Find index max in Frequency domain

Hi everyone!

I have one problem, I need to find the index max in frequency, on this signal

if you can see, it's centered in zero in X axis and Y axis, but when I try to find the index, this take the value when the signal doesn´t center.

to center the signal I used this, having Fs and Ts

m=abs(fft(signal));
magxF=fftshift(m);

f_esp = Fs/(length(signal));
f_inicial = -(Fs/2);
f_final = (Fs/2) - f_esp;

f_eje = f_inicial:f_esp:f_final;
plot(f_eje,magxF)
xlabel('(Hz)')

and to find the index I used

[maxval,idx] = max(magxF);

but returns values of this signal

How can I find the index Max for the signal centered ?

[Azzi Abdelmalek]

It's centered in X axis and Y axis : What does that mean?

[Camilo A Rincón]

The signal must be symmetrical, having positive and negative frequencies. it means that the center should be in zero for X axis and Y axis.

Answer 1

[Walter Roberson]

No it is finding the index correctly, but what you want as output is not the index but rather the frequency. You will get the frequency as

 f_eje(maxidx)