When to simulate Noise Figure, the noise source is usually a port, in which the default impedance is 50 Ohm. More importantly, source noise comes from the impedance of the port. So a larger source impedance produces larger source noises. With respect to the Noise Factor equation, N=(No^2-Nl^2)/(Ns^2), larger source noises will make a smaller Noise Factor.
Q1. In a Noise Figure simulation, such as in design of LNA or Mixer, what value of the port impedance should be?
In my opinion, Noise Factor is under the assumption that the transeiver system has a input impedance of 50 Ohm, so is its output impedance. Every module in such a system should consider a 50 Ohm noise source. Otherwise, one cannot get the following equation:
NF = 1 + (NF1-1) + (NF2-1)/G1 + ...
Q2. Am I correct? If anything is wrong, please point it out.
Thanks in advance.
You're right. Sorry, my mistake - I didn't think this through. You can see this quite clearly if you change the noise temperature of the noise source - if it truly was independent of the source impedance (other than due to matching), then changing the noise temperature would also have no effect.
This is discussed (to some level) in RF Microelectronics by Behzad Razavi, 1997 (pages 39-45). I'm sure it's covered in many other books too. I think the primary requirement on the kind of combination of noise figure you describe is that the impedances are consistent.