hero) your oscillator has an octave switch — one with an accurate voltage reference and resistors that have been matched at the 0.1% level. Then, again, you should get a voltage change of 2.000 V when you go up or down two steps on the switch. Use a multimeter on the V/oct input jack (if using a keyboard) or on the octave switch (if using that) to verify the voltage change is 2.000 V.

There are three trim pots that affect tuning: Tracking, Center Frequency (CF), and High Frequency Tracking. To start with we’ll ignore the HFT pot. Start with the lower voltage and adjust the center frequency pot, if necessary, to get a frequency of around 100–200 Hz. (Your oscilloscope may be able to measure frequency, or you may be able to find a tuner device or app that will read out frequency.) Now what you do not do is something like this:

• With the lower voltage, measure the frequency, say 100 Hz.
• With the higher voltage, measure the frequency, say 440 Hz. Too high! So:
• Turn the Tracking pot until the higher voltage produces 400 Hz.
• Go back to the lower voltage. Oops, now it gives 93 Hz. Too low! So:
• Turn the Tracking pot until the lower voltage produces 100 Hz.
• Go back to the higher voltage. It’s producing 440 Hz again! ARRRRGGGH.

Your mistake there is you’re trying to tune frequencies. Ignore the frequencies, and tune the frequency ratios. Instead of the above, do this:

• With the lower voltage, make a note of the frequency, call it f₁.
• Go up 2 V and make a note of the new frequency, call it g₁.
• Use a calculator to get the frequency ratio R₁ = g₁/f₁. Now forget about g₁ and f₁, it’s this ratio that’s important. What you want is for it to be 4.000. Let’s just say for example you get R₁ = 3.923.
• Now turn the Tracking pot by some amount — let’s say 1 turn clockwise.
• Go back to the lower voltage, make a note of the frequency, call it f₂.
• Go up 2 V and make a note of the new frequency, call it g₂.
• Calculate the frequency ratio R₂ = g₂/f₂. Let’s suppose you get R₂ = 3.964. That’s closer to 4.000 so clockwise was the right direction but you didn’t go far enough. So turn Tracking another turn clockwise. If on the other hand you got R₂ = 3.896, that would be further from 4.000, so clockwise was the wrong direction. So turn Tracking two turns counterclockwise.
• Go back to the lower voltage, make a note of the frequency, call it f₃.
• Go up 2 V and make a note of the new frequency, call it g₃.
• Calculate the frequency ratio R₃ = g₃/f₃. If you haven’t reached 4.000 yet, turn Tracking some more the same way. If you’ve passed 4.000, turn Tracking half a turn back the other way.
• Keep repeating measuring two frequencies, calculating their ratio, and adjusting Tracking one way if the ratio is too low, the other way if it is too high, until the ratio is 4.000.

Next, you get to do all that over again, sort of. But with two differences:

• Either use higher notes on the keyboard, or re-adjust the center frequency trimmer, so you’re a couple octaves higher — up around 1000 Hz.
• Instead of adjusting the Tracking pot to get the ratio right, adjust the HFT.

Otherwise it’s exactly like before. Check the frequency ratio, tweak HFT, check the ratio again, and so on.

Once you get the ratio to 4.000 at the higher frequency range, go back to the 100-ish Hz range and check the ratio there. If it’s no longer 4.000, tweak the Tracking pot just like before until it is; then go back around 1000 Hz and check the ratio there and use the HFT pot to correct the ratio if needed. Go back and forth between low frequency / Tracking and high frequency / HFT until the ratio is 4.000 for both.

In reality you might have to settle for frequency ratios that aren’t quite exactly right, and it’s likely you won’t have to go back and forth on the frequency ranges more than once. Part of that is using your ears and deciding if it sounds close enough to you.

Finally, once you have low and high frequency tracking working properly, use the CF trimmer to set the frequency range as desired. There is unfortunately no standard for this — a change of 1 V is a change of 1 octave, but the actual octave for a particular CV, say 0 V, isn’t standardized. Some CV keyboards and MIDI to CV converters put out 0 V for C₄ (middle C), some put out 0 V for C₁ (the lowest C on the piano keyboard) or C₀ (an octave lower than that)… or maybe something else. So just set your fine tune knob in the middle, turn the octave switch or coarse tune pot to the middle, and adjust CF until the the middle C key gives middle C (261.6 Hz) or… something. Whatever works for you. And now you’re done.