# Spectrasys : NF in mismatch conditions

Question asked by JeanBa on Jul 19, 2010
Latest reply on Jul 21, 2010 by rulonv
Good Morning,

I am trying to understand how the NF of an gain block evolves in Spectrasys whit mismatch impedance.

So I have, in Spectrasys 3 blocs Source - Gain Block - Output.

When all I/O impedances are the same the NF of the block equals NF measured. So it's ok.

The problem is when I modify the input impedance of the Gain Block or impedance of the source. I have seen in the documentation that the NF of a block evolves with the output impedance of the preceding block.

F = F min + ( r n / g s ) * ( | Y s - Y o | 2 )

I am trying to understand the assumptions taken in Spectrasys to apply this formula.

In the litterature, it said that to characterize the NF, four parameters are needed. In Spectrasys, we can only specify I impedance, O impedance and NF...

I have try to calculate the F when I take Fmin = 10(NF/10), rn = Zin, gs = 1/Zsource, Ys = 1/Zsource and Yo = 1/Zin. When the NF is below 5dB the NF calculated equals the NF measured but not when NF > 5dB.

Please can you help me to understand this problem.

Thank you