plot a signal in the freq domain

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

[Honglei Chen]

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

[Thomas]

what are 'm' and 'n' here?

Answer 1

[Dr. Seis]

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)

[Nicolas]

Hello!!

Thanks, now I am plotting the image but is not what I was expecting, but this is another thing...

thanks you

[Nicolas]

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]

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

Answer 2

[Thomas]

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)

[Nicolas]

Hi!

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