Pwm & dac

[J. Mark Wolf]

Is the 1.0 to 20.0 Hz signal also a 50% duty cycle square wave? And would it need to be on at the same time as the other two signals?

Mike

<added>

Sorry! Just noticed you said 1.0 to 20.0 Hz signal was going to D/A. So, is that a sine wave output?

The 1Hz to 20Hz doesn't need to be a high-fidelity sine wave, but a reasonable facsimile (although I'll be using only the positive alternation). I've been experimenting with a 4-bit R2R ladder and filtering with surprisingly good results, and I think the DAC on the 16F1827 with a sine lookup table should be fine.

The higher freqs will be just audible tones, low-pass filtered to knock off the corners.

The positive alternations of the 1Hz - 20Hz will serve as an amplitude envelope for the higher freqs. I tried simply turning them on and off at a 1Hz - 20Hz rate, in a tone-burst fashion, but it produces an objectionable "popping" audio artifact. I think the envelope will solve that.

Your DDS approach is very interesting, but I wouldn't have a clue how to implement it, and I fear it would make a bigger project out of it. I'll keep it in the back of my mind.