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cannot get fftshift to shift frequency

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rodger herbst
rodger herbst 2019 年 11 月 22 日
Commented: Star Strider 2019 年 11 月 23 日
I am trying to verify an FFT computation by producing the original signal. Below I explain the situation. My question is in CAPS below
VACCL.csv has the following format. (no line spaces)
sec VACCL
0 1
0.125 1.002
0.25 1.002
0.375 1.007
0.5 1.002
0.625 1.002
0.75 1
0.875 1.002
1 1.005
1.125 1.005
1.25 1.005
1.375 1.002
1.5 1.002
in command line I typed:
TT = readtimetable(VACCL.csv');
Using signal analysis tool on TT, I was able to plot the original signal and a frequency response. The frequency response was plotted in Hz, but signal time was plotted in hours. In input file VACCL.csv, time was given in seconds.
I then Imported VACCL.csv as matrix with 64 bit double precision and typed
Y = fft(VACCL)
Y was generated in the workspace as complex double: 2 columns: one real, one Imaginary
When I plotted Y(:,2) on sig analyzer, the display was identical to original signal analyzer result except for axis labels: the signal was plotted in samples, while the normalized frequency was plotted from 0 to 1.0
Then I typed:
Y_SHIFTED = fftshift(Y)
When I plotted Y_SHIFTED in signal analyzer,
the time signal was broken in two halfs, left and right, and the two halves switched, while
the frequency remained normalized from 0 to 1.0.
HOW DO I GET THE FREQUENCY SHIFTED RATHER THAN THE SIGNAL?

  2 件のコメント

rodger herbst
rodger herbst 2019 年 11 月 23 日
Thanks; this code works great !
Star Strider
Star Strider 2019 年 11 月 23 日
As always, my pleasure!

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Star Strider
Star Strider 2019 年 11 月 22 日
You need to create the frequency vector yourself.
Probably the easiest way is to use the linspace function.
Try this:
VACCL = [ 0 1
0.125 1.002
0.25 1.002
0.375 1.007
0.5 1.002
0.625 1.002
0.75 1
0.875 1.002
1 1.005
1.125 1.005
1.25 1.005
1.375 1.002
1.5 1.002];
Ts = mean(diff(VACCL(:,1))); % Sampling Interval
Fs = 1/Ts; % Sampling Frequency
Fn = Fs/2; % Nyquist Frequency
L = size(VACCL,1); % Signal Length
VACCLv = VACCL(:,2); % Signal Vector
Y = fft(VACCLv)/L; % Fourier Transform
Y_SHIFTED = fftshift(Y); % Shift
Fv = linspace(-Fn, Fn, size(Y,1)); % * Frequency Vector *
figure
plot(Fv, abs(Y_SHIFTED))
grid
Experiment to get different results.

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