multiple fisher criteria for periodic components

i have following code

clear all;
clc;
close all;
fs=100;
ts=1/fs;
t=0:ts:2.93;
x=24*sin(2*pi*20*t)+randn(size(t));
[Pxx,f]=periodogram(x,[],[],fs);
[maxval,index] = max(Pxx);
fisher_g = Pxx(index)/sum(Pxx);
N = length(Pxx);
    upper  = floor(1/fisher_g);
    for nn = 1:3
        I(nn) = ...
            (-1)^(nn-1)*nchoosek(N,nn)*(1-nn*fisher_g)^(N-1);
    end
pval = sum(I)
%end program;

pval is

pval =

     1.2667e-70

what about when we more then one periodic component? please help me