Bob Eld wrote:
"dave.harper" wrote in message
...
I'm trying to come up with a filter or converter that takes a square
wave input and outputs something closer to a sign wave (varying
between ~1.9kHz and 2.1kHz). It can have some distortion, but I'm
trying to eliminate the sharp leading and trailing edge. One option
I'm pursuing is a bandpass filter (2 caps and 2 resistors), which
looks to give a reasonable output, but still not quite as smooth as
I'd like. I've also considered using a counter feeding a bank of
resistors, but finding a method for it to start counting up with it
hits 0 and down when it hits the high value might be more difficult.

Originally, I had used a wein-bridge oscillator with op-amps to make
the sine wave, but due to the environment, EMI was a problem and it
damped out the oscillations (even with ferrite beads and modest
shielding). So I'd like to avoid using op-amps (since they are
apparently sensitive to EMI) if possible.

Any suggestions?

Thanks in advance!
Dave

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.




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--
Many thanks,

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