Pseudo-random LFO

Sometimes there’s a need to get random voltages, you can do that with a S&H and a noise source (see this previous post), or you could do it with a pseudo-random voltage source. This is a pseudo-random control voltage source that uses four square wave generators, of one with controllable frequency, and mixes these four signals to form one moderately random pulse wave at the output. This illustration shows the mix of four square waves with different frequency.

Summing square waves. From Digital Generation of LFO’s for Modulating Effects. Copyright 2000 R.G. Keen

My design of a pseudo-random LFO is inspired by the design by R.G. Keen as well as a of Ken Stone’s design, and there are probably many other similar designs out there. It has four controls (Coarse rate and fine rate, Range of the pseudo-random CV (1-10V), and Slew (stepped CV or smooth CV), it also has two outputs (pseudo-random CV out and pseudo-random clock out (positive going clock pulses at +5V)).

pseudo-random-lfo

Pseudo-random LFO

The module uses +5V, +12V, GND, -12V, and needs a +5V option installed in the modular system, or have a +5V voltage regulator (e.g. 7805…). It would also be possible to use a HEF40106B Hex inverting Schmitt trigger instead of a 74HC14, and this would remove the need of +5V.

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