[sdiy] Modulating a square wave

Scott Juskiw scott at tellun.com
Mon Mar 10 23:47:32 CET 2008

As others have already said, an analogue switch with a comparator will  
do this for you. I've got a circuit that does this:


On 10-Mar-08, at 2:34 PM, Justin Owen wrote:

> 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.

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