"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

One possibility is to use a digital filter. These have very sharp roll-offs
(e.g. 8th order for MAX292), you basically feed the clock of the filter with
a multiple of the fundamental frequency of the filter (say 64x), and the
output is a very pure sine wave. The beauty of this is the amplitude of the
output is independent of frequency and you don't have to change filter
components:
http://electronicdesign.com/content....nd-measurement

Another solution is using a multiplexor to create an approximate sine wave
using an 8x clock followed by a filter (the example below uses a pair of
cascaded 2nd order filters). This may suit you better:
http://www.maxim-ic.com/app-notes/index.mvp/id/21

Mark.