How can I ameliorate the trend of a curve ?

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Mallouli Marwa
Mallouli Marwa 2018-6-12
Hello
I have to calculate the FFT of the voltage but the trend found is not well.
The curve FRF wanted attached is the best trend.
So How can I ameloirate the trend of this FRF found (figure3 in the program).
Please help me.
The program is:
%% %script traitement signal %objet : traitement signal MPF clc; clear; close all; cd Z:\Matlam\Curves_oscillo_Pc %%
%% load 'data1.txt';
tmp = data1(:,1);
disp = data1(:,2);
acc = data1(:,3);
figure(1)
plot(tmp,acc,'b',tmp,acc,'r.');
xlabel('time (sec)');
ylabel('Acc (ms^{-2})');
%on récupère les données de Oscilloscope en Volt
load 'data1V.txt';
tmpV = data1V(:,1);
Volt = data1V(:,2);
%Voltb = 40e-3*ones(size(tmpV));
figure(2)
plot(tmpV,Volt,'b',tmpV,Volt,'r.');
xlabel('time (sec)');
ylabel('Voltage(V)'); %%
L=length(tmp)
Lv=length(tmpV)
Fs = 5000; % Sampling frequency
NFFT = 2^nextpow2(L) % Next power of 2 from length of y
NFFTV = 2^nextpow2(Lv) % Next power of 2 from length of y
figure(3)
VOLT = fft(Volt,NFFT)/L; % Calcul de la FFT
amplitude = 2*abs(VOLT(1:NFFT));
f = Fs/2*linspace(0,1,NFFT/2);
semilogy(f(1:NFFT/12),2*abs(VOLT(1:NFFT/12)))
xlabel('frequency')
ylabel('FRF Volatage')

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