design low pass filter with Matlab.

[triquent]

I was asked to design a low pass filter that would attenuate the aliased frequencies by at least 60dB. I am using the fir1(n, wn) function. But no matter how I adjust the order n and corner/cutoff frequency wn, I can not make the attenuation less than 60dB, the highest point of the stop-band ripple is always around -52dB. The lowest point of the ripple is about -80dB. I am wondering if I should look at the middle of the stop-band ripple, then the attenuation is larger than 60dB compared to the pass band 0dB. Am I right? Or how to change the order and corner frequency to make the attenuation larger than 60dB?

 

[vadkudr]

For proffesional design of FIR filter you should use "qremez" function (uses optimization technique)
If you don't care about linear phase, you can try to design IIR filter by "iirlpnorm" function (uses optimization technique)

 

[claudiocamera]

When you use fir1(n,wn) you are not specifing the window, so Matlabs uses the default window wich is the Hamming window. It Happens that Hamming window can get the maximum stopband attenuation of 53 dB, in Order to get more than 53 db of attenuation you shoud use ,for instance, the blackman window which provides an attenuation up 75 dB or you can use either the Kaiser window which can provide an attenuation from 50 to 90 dB, the kaiser window is more flexible because it incorpore the ripple parameter control.

In order to use these windows and get bigger attenuation you should use fir1(n,wn,'window') The vector window must be n+1 elements long and constructed by window approprieted command, refer to Matlab Help.

The remez command as quoted by our pal vadkudr offer more flexible design than a filter that uses window method ( fir1()) , and its use should be strongly considered.

 

[Circuit_seller]

Hi

Try using "fdatool" in MATLAB, which is a GUI filter design software.

Regards

 

[shetoos_asu_65]

hi
i really think that "fdatool" can satisfy that
bye