Non symmetrical filter fda tool

Hi,

I have designed a Notch Filter with the fdatool. The filter is symmetrical to the frequency 0. I would like to have a non symmetrical filter, because I just want to fade out a part of the positive frequencies.

Has someone an idea how to realize this?

Best regards

Answer 1

[Star Strider]

Not really certain I understand what you want to do, but if you want different filter characteristics on each side of the stopband, consider a lowpass filter with one set of parameters, and a highpass filter with a different set. Then set the cutoff frequencies to give you the stopband performance you want. (Instead of a highpass filter, I suggest a bandpass filter with an appropriate high-frequency cutoff so you can filter out powerline interference and other noise at the high end without compromising the notch characteristics.)

[Star Strider]

My pleasure!

Actually, ‘negative frequencies’ don’t actually exist. (Their appearance are due to the symmetry properties of complex integral transforms that create complex conjugates.) Simply concentrate on the positive frequencies and the ‘negative’ frequencies will take care of themselves. (They will be mirror-images of the ‘positive’ frequencies.)

[Star Strider]

I have no recent experience with ODFM (one digital communication class long ago), and have never attempted to do what you are doing. (I just now did an Internet search and found only two conference papers on ODFM digital filtering that I didn’t want to pay to download. As conference papers, they likely did not go into any detail anyway.) I am still not certain that what you want to do is possible, since my search did not reveal any method to do negative-frequency filtering on any type of signal.

Digital communication is definitely not an area of my expertise. That said, if you are filtering a baseband signal, one way around the problem could be to use it to modulate a constant carrier to create completely positive frequencies and then filter each sideband with your filter. Then demodulate to baseband. That is the only way I can think of to do it.

[Star Strider]

My pleasure!