Average value of power spectral density
You should remove low frequency content of the diagram. Simply use moving average filter. find the optimum filter size that produces your desired result. Here is a matlab example code.
t=0:0.001:1;
PSD=2*sin(2*pi*1*t)+10*randn(size(t))+5; %assume as the power spectrum
filtered=zeros(size(t));
M=20 ;% Moving average filter size
for i=1:length(t)-M
    filtered(i)=mean(PSD(i:i+M));
end
plot(PSD);
hold on
plot(filtered,'r')