FFT Plot different Vector not same length?

Question

Alia Hicks 2020-7-28

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此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/571528-fft-plot-different-vector-not-same-length

评论： Star Strider 2020-7-28

I am not getting any plots from my FFT of a 2X60000 database of the i and q channels of a dobbler radar measuring movment. I recieve "Error using plot Vectors must be the same length. Error in Julytwentyseventh (line 21) plot(t, f), hold on" for the first plot line. I've changed my value of t from t = 0:dt:1, and to t = 0:dt:60000. Same results.

dt = .001;

t = 0:dt:600;

f = data_output;

%%Compute the Fast Fourier Transform FFT

n = length(t);

fhat = fft(f,n); % Compute the fast Fourier transform

PSD = fhat.*conj(fhat)/n; % Power spectrum (power per freq)

freq = 1/(dt*n)*(0:n); % Create x-axis of frequencies in Hz

L = 1:floor(n/2); % Only plot the first half of freqs

%%Use the PSD to filter out noise

indices = PSD>100; % Find all freqs with large power

PSDclean = PSD.*indices; % Zero out all others

fhat = indices.*fhat; % Zero out small Fourier coeffs. in Y

ffilt = ifft(fhat); % Inverse FFT for filtered time signal

%%PLOTS

plot(t, f), hold on

plot (f, fft), hold on

plot (t,ffilt);

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Answer 1

Star Strider 2020-7-28

1
链接

此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/571528-fft-plot-different-vector-not-same-length#answer_471829

在 MATLAB Online 中打开

Try this:

plot(t, f(1:n)), hold on 
plot (freq(L), fhat(L)), hold on 
plot (t,ffilt(1:n));

Note that with no knowldege of ‘f’ or ‘data_output’, writing exact code is not possible.

There may still be problems if there is a discrepancy in the sizes of ‘t’ and ‘f’.

.

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Alia Hicks 2020-7-28

Thanks for your answer.I really appreciate your writing a different few lines of code to try. I did change it to what you suggested and I get the error, "Index exceeds the number of array elements (120000). Error in Julytwentyseventh (line 47) plot(t, f(1:n)), hold on" Do you think it matters that t is being stored as a row (1x600000 double) while my f and fhat and ffilt are stored as colomns and rows (60000x2 double) (complex on the fhat)? If so how do I fix this?

Star Strider 2020-7-28

在 MATLAB Online 中打开

Try this:

plot(t(1:min(length(t),length(f))), f(1:min(length(t),length(f)))), hold on

The difference between ‘t’ and the others being row and column vectors should not mattter for the plot function in this instance, although in other situations it definitely would.

Nevertheless, it would be best to transpose ‘t’ to a column vector to avoid potential problems.

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