MATLAB Answers

Plotting multiple Magnitudes in one plot

17 views (last 30 days)
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 ?

  0 Comments

Sign in to comment.

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 !

Sign in to comment.

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

Sign in to comment.

Community Treasure Hunt

Find the treasures in MATLAB Central and discover how the community can help you!

Start Hunting!

Translated by