My Matlab FFT plot keeps showing the mirrored values of my frequency of interest. In this example I am looking for 250Hz with a 3200Hz sampling freq. The FFT plots my value at 266Hz as well as 1330 Hz. I assume my sampling freq is a little off which is why it isn't exactly at 250Hz. Matlab code is below, any help is appreciated.

% Tmax = (N-1)*Ts
% largest freq = Fs/2
% smalles = -fs/2

fid = fopen('250Hz.bin');
AA = fread(fid, 'short');
flenbytes = ftell(fid);
flenshorts = floor(flenbytes/2);
FF = floor(flenshorts/16)*16;
fseek(fid,0, 'bof');
BB = fread(fid, [8 (FF/8)], 'short');
CC = BB';
DataSet = CC ./2^3;
Z1Axis = DataSet(:,4);

N = length(DataSet)
Fs = 3200;
ts = 1/Fs;
tmax = (N-1)*ts; 
t = 0:ts:tmax;  %start at 0 increment by ts up to tmax

f = (0:N-1)*(Fs/N)/2; % start with -Fs/2, increment by Fs/(N-1), goes to Fs/2
z = fft(Z1Axis);
%zreal =

1 Answer 1


It is a mathematical property of the DFT that its output is conjugate symmetric if its input is real-valued. That means that for the magnitude of the DFT output signal $X_k$ of length $N$ it holds $$ |X_{N-k}| = |X_k|. $$ So what you see is to be expected. (In your example $3200\,\mathrm{Hz}/2 - 266\,\mathrm{Hz}\approx1330\,\mathrm{Hz})$ If you're only interested in the positive frequencies, just plot the first $N/2$ elements of the DFT output, the rest is redundant.


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