MATLAB: How to get a magnitude respons with frequency at the x-axis

frequencyMATLAB

When I use the function freqz() I'll get a response with normalized frequency at the x-axis. Is there a way to get the freqency instead or optionally calculate it by hand?

Best Answer

Yes!

Supply the sampling frequency (here ‘Fs’) as an argument to freqz to get the plot in terms of actual frequency (from 0 Hz to the Nyquist frequency), where ‘NFFT’ is the length of the Fourier transform you want to use (a good choice being 2^16):

freqz(sos, NFFT, Fs)

or:

freqz(b, a, NFFT, Fs)

or others, depending on how you implemented your filter.

Related Solutions

MATLAB: How do i normalise frequency axis after computing fft

If you have a 50 Hz sampling frequency, the Nyquist frequency — in this instance the normalising frequency if you are designing a digital filter — is 25 Hz. So if you want a 15 Hz cutoff for a lowpass filter for instance, the normalised frequency, ‘Wn’ would be:

Fs = 50;                    % Sampling Frequency (Hz)
Fn = Fs/2;                  % Nyquist Frequency (Hz)
Fc = 15;                    % Cutoff Frequency (Hz)
Wn = Fc/Fn;                 % Normalised Cutoff Frequency (Dimensionless)

So Wn = 15/25 = 0.6

EDIT — For fft scaling, the best documentation is in the R2015a fft version. See specifically the code between the first two plot images.

MATLAB: How can i plot zero and its frequency responsE

The numerator should be

a = 0.0686

To find poles and zeros

[z,p,K] = tf2zp(a,c); % not tf2zp(c,a)

Add

b = conv([1 -1.08 0.3132],[1 -1.315 0.6182])
c =b;
a = 0.0686
[z,p,K] = tf2zp(a,c);
figure(1)
zplane(a,c)
xlabel('Re(z)');
ylabel('Im(z)');
title('POLE DIAGRAM OF THE TRANSFER FUNCTION H(z)')
w = -pi:(pi/500):pi;
H = freqz(a,c,w);
figure(2)
subplot(2,1,1); 
plot(w, abs(H),'r','LineWidth',2);
grid on;
xlabel('frequency in pi units');
ylabel('Magntiude');
title('MAGNITUDE FREQUENCY RESPONSE OF THE TRANSFER FUNCTION H(z)')
subplot(2,1,2);
plot(w, angle(H),'r', 'LineWidth',2);
grid on;
xlabel('frequency in pi units');
ylabel('phase');

Best Answer

Related Solutions

MATLAB: How do i normalise frequency axis after computing fft

MATLAB: How can i plot zero and its frequency responsE

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