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#10
March 25th 10, 09:02 PM
posted to sci.electronics.design,rec.radio.amateur.homebrew
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Square wave to psuedo-sine wave?
On 25 Mar, 20:26, "dave.harper" wrote:
On Mar 25, 10:57*am, wrote: On 25 Mar, 02:37, "dave.harper" wrote: 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. They're about the size of 0.4W resistors but if you're stuck for space then it's not an option. I had in mind this ... 'Epcos' part# *B82144A2156J000 A pic of *a similar item ...http://uk.farnell.com/epcos/b78108s1...-4700uh/dp/608... Or a different body style but more expensive ...http://uk.farnell.com/murata-power-s...ductor-15mh/dp... Hm, what about this one? *Looks promising: http://search.digikey.com/scripts/Dk...tail&name=811-.... Yep. Fine!. (DigiKey 811-1302-ND for the 15mH) |
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