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From: david on 1 Jul 2010 08:03
Hello, i want to implement a DC filter with Fdatool but i do not know how to do it?
the second question is how to do a notch filter with fdatool too?
From: astro mmi on 1 Jul 2010 09:04
Hi,
You can create a notch filter using fdatool using the bandstop option under which you can select notching and then give appropriate fs and design filter. I hope this helps.


"david " <int78005(a)stud.uni-stuttgart.de> wrote in message <i0i05o$1uv$1(a)fred.mathworks.com>...
> Hello, i want to implement a DC filter with Fdatool but i do not know how to do it?
> the second question is how to do a notch filter with fdatool too?
From: Wayne King on 1 Jul 2010 09:35
"david " <int78005(a)stud.uni-stuttgart.de> wrote in message <i0i05o$1uv$1(a)fred.mathworks.com>...
> Hello, i want to implement a DC filter with Fdatool but i do not know how to do it?
> the second question is how to do a notch filter with fdatool too?

Hi David, you won't be able to create a bandstop filter with fdatool around zero frequency, because fdatool doesn't allow you specify negative frequencies. If you want to filter out DC alone, you simply make your signal zero mean.

Otherwise, you CAN create a highpass filter using fdatool that passes everything above a few hertz. Let's say you design a highpass filter that passes everything above 2 hz. fdatool will return a filter with magnitude reponse [-Nyquist, -2] U [2,Nyquist] (U means union-- obviously it will be an approximation of an ideal filter with that support). However, keep in mind that your sampling frequency will not have to be that high at all before you will get a very expensive filter this way.

There is not a notch filter option on fdatool. Filter Design Toolbox has a command line function for notch filters called iirnotch that does a very nice job of creating notch filters but that will not work at zero frequency.

Again, I'm not sure why you cannot just make the signal zero mean to remove DC. Can you provide code that constructs a signal where that does not work?

Wayne
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