No PIC (that I'm aware of) has real analog outputs - you'll need to produ=
ce
a PWM output (on/off at varying duty cycles) at a significantly higher
frequency than the highest you need to generate, and filter it externally=
.

3 separate PWM modules are generally not found except in extremely high
pin-count PICs, so you'd probably want to combine the 3 sinewaves
internally and output them on a single PWM channel.  Fortunately, this is=

no problem - the standard PWM module has 10-bit resolution, so you could
simply add up to 4 8-bit signals.

An 18Fxxx PIC would be a good choice.  Its hardware multiplier is exactly=

what you need to adjust the amplitude of the individual sinewaves.

The sinewave itself comes from a table of precalculated values.  Normally=
,
a table covering a quarter cycle is used - you play it forwards, then
backwards, then forwards with the value negated, then backwards negated. =

However, a table covering a full sine cycle might be easier to work with,=

and any 18F PIC should have plenty of room to hold that at reasonable
resolution.

You'd have three pointers into that table, one for each sinewave.  At eac=
h
step, you'd grab the values from each pointer, multiply each by its
corresponding amplitude, add them together, and send to the PWM module. =

You'd then advance each pointer by an amount depending on the correspondi=
ng
frequency, wrapping around at the end of the table.  That's all there is =
to
it.
        Jason Harper

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