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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