# Thread: NF in mismatch conditions

Replies: 1 - Pages: 1 - Dec 17, 2010 10:29 AM Last Post By: rulonv
 Posts: 3 Registered: 07/19/10
NF in mismatch conditions
Posted: Jul 19, 2010 2:27 AM
 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
 Posts: 61 Registered: 06/08/09
Re: NF in mismatch conditions
Posted: Dec 17, 2010 10:29 AM   in response to: JeanBa
 The current amplifier model in Spectrasys does not support NFmin and NFopt so proprietary adjustments are made to ensure that the amplifier model remains physical.