Oops.

Vern's comment drew my interest, and I took another look at the
circuit.  I didn't see capacitor C10 before, and now it looks
like the designer isn't after the DC component of the square wave
after all, but instead is using the PWM output to generate audio
by modulating the PWM duty cycle.

The low-pass filter between the PWM is still doing what I
suggested below, though: removing the PWM fundamental frequency
and its harmonics from the waveform, leaving only lower
frequencies in the output.

Sorry for the mixup.

DD

I previously wrote...

>Lindy wrote...
>
>>I remember LPF's and HPF's from studying about synthesizers.
>>Like, for instance, passing a noise through it and getting
>>a cymbal or drum sound.
>>
>>But you mention DC components.  I don't understand that.  What
>>does that have to do with the DC PWM signal?  Where was the AC
>>in the mix?
>
>Say I have a PIC with a PWM output that's running at 1 kHz with a
>50% duty cycle; it produces a square-wave output which alternates
>between zero volts (logic "0", or ground) and 5 volts (logic
>"1"), spending half its time in each state.
>
>That square wave consists of a 2.5 volt DC component plus a 1 kHz
>sine wave of 5 volts p-p amplitude, plus an infinite series of
>the odd harmonics of 1 kHz with amplitude inversely proportional
>to frequency.  Any of these components of the square wave-- as
>well as the DC average value-- can by isolated by an appropriate
>filter.
>
>>And most importantly, why or what about the PWM signal made it
>>necessary to low-pass-filter it?
>
>It looks to me like the PWM is being used as a digital-to-analog
>converter, where the analog output consists of the average value
>of the PWM's pulse train output.  The PWM output is a square
>wave, and the filter gets rid of everything but the DC part of
>it.
>
>>What where they trying to get by it?
>
>A smooth DC output voltage with little high-frequency "hash" or
>ripple on it, that can be adjusted simply by loading different
>numbers into the PIC's PWM registers.
>
>Hope this clarifies instead of obscures.  What might help is to
>just try it on the nearest handy PIC: put a single-stage RC
>filter on the PWM output (try a 10K resistor and a 10uF
>capacitor, for starters), stick your scope probe on the filter
>output and try adjusting the PWM register values.  See what you
>get.
>
>Dave D.

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