MATLAB: FFT of sensor data

digital signal processingfft

Hello everyone!
I am a beginner in Signal processing. I have a time series plot of sensor data that has 200000 values. I need to calculate the FFT of the same data. When I perform fft(x), (assuming x is the time series signal), I get a plot with two peaks , one at 0 and one at 200000. The peak at 0 has an amplitude of 110 and the peak at 200000 has an amplitude of 7. The FFT should be mirror images. Why am I not observing any mirror images on this plot? The sample time of sensor is 0.01s (1000Hz).
Also I expect a resonant frequency of the sensor to be around 70-80 Hz. I do not see any peaks in the FFT at this frequency range. Please help.

Best Answer

The peak at 0 is equal to the sum of your readings. It would disappear if you were to subtract the mean of your readings from your input.
If your readings are real-valued, then the peak at the last bin is the complex conjugate of the peak at bin #2; both correspond to the lowest non-zero frequency.
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