6/1/16: Why not just spec it as Noise Figure instead of RMS voltage?

Question

Why not just spec it as Noise Figure instead of RMS voltage? It would be much easier to figure out system performance if everything is spec'd as Noise Figure in dB.

ScottH · Accepted Answer

I agree that Noise Figure is a handy figure of merit when you have a constant, known source impedance. But when you generalize NF to variable source impedances, it gets a bit more confusing because NF isn't actually what you want to optimize. 
 I would like to quote Horowitz and Hill on this: 
 "Big Fallacy: Don't try to improve things by adding a resistor in series with a signal source to reach a region of minimum NF. All you're doing is making the source noisier to make the amplifier look better! Noise Figure can be very deceptive for this reason." .... "In general, when evaluating the performance of some amplifier, you're probably least likely to get confused if you stick with SNR calculated for that source voltage and impedance." 
 I have found that it's better to explain noise in terms of the fundamentals NSD, RMS, and SNR in introductory lectures like this webcast, so as not to confuse the less experienced engineers with higher levels of abstraction like NF.)