From: christopher@librehacker.com   
      
   > JL has beaten me to it. In full detail: imagine the sawtooth or   
   > triangle wave swings between 0V and 1V and is nicely linear, i.e.   
   > straight sloped. The comparator in my circuit (A) compares that with   
   > the input voltage to be squared Vx. When Vx is 0 the output is always   
   > low; when Vx is 1V the output is always high; and when Vx is 0.5V the   
   > output is toggling with 50% duty cycle.  This pulse width modulation   
   > can then be used to switch the desired output between input Vx and 0V   
   > so when Vx is 0V then output is zero, when Vx is 1V the output is Vx   
   > and when Vx is 0.5V the output is 50% of the time that 0.5V and 50% of   
   > the time zero or in other words an average of 0.25V. The low pass   
   > filter averages the pulsating output to a steady level.   
   >   
   > In circuit (A) the switching isn't done by an explicit analog switch   
   > but by shorting the output to ground - very conveniently done by   
   > choosing a comparator with open-drain output.   
   >   
   > I had multiple square and root channels so it made sense to have one   
   > common triwave generator serve them all. Instead of 1V full scale   
   > unity I scaled to use 5V so ordinary comparators can be used. If you   
   > had no other need for a sawtooth/triwave elsewhere one could build a   
   > self-oscillating PWM generator and use that to drive the chopper. Let   
   > me know if you want that detailed more fully.   
   >   
      
   Okay, thank you. I think I grasp the core idea now.   
      
   My THAT comes with a few comparators, but already less than I need for   
   other things, so I don't want to use those. But I've got a lot of 741 op   
   amps to spare.   
      
   I do have access to a good benchtop signal generator, which I could use   
   for generating the triangle wave. But having a separate triangle wave   
   generator built in to the project also has some appeal.   
      
   --   
   Christopher Howard   
      
   --- SoupGate-Win32 v1.05   
    * Origin: you cannot sedate... all the things you hate (1:229/2)