plot a signal in the freq domain

Question

Nicolas 2012-3-7

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Hello!! I am trying to plot this function X= m*((sin(pi*f/2*m)*sin(pi*f/n))/pi*f*cos(pi*f/2*m))^2;

that is in the freq domain, but when I get the plot is empty. This function is the theorically spectrum of a BOC modulation, so I guess that I can not do fft or pwelch or stuff like that, so I dont know how to plot it in the f domain. I thought is gonna be samething easy as

for f=1:100:100000;

X= m*((sin(pi*f/2*m)*sin(pi*f/n))/pi*f*cos(pi*f/2*m))^2;

end

plot(f,X)

but is not working I will appreciate a lot is someone can give me a hint Thanks in advance Nicolas

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Honglei Chen 2012-3-7

When you say plot is empty, did you check what's in X?

Thomas 2012-3-7

what are 'm' and 'n' here?

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Answer 1

Dr. Seis 2012-3-7

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If the code is setup exactly the way you say above, then there is only one element in "f" and one element in "X". You need to do something like this instead:

% Define m and n
f = 1:100:100000;
X = zeros(size(f));  % Preallocate
for ii=1:numel(f)
   X(ii)= m*((sin(pi*f(ii)/2*m)*sin(pi*f(ii)/n))/pi*f(ii)*cos(pi*f(ii)/2*m))^2;
end

Or... skip the for loop

X = m*((sin(pi*f/2*m).*sin(pi*f/n))/pi.*f.*cos(pi*f/2*m)).^2;

Then

plot(f,X)

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Nicolas 2012-3-8

hello elige!!

now, I am doing something similar but I am getting an error cause of the difference vectors length but look what I wrote, so I dont know where is the mistake

Tc=1*10^-3;

a=1:100:1000000;

R=zeros(size(a));

for f=1:numel(a)

R(f)=i*Tc*sinc(f*Tc/2)*sin(pi*f*Tc/2);

end

plot(a,R);

where i is the imaginary unit

Dr. Seis 2012-3-8

You are using "f" inside your sinc and sin functions, but I think you meant to use "a(f)" instead.

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Answer 2

Thomas 2012-3-7

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在 MATLAB Online 中打开

try the following:

define m and n;

m=1;
n=1;
format long
for f=1:100:100000;
X=[X m*((sin(pi*f/2*m)*sin(pi*f/n))/pi*f*cos(pi*f/2*m))^2];
end
plot(f,X)

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Nicolas 2012-3-8

Hi!

The code that you gave me had a problem, but I solved. Thanks a lot anyway

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