# maximum load amplitude from the spectrum curve

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Mohammad Haghnejat on 13 Oct 2022
Answered: Gokul Nath S J on 26 May 2023
It would be helpful if you could tell me how to calculate the maximum load amplitude from the spectrum curve, so that I can obtain the same result when I check with the time domain function.

Gokul Nath S J on 26 May 2023
To calculate the maximum load amplitude from the spectrum curve and obtain the same result as in the time domain function in MATLAB, you can follow these steps:
Obtain the spectrum curve of the signal using the Fast Fourier Transform (FFT) in MATLAB. Let's assume you have a signal x with a sampling frequency Fs:
X = fft(x);
N = length(x); % Length of the signal
f = (0:N-1)*(Fs/N); % Frequency axis
P = abs(X/N).^2; % Power spectrum
Find the index k corresponding to the maximum value of the power spectrum. Calculate the frequency f_max corresponding to the maximum power and convert the frequency f_max to angular frequency w_max. Calculate the period T_max corresponding to the angular frequency w_max. Finally, find the maximum load amplitude A_max by finding the peak value of the time domain signal x:
[~, index] = max(P);
f_max = f(index);
w_max = 2*pi*f_max;
T_max = 2*pi / w_max;
A_max = max(abs(x));
The A_max value obtained using the maximum load amplitude from the spectrum curve should be equivalent to the peak value in the time domain signal.
with regards
Gokul Nath S J

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