i have declared an FIR filter as fb as given below. Now, i want to find its Fourier transform as FB(jw) with respect to frequency w (omega). How can i do that? I dont think its the same as taking DFT.

fb=fir1(50,[0.2500 0.8750]); %FIR filter fb

Answer 1

[Star Strider]

Use the freqz function:

fb=fir1(50,[0.2500 0.8750]); %FIR filter fb

figure(1)
freqz(fb, 1, 65536)

[Saiankita Anand]

Hi Star Strider. That was pretty quick :) freqz(fb, 1, 65536) --> what does 65536 mean here? I meant, why specifically 65536 here? is there a significance?

[Star Strider]

The 65536 is just the length of the fft, here designed to show all the detail in your transfer function. Any power of 2 will work most efficiently (powers of 2 because of the way the fft is calculated). So using 2^N as a length, where ‘N’ is any integer above 10 will give you good resolution.

See the documentation on freqz for all the options, including (in more recent MATLAB releases) of scaling the frequency to the Nyquist frequency of the signal you are filtering, instead of [0,pi].

[Saiankita Anand]

Thank you :) I saw the documentation as you suggested.

[Star Strider]

My pleasure!

You will find that FIR filters will generally require higher than the default resolution to show all their details. I usually choose ‘N’ to be between 14 and 16 for them.

[Saiankita Anand]

gotcha. I have just started with Matlab some time back. Girls in coding are far and few in between around me. want to be as good in Matlab as you are :) Any suggestions?

[Star Strider]

Hang out here!

Then start Answering Questions. It will definitely build your knowledge and confidence with MATLAB.

Thank you! I learned more about MATLAB from hanging around MATLAB Answers for the last three years (total) than in the 20 years I used MATLAB before that (or so it seems). It’s quite intellectually challenging.

I don’t know where you are geographically, but if you’re near the University of Missouri Rolla campus, talk with Dr. Cheryl Schrader. She introduced me to MATLAB, and is the best control system professor you’ll ever have the privilege to study with. One woman who is relatively active here on MATLAB answers who are truly excellent is Kelly Kearney. She has a number of File Exchange contributions you could look through and learn from. I mention women here because women face different problems (and some discrimination) in the sciences, as much as we might want to deny it, so those who are successful at dealing with it are those it would be best to consult with. (FWIW, I’m male, so my perspectives in this regard are limited.)