Unable to get Zero Phase shift after filtering PWM

Hi,
After filtering the PWM signal i'm not getting the exact shape of Sine wave, i'm using the Zerophaseshiftfilter.vi in the blockdiagram.I've also attached the resulting waveforms that i'm getting. Anybody help me out.
Thanks
sayaf
Attachments:
PWM- Sine.bmp ‏486 KB

sayaf,
Thanks for including the image -- it's a big help. Now I'm not the greatest expert at filtering, but to my eye you're getting a pretty reasonable approximation. I don't know how much better you can get when you only have about 8 PWM cycles for defining a half-cycle of the sine wave you want to see. At least, it looks pretty good if you have to handle a very general case of modulation.
Can the modulation always be considered a pure sine wave with a constant frequency throughout your sampling time? If so, then you can use that knowledge to your advantage -- for example, pass a few cycles of your filtered signal into one of the tone-extraction vi's that ship with LabVIEW. I'm sure there are other good (probably better) methods as well.
-Kevin P.

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I admit this is not 100% on mark but may give you some ideas. In this thread I suggested using a zero-phase shift filter. Ignore this post if it does not help.
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Message Edited by JimmyinCT2 on 01-23-2008 07:17 PM

dahhh, okay, i think i finally figured out what is going on here, and i think im just being somewhat dumb or maybe just assuming to much.
Looking at matts example helped a lot as i see why.
with one of the old VIs for measuring phase shift, we were getting a phase shift range of [-360, 360] or more.    We were just assuming that was the range here with this new VI.
With the new dual channel measurement (or the example you posted), when phase is measured, there is an inflection point at some angle of phase difference.. The way your example works (and the express vi) is with an inflection at 180 degrees. So, lets imagine we have a measurement of phase shift that is 178 degrees, as this difference increases, the next is 179, and then 180.. Now as it continues to move, the next phase measurement will not bet 181 but instead -179.   As the signal continues to move in the same direction, its then 178, 177, 176.. So there is an inflection point at 180 degrees with two sinusoids, in simple terms, its because the phase difference of one sinusoid to the other has shifted and is now being measured in reference to the next peak in the series.
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I think for now, we are going to have to work inside the range [-180,180] and initialize our hardware to 0 phase so we dont start near the inflection point and see if that works.
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i have a signal with phase,frequency n amplitude. To this signal i have added noise and i filtered that signal
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Attachments:
report(2).vi ‏89 KB

Unable to get Zero Phase shift after filtering PWM

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