# Questions about some equations in Rohde's book - RF Cafe Forums

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mmaassel

Post subject: Questions about some equations in Rohde's book Posted: Thu Oct 30, 2008 10:41 am

Colonel

Joined: Mon Apr 10, 2006 12:43 pm

Posts: 25

Good day.

I have some questions about an equation in Rohde's book - Communications Receivers (2nd Edition). I am hoping someone can help me unlock my brain.

My first question deals with figures 3.2a and 3.2b (Page 96).

Figure 3.2a

Signal generator (Eg) -> source impedance (Zg) -> voltage noise source 1 (En1) -> noise free network -> voltage noise source 2 (En2) -> load impedance (Zl)

Figure 3.2b

Signal generator (Eg) -> source impedance (Zg) -> voltage noise source (En)-> current noise source (In) -> noise free network -> load impedance (Zl)

The noise free network is described by Z-parameters (Z11, Z12, and Z22)

On page 101 the authors state that the output voltage source (En2) can be replaced by an input voltage source that will produce the same output. The source they come up with is -(E11 + Zg)(En2/Z21). They continue by saying that the open circuit voltage ZgEn2)/Z21)] is just that from an input current source (-En2/Z21)

How do they get the source -(E11 + Zg)(En2/Z21)?

My brain is stuck on this problem and to make matters really annoying I have solved this problem in the past.

Thank you for your time and help.

Michael

Posted  11/12/2012