From: jl@glen--canyon.com
On Fri, 13 Feb 2026 13:00:55 -0500, bitrex wrote:
>On 2/13/2026 11:34 AM, john larkin wrote:
>> On Fri, 13 Feb 2026 06:40:05 -0900, 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.
>>
>> A fairly simple single-opamp or comparator circuit can convert input V
>> to a duty cycle n. A transistor or analog switch or even a diode can
>> multiply V * n. Lowpass filter that and you have V^2.
>>
>>
>> John Larkin
>> Highland Tech Glen Canyon Design Center
>> Lunatic Fringe Electronics
>
>It has the advantage that the analog switch and passive low pass filter
>don't affect the DC performance, so long as you can afford the settling
>time of the filter.
>
>The hassle of trying to kludge cheap parts into decent multipliers,
>particularly ones with decent DC performance, and keep the signal path
>totally analog is that every cheap op amp and resistor you add to try to
>correct for the deficiency of all the other cheap parts tends to add
>noise and offset and soon enough you lose more than you gain.
The PWM thing I suggested should be pretty good. But we don't have
firm requirements for range, accuracy, or speed.
Cheap opamps can have sub-millivolt offsets, and 1% resistors cost
less than a penny nowadays. We don't even buy 5% resistors.
My PWM circuit doesn't much care about resistor tolerances.
John Larkin
Highland Tech Glen Canyon Design Center
Lunatic Fringe Electronics
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