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Plotting multiple Magnitudes in one plot

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Mehdi Jaiem
Mehdi Jaiem on 21 Jan 2021
Commented: Mathieu NOE on 21 Jan 2021
Hello everyone,
I intend to use freqz() function I need to plot the magnitude for different border frequencies. But first I need to determine the cut off frequencies.
I used this code but which seems to not give me even a close response to this
%% INitializing Matlab Environment
close all;
clc;
clearvars;
%% 1.1
b=log10(8000);%stop frequency and 100Hz is the pass frequency
yc1=logspace(2,b,23);
yd=stem(yc1);
grid on
xlabel('Borders','FontSize',12);
ylabel('F (Hz)','FontSize',12);
set(gca,'YScale', 'log')
%%
m={};
n={};
h={};
ph={};
j={};
fs=21e3 %sampling frequency
for i= 1:1:23
[u,o] = butter(1,[2*100/fs, (2*yc1(i))/fs],'bandpass');
m{i}=u;
n{i}=o;
%freqz(m{i},n{i});
[h{i},ph{i}]=freqz(m{i},n{i});
figure
subplot(2,1,1)
hold on
plot(ph{i},20*log10(abs(h{i})));
end
hold off
grid
set(gca,'XLim',[0 1e4],'XTick',10.^(0:4),'YLim',[-12 0],'YTick',-12:2:0,'XScale', 'log')
Any Idea on what I am missing ?

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Accepted Answer

Star Strider
Star Strider on 21 Jan 2021
Put the figure call before the loop, and remove the subplot call:
figure
hold on
for i= 1:1:23
[u,o] = butter(1,[2*100/fs, (2*yc1(i))/fs],'bandpass');
m{i}=u;
n{i}=o;
%freqz(m{i},n{i});
[h{i},ph{i}]=freqz(m{i},n{i});
plot(ph{i},20*log10(abs(h{i})));
end
hold off
This does not exactly produce the plot you posted, however it will get you closer.
I leave the other details to you.

  2 Comments

Mehdi Jaiem
Mehdi Jaiem on 21 Jan 2021
Thank you it solves at least the multiplot issue !

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More Answers (1)

Mathieu NOE
Mathieu NOE on 21 Jan 2021
hello
according to the picture, your code needed a revision, see below
the bandwith of your filters (BW) should be proportionnal to the center frequency to have identical roll off curves when plotted in log axis
you can test with a fixed constant bandwith , it's no the right option
hope it helps !
%% INitializing Matlab Environment
close all;
clc;
clearvars;
%% 1.1
b=log10(8000);%stop frequency and 100Hz is the pass frequency
BW = 100; % fixed bandwith (Hz)
yc1=logspace(2,b,23);
yd=stem(yc1);
grid on
xlabel('Borders','FontSize',12);
ylabel('F (Hz)','FontSize',12);
set(gca,'YScale', 'log')
%%
fs=21e3 %sampling frequency
for i= 1:1:23
BW = 100; % fixed bandwith (Hz) : NO !!!
% BW should be proportionnal to center frequencies (not fixed)
BW = yc1(i)/5;
f_low = yc1(i) - BW/2;
f_high = yc1(i) + BW/2;
[B,A] = butter(1,[f_low, f_high]*2/fs,'bandpass');
freq = logspace(log10(f_low/4),log10(f_high*4),100),
[h]=freqz(B,A,freq,fs);
figure(2)
hold on
semilogx(freq,20*log10(abs(h)));
end
hold off
grid
set(gca,'XLim',[0 1e4],'XTick',10.^(0:4),'YLim',[-12 0],'YTick',-12:2:0,'XScale', 'log')

  2 Comments

Mehdi Jaiem
Mehdi Jaiem on 21 Jan 2021
Awesome thank you!
I have to review some notons then since I am new to this topic.
But there are two ine that I did not understand:
BW = yc1(i)/5; %why did you divide by 5
freq = logspace(log10(f_low/4),log10(f_high*4),100) % what is the perpose of 100?
semilogx(freq,20*log10(abs(h))); % I read about semilogx but I did not understand it. why not plot ?
Mathieu NOE
Mathieu NOE on 21 Jan 2021
BW = yc1(i)/5; %why did you divide by 5
bandwith is a fraction of center frequency , the higher the division factor, the narrower the filter ; just adapt to your own needs
freq = logspace(log10(f_low/4),log10(f_high*4),100) % what is the perpose of 100?
this is the length we want for freq vector; try another value , like 10, and see the plot being not so smooth
semilogx(freq,20*log10(abs(h))); % I read about semilogx but I did not understand it. why not plot ?
try plot to see the difference

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