From: jl@glen--canyon.com   
      
   On Fri, 13 Feb 2026 19:36:23 +0000, piglet    
   wrote:   
      
   >On 13/02/2026 3:40 pm, Christopher Howard wrote:   
   >> piglet  writes:   
   >>   
   >>> If high precision is not needed and only LF response is enough then I   
   >>> have used PWM techniques:   
   >>>   
   >>> Circuit A is the basic squaring concept, relies on modern open-drain   
   >>> output comparators being pretty good switches to ground. Assumes you   
   >>> already have a source of sawtooth or triangle waves with defined zero   
   >>> and peak values in the system.   
   >>>   
   >>   
   >> Could you please explain better how the PWM squarer circuit works? It   
   >> look like it would be easy to build, but I don't grasp what is going on.   
   >>   
   >   
   >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.   
   >   
   >piglet   
   >   
      
   You can do a pretty good voltage-to-PWM with a single opamp and a few   
   passives.   
      
      
   John Larkin   
   Highland Tech Glen Canyon Design Center   
   Lunatic Fringe Electronics   
      
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