strange behavior of plot and freqz
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Hello,
Does anyone knows why the freqz and the plot function display diffrent things?Shouldnt be the same?
For example in the first case everything seems fine but in the second the phase doesnt look the same.
This is my code:
close all;
clear all;
w = -pi:pi/128:pi;
z=tf('z');
G=(0.2*z/(z+0.2))*(1/(z-0.9));
[num,den]=tfdata(G,'v');
figure(1);
freqz(num,den,w);
[h, freq] = freqz(num, den, w);
phase = angle(h);
figure(2);
plot(freq, phase);
clear all;
w = -pi:pi/128:pi;
z=tf('z');
G=(0.2*z/(z+0.2))*(1/(z-0.9))*(1/(z-1));
[num,den]=tfdata(G,'v');
figure(3);
freqz(num,den,w);
[h, freq] = freqz(num, den, w);
phase = angle(h);
figure(4);
plot(freq, phase);
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Accepted Answer
Star Strider
on 4 Nov 2023
Thje angle functon returns angles in radians. If you add a call to the rad2deg function, the results will be in degrees, and the plots should look similar. It might also be necessary to use the unwrap function first.
w = -pi:pi/128:pi;
z=tf('z');
G=(0.2*z/(z+0.2))*(1/(z-0.9));
[num,den]=tfdata(G,'v');
[h, freq] = freqz(num, den, w);
phase = angle(h);
figure(2);
plot(freq, rad2deg(unwrap(phase)))
w = -pi:pi/128:pi;
z=tf('z');
G=(0.2*z/(z+0.2))*(1/(z-0.9))*(1/(z-1));
[num,den]=tfdata(G,'v');
[h, freq] = freqz(num, den, w);
phase = angle(h);
figure(4);
plot(freq, rad2deg(unwrap(phase)))
The aspect ratios are different because freqz produces subplot plots.
.
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