Tim Forcer wrote...

>You are not restricted to PICs with PWM outputs!  You can use a look-up
>table for a one-bit oversampled waveform just as easily as for multi-bit
>sampling.

The main advantage of using a software DDS approach (i.e., a sine
lookup table together with a PWM output or DAC) is that you can
get a sine wave which is both easily tunable--simply by varying
the phase increment register value--and capable, with appropriate
output filtering, of generating a high-quality sinusoidal output.
And in contrast to a tunable analog oscillator (a Wein bridge
arrangement, for example), the tuning is repeatable as well.

In one implementation I'm using a PIC16F877 and a 12-bit DAC to
generate a 0-10KHz sine wave, tunable in 0.002Hz increments with
THD around 0.1% over the range.

You wrote of using 1-bit oversampled binary coding, 64 samples
per cycle; I'd be interested in knowing how well this lends
itself to tuning over a wide frequency range while maintaining
spectral purity.

My guess would be that while generating fixed-frequency sine
waves with this technique is straightforward, generating sine
waves continuously tunable over a wide range is not; but I'm open
to being corrected on this point.

Dave D.

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