[sdiy] Modulating a square wave

Justin Owen juzowen at googlemail.com
Mon Mar 10 21:34:38 CET 2008

Hey all,

I could do with some advice on a particular way of modulating a square-wave.

A 'standard' square-wave goes up and down between, say, 5 volts (mark) to 0 volts (space). I'd like a square-wave where I had independent control over the voltages/amplitude of (ideally) both the 'mark' and 'space' - so I could have, say, a square-wave that was made up of between 5 volts (mark) and 2 volts (space) or 4 Volts and 1 Volt or 1 Volt and 5 volts - or any other combo.

So instead of adjusting the width - you're adjusting it's 'height'.

At this stage it would be for control voltage purposes (e.g. to control the output of a VCA) - so the frequency wouldn't have to be super-fast/high. I'd also be happy with having control over just the 'space' (i.e the part of the square-wave that is normally at 0 Volts) and in a pinch I'd accept 'stepped' control rather than linear if that was going to be much easier.

I figured I could use a 'normal' square-wave to switch between two different 'reference' voltages (via a CMOS switch or a Multiplexer?) or I could use a comparator and drop the High Voltage and boost the Low one - but I was hoping there might be a simpler/more established way.

(oh - I posted this earlier today in Rich text - so it didn't go through - apologies if this turns up twice)

Many thanks,


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