Getting magnitudes from frequency spectrum

The amplitude of a variable is the measure of how far, and in what direction, that variable differs from zero. Thus signal amplitudes can be either positive or negative. The magnitude of a variable, on the other hand, is the measure of how far, regardless of direction, its quantity differs from zero. So magnitudes are always positive values.

I assumed, if i apply a fft on my signal, then i get a two-sided frequency spectrum, therefore i cut off the one side, which only contains complex values. Then i used the fucntion "abs" on my one-sided spectrum. So, that all values are converted to positive values. How can I get all the positive values from one frequency spectrum and get the next values from another frequency spectrum, when i plot several of them, every 100 ms?

Answer 1

[Cyriac Jose]

I have a set of 50Hz Voltage and Current values stored as arrays under variable name 'Volt' and 'Cur' sampled at 1000 Hz

Things to be obtained:

1. Extract the 50 Hz component out of the whole load data in turn. Require a sliding window of 20 ms taking the 50 Hz component of the vector for that sliding window and do the fft of that vector and store it in a separate array.

2. Simliary take 50 Hz component of every 20 ms window and store the same to the array. Can spectrogram do the same?