High speed VCO. www.haraldswerk.de

Motivation

This is a high speed VCO build for use with my Walsh 32 Generator. It is build around the LM331. The circuitry is derived from the TI datasheet fig. 17. I added a V/Oct and a FM log input, Octave switch and fine tune as well. The LM331 is specified in the datasheet for a maximum frequency of 100kHz. I tested a few and all of them ran very well up to 200kHz. So, I decided to use it slightly out of spec and ran it with 120kHz. This gives you a upper limit for the Walsh 32 Generator output of 7,5kHz max. If you want to go higher, go for 140kHz which gives you about 8,75kHz max for the Walsh 32 Generator. This should be enough for audio use. This VCO is not intended for separate use. So it has no output on the front. The output is on the back and there directly connected to the Walsh 32 Generator.

Specs and features

Output frequency specified for 100kHz. Goes much higher. 200kHz measured.
V/Oct and FM log input
Octave switch
Fine tune
Power consumption below 25mA each rail

Implementation

Schematic

High speed VCO schematic main board — High speed VCO schematic: Main board

Description:

The main circuitry is derived from the TI datasheet fig. 17. Everything else around is pretty standard.

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Calibration

First make a gross test. You should have a signal at every output, pots and inputs reacting.

Adjust the voltage reference with TR2 on PCB 2.0.1-B for 10V

VCO calibration: Initial frequency
- Set the octave switch to max
- Fine potentiometer to center
- Measure frequency output
- Adjust TR_2 on PCB A for initial frequency 120kHz

VCO calibration: V/Octave
- The calibration procedure for the VCO is the same as described here: NGF VCO Core two
- TR1 on PCB 2.0.1-B is the V/Octave low end trim. TR3 on PCB 2.0.1-B is the V/Octave HF trim.
- Check initial frequency

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