I can't place the fft curve at the signal frequency exactly. There's been a slight deviation of the fft peak at that frequency

clear all
close all
clc
L=10;
n=1.45;
c=2.9979e8;
dt=6e-12;
T=10*2*L*n/c;
fmax = 1e9;
fs=80*fmax;

TA=-T/2:dt:T/2;

%t = (-T/2/dt:1:T/2/dt)*dt;
Nt=round(T/dt);

vsine = 1;
phi = vsine*sin(2*pi*fmax*TA);
EL1t=1.274e7*exp(1i*phi);
plot(TA,(EL1t));

Warning: Imaginary parts of complex X and/or Y arguments ignored.

%FA = ((0:Nt-1)-floor(Nt/2))/Nt*fs;
FA = (-Nt/2:Nt/2-1)/Nt*fs*2;
FP=fft(phi);
%fs=1/dt/Nt;
figure;
Z=plot(FA,fftshift(abs(fft(EL1t/Nt))));
xline(1e9);
xlim([-2e9 2e9]);

As per the logic , each peak should have been at the multiples of 1GHz like the first peak should have been at 1GHz and the next at 2GHz and so on.

But evidently there has been a slight devition.

Answer 1

[Paul]

The code has

fs = 80*fmax;

but the TA vector is space by dt, so fs should be

fs = 1/dt;