how can i find the value for the amplitude and frequency using fft?

Hi everyone,

please i want to find the value for the maximum amplitude and the frequency. i used the abs of the fft for the amplitude but the amplitude value i am having does not tally with that from the figure (from the figure is around 2-2.2 while am having from the fft 8900). also, i used several methods from this page for the frequency value but its returning with a vector not value. The oscillations have no sampling frequency, so i used fs = 1/T.

Time = 0.2-0.3ms  %the vector values= 250000x1 double
% to find the amplitude
p=signal
pt=signal time 
t1=0.2
t2=0.3
pttt=find(pt>ti&pt<t2)
a=fft(pt(pttt))
aa=abs(a)
aa_max=max(aa)   % it will give value of 8900
% to find the frequency value
f=find(aa==aa_max)
f_exst=(f(1,1)-1)*fs/N    % but am not sure of the value it gives

Please i need your help

thanks

Answer 1

[KSSV]

Note that max gives you the index of the max value also.

[aa_max,idx] = max(aa) 

[Abba Alhaji Bala]

Thank you for your reply. i did that but the idx is 301

[KSSV]

you have to extract the frequency of that index.

[Abba Alhaji Bala]

I use this code to extract

>>[aa_max,idx] = max(aa)

>>faa = max(aa);

>>f_exst=(faa(1,1)-1)*fs/N;

i dont know if this is how you mean i can extract the frequency

Thanks,