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A Voltage Controlled Oscillator (VCO) is one of the basic building blocks of a synthesizer. A VCO produces alternating outputs in several waveforms with the frequency (tone) controlled by an input voltage. The higher the input voltage, the higher the output frequency. The input voltage might come from a manual source like a knob, from an instrument like a keyboard, from another synth module, or some esoteric control surface.

This CEfx VCO module is based on the Coolaudio V3340M Voltage-Controlled Oscillator. It features standard 1V-per-octave tuning, and has three simultaneous outputs: triangle, ramp, and pulse, with a manual- or voltage-controlled pulse width. Outputs all range from 0V to +4V. We expect to offer a companion triangle-to-sine converter module soon. Note that outputs are not buffered. Output impedance is 10K for triangle out, 5K for ramp out, and 3.6K for pulse out.

Features

• Based on the Coolaudio V3340 VCO chip
• Triangle, ramp, and pluse outputs
• Two summed CV inputs plus on-board frequency offset pot
• Socketed frequency range capacitor can be swapped out to create a LFO
• FM input

More CEfx Information

Usage Notes

For voltage controlled pulse-width modulation, remove the jumper labeled PWM and inject a control voltage between 0V and +5V into the testpoint closest to trim pot R11.

C23 (the socketed capacitor) determines the frequency range of the VCO. The standard CEfx-VCO uses a 100pF capacitor here to set the base frequency range (adjusting FREQ offset with no external CV) to roughly 8 Hz to 220 Hz. You can easily turn the module into an LFO by replacing C23 with a higher-value capacitor. 0.1uF is a good choice for an LFO. Use a film capacitor here, not a ceramic, for frequency stability.

The FM input is DC-blocked, and accepts CV signals above 1 Hz and up to +/-10V, or audio at frequencies up to the VCO's output frequency.

Calibration Process

1. Apply a calibrated source of 1.000V to CV1 in.
2. Read pulse output on a frequency counter.
3. Adjust manual offset until frequency counter reads 200Hz.
4. Change voltage source to 2.000V.
5. Frequency should read 400 Hz. If it's low, turn "8va TUNE" clockwise. If it's high, turn counterclockwise.
6. Change voltage source back to 1.000V and readjust manual offset until frequency reads 200 Hz.
7. Continue toggling between 1V and 2V, adjusting the manual offset and the 8va tune until you get to 200 and 400 Hz.
8. Change voltage source to 3.00 V and verify frequency of 800 Hz. It may be necessary to tweak the 8va TUNE pot for a more accurate reading. As you check higher octaves, you will need to adjust the "HV TRACK" pot for the higher octaves, then go back and re-measure the lower octaves, then re-tweak the 8va TUNE pot.

Design Notes

• A few minor tweaks were made for +/-12V operation and we adjusted the output voltage dividers so that all three waveforms would go from 0V to +4V.
• A hysteresis resistor between pins 4 and 5: 2.5 MOhm was the value needed to stop square wave glitches
• Outputs are not buffered (trying to keep things simple), but the result is fairly high output impedances, so watch out for that (cannot drive a lot of load).
• This design does not allow for hard or soft sync inputs, in the name of simplicity, but if there's enough interest, we might try to add some hooks in a future rev.
• Using a breadboard friendly audio jack makes it easier to receive CV signals from an external controller (such as a keyboard that has CV outputs).
• Some ideas for enhancing this on your breadboard:
  • Individual op-amp buffers on all outputs
  • Triangle-sine converter

Schematic

Download Schematic PDF