On Mar 24, 3:56*pm, wrote:
On 24 Mar, 18:22, "dave.harper" wrote:





On Mar 23, 3:00*pm, John Larkin

wrote:
On Tue, 23 Mar 2010 12:39:59 -0700 (PDT), "dave.harper"

wrote:
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.

A lowpass filter is appropriate. You're trying to kill the higher
(odd) harmonics but there's no signal below the fundamental, excepting
DC maybe.

A dual-stage RC lowpass would be pretty good, if you set the -3 dB
point around 2 KHz. A higher-order filter, active or LC, would be even
better. An LC filter isn't unreasonable at 2 KHz.

John

I modeled a few low pass filters, and think this is the best
solution. *I have 3 RC low-pass filters and the output looks
relatively sine-like. *Thanks to everyone that provided good
suggestions! *Here's the schematic in LTSpice (comments appreciated):

An inductor really does make a big difference for these kind of jobs
and only costs pennies.
I've tacked on an LC filter to your drawing.
Output is *5Vpkpk sine at very low distortion.
(filter is a 3rd order, Chebyshev, low pass, with 6db passband ripple
and a Zero notch at 6kHz. Components rounded to preferred values)
[replacement for LTlist...]

That is a nicer output, but that also requires a 15mH coil. Wouldn't
that be a little big? This will be going into a small payload
container with 3" diameter.