It’s really all very simple — once you understand it. Then， here’s the inside story on noise for those of us who haven’t been designing low noise amplifiers for ten years. You hear all sorts of terms like signal-to-noise ratio， noise figure， noise factor， noise voltage， noise current， noise power， noise spectral density， noise per root Hertz， broadband noise， spot noise， shot noise， flicker noise， excess noise， I/F noise， fluctuation noise， thermal noise， white noise， pink noise， popcorn noise， bipolar spike noise， low noise， no noise， and loud noise. No wonder not everyone understands noise specifications. In a case like noise， it is probably best to sort it all out from the beginning. So， in the beginning， there was noise; and then there was signal. The whole idea is to have the noise very small compared to the signal; or， conversely， we desire a high signal-to-noise ratio S/N. Now it happens that S/N is related to noise figure NF， noise factor F， noise power， noise voltage en， and noise current in. To simplify matters， it also happens that any noisy channel or amplifier can be completely specified for noise in terms of two noise generators en and in as shown in Figure 1.