Highpass Blackman Filter Design

Hello, I'm looking for some help designing a highpass filter that uses a blackman window and has a cutoff frequency at 0.001Hz for an input signal sampled at 1Hz. I've tried playing around with designfilt(), but can't seem to get the magnitude response quite right. Any suggestions?

Answer 1

[Star Strider]

There is no relevant documentation for designing with Blackman windows with fir1, or in my references.

This took a bit of experimentation to get to work:

Fs = 1;
Fn = Fs/2;                                  % Nyquist Frequency
Fc = 0.001;
N = 120;
N = 2*fix(N/2);
wndw = blackman(N+1);
b = fir1(N, Fc/Fn, 'high', wndw);

figure(1)
freqz(b, 1, 2^17, Fs)

The filter order ‘N’ must be even, so I included a line that enforces that. Longer filters are more likely to match your requirements, however they are less efficient.

[Craig Kelley]

Thanks a bunch.

[Star Strider]

My pleasure.

Answer 2

[Samuel Low]

if true
clc; clear all;
fs = 10000; % Sampling frequency in Hz
Wp = 2000; % Passband frequency in Hz
Ws = 2500; % Stop band frequency in Hz
f = [Wp,Ws];
fStop = 0.5*(Wp+Ws)/fs;
dev = [0.005,0.005];
a = [1,0];
[n,Wn,beta,ftype] = kaiserord(f,a,dev,fs);
% We use the order estimated in kaiserord
BMWindow = blackman(n+1)
b = fir1(n,fStop,'high',BMWindow) % Normalise the frequencies
freqz(b,1,512)
title('Highpass Filter Design')
end

I determined the filter length using the Kaiser Order function (which determines what my filter length should be) but you can just set any arbitrary N that you like, it just happens to have this requirement for one of my assignments and I was lazy to change it. I then defined a Blackman window using the 'blackman' function, and used this window function as one of the arguments in the variable 'b'.

Hope this helps.