Hi,
I've come across an interesting conundrum. I'm using the tone() function to, of course, generate a tone. I've also made a 2nd order Sallen-Key filter to clean up the harmonics a bit and get a bit closer to a sinusoidal wave. I've tested the filter against a square wave generator and it seemed to do the trick very well, however when I hook the filter up to the arduino tone generator, the filter output still remains a square. The cutoff freq is set to about 50 Hz and the testing frequency is 200 Hz. All Caps are 0.22 uF and Resistors are 15k. Am I missing a key piece of knowledge in the arduino's tone generation?