> 40 KHz

That helps.

> It needs to vary one side to the other of that 40 KHz just for tweaking
> purposes

That's a bunch of meaningless weasel words.  What does "tweaking" mean,
+-1%, 10%, 100%?  What resolution?

>>> How pure does the sine wave have to be?
>
> Not necessarily pristine, but the cleaner the better, probably.  At
> least to the point of diminishing returns.

More qualitative weasel words that are useless in a specification.

Let's say you need 40KHz +-1% with no more than 5% distortion.  The PIC
should be able to generate a square wave to match the frequency specs.
Now you need to convert that to a sine wave, which is pretty easy to do
with analog electronics.

The frequency content of a square wave is the fundamental with rather
strong odd harmonics.  That means the lowest frequency that must be
filtered out it 3 times higher than the one to pass.  I don't remember off
hand the amplitude of the 3rd harmonic, but let's say it's equal to the
fundamental and ignore the remaining harmonics just to get a ballpark
estimate.  You therefore need to attenuate the 3rd harmonic 20x compared
to the fundamental.  A perfect integrator will do a relative attenuation
of 3x, so at least 3 such stages are required.  This means a 3 pole R/C
low pass filter could do it, but 4 poles would allow the rolloff frequency
to be higher and the resulting in a little less amplitude variation as the
frequency is tweaked.

So, something as simple as 4 resistors and 4 capacitors could do the job
within your very loose "specification".  A better answer requires a better
specification.


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