From: erichpwagner@hotmail.com
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
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