cannot get fftshift to shift frequency

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?

[Rodger Herbst]

Thanks; this code works great !

[Star Strider]

As always, my pleasure!

(Link)

Answer 1

[Star Strider]

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.

Answer 2

[Sk Group]

Frequency Shifting Property in MATLAB Complete Prove (Code + Output)

Frequency shifting Prove: DFT{x(n)e^(j(2*pi/N)nl = X(K-l)

For detailed post and complete code visit: https://www.swebllc.com/frequency-shifting-property-in-matlab- complete-prove-code-output/