Errors when trying to computer DTFT?

Question

kishintai 2014-2-15

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https://ww2.mathworks.cn/matlabcentral/answers/116261-errors-when-trying-to-computer-dtft

评论： Wayne King 2014-2-15

Hello everyone,

I just started a MATLAB course and my first excercise is to write a MATLAB function that computes the DTFT of a signal x[k] at a given number of frequencies. Also a magnitude and phase plot must be made.

My code:

% X=dtft(x,phi);
%
% discrete-time Fourier transform
%
% INPUTS
%   x   : time-domain signal
%   phi : vector containing the frequencies relative to the sampling frequency
%         for which the DTFT has to be evaluated
% OUTPUT
%   X   : DTFT of x
function X=dtft(x,phi)
X=zeros(length(phi),1);   % initialize X
for k=1:1:lentgth(x)-1                % compute the DTFT for all phi
    for phi=0:1:360
        x(k)*exp(1)^(1j*2*pi*phi*k);
    end    
end
subplot(2,1,1)          % make a Bode plot of X
plot(phi,abs(X))
xlabel('frequency relative to sampling frequency')
ylabel('magnitude)')
grid
subplot(2,1,2)
plot(phi,unwrap(angle(X))*180/pi)
xlabel('frequency relative to sampling frequency')
ylabel('phase (degrees)')
grid

I get the following errors:

1 Undefined function 'lentgth' for input arguments of type 'double'.

2 Error in dtft (line 16) for k=1:1:lentgth(x)-1 % compute the DTFT for all phi

This is my first experience with MATLAB, so my apologies for such questions. Thanks in advance

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Answer 1

Wayne King 2014-2-15

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https://ww2.mathworks.cn/matlabcentral/answers/116261-errors-when-trying-to-computer-dtft#answer_124488

编辑：Wayne King 2014-2-15

在 MATLAB Online 中打开

you misspelled

length()

There is no MATLAB function, lentght()

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kishintai 2014-2-15

编辑：kishintai 2014-2-15

Of course, yes excuse me for that.

Is it normal that when I run the function, nothing is plotted?

EDIT: Nevermind, my pc is just slow. The output of X is 0, so my function isn't correct

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Answer 2

Wayne King 2014-2-15

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在 MATLAB Online 中打开

You're not assigning X anything in your function except zeros

Minimally, in your for loop, you should be doing something like

X(k) = x(k)*exp(1)^(1j*2*pi*phi*k);

But even then you have a number of other problems with this function.

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kishintai 2014-2-15

What do you mean by numer of other problems?

Wayne King 2014-2-15

I don't want to simply do your homework problem for you, but for example, why do you even ask for an input argument of phi, when you simply create a vector of phi in the for loop? In other words, you use the phi as a loop index, so you're not using the input argument at all.

I answered your original question, so you should accept that answer.

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