Bob Eld wrote:


I would make a five pole elliptic low pass active filter breaking at about 3
kHz with a deep 80 dB notch at the third harmonic of 6 kHz. This is a dual
op amp (2 section) solution and uses a small number of resistors and caps.
You should be able to attenuate all of the harmonics at least 80 dB leaving
a pretty good sine wave.

Here is minimal solution:

http://www.abvolt.com/misc/square_to_sine.jpg

This filter makes for about 0.65% of THD; it is probably as good as it
could be done in reality with this number of components.


Vladimir Vassilevsky
DSP and Mixed Signal Design Consultant
http://www.abvolt.com