<p>The big big use of XOR GATES with square waves is a 4 gate pseudo ring mod. Thats typically what you find most people using chips like the CD4011 or CD4030 for at first. Its true you can do pulse width control or phased pulse width control, but in most cases I have found its a wicked gate to use for pseudo ring modulation without needing to go buy two CT coupling transformers and 4 germanium diodes (sorts sucks that basically no other diode will work).</p>
<p>Actually if you are wiring up the 2 waves straight to the 4 inputs on a CMOS XOR it is producing a ring mod effect.</p>
<p>A easy way to do pulse width modulation is to do this oscillator mod....Riding on the same lines your pitch control are is another pot. Center lug tied to inverted side (out) and the the two outside lugs have a diode in opposing directions that connect to input side.</p>
<p> The super awesome best way I have seen is a tiny circuit Synthmonger made using a 4007 and a 40106. It use the parts of the 40106 after the 4007 has been set the pulse width by  pot. Will need to send a diagram of it. It has insane control of the duty cycle.
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