Most of the time we check signal level proportional to noise level (such as noise figure) not singly.
I have obtained PSD of my desire system's output and it is a combination of
a) Dc term
b) reaction of input noise to itself
c) reaction of input signal to itself
d) reaction of input signal and input noise.
At system's output at frequency $\omega$ should I assume terms (c) and (d) as the signal level? Or c is enough?