# Input Impedance - RF Cafe Forums

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Measures
 Post subject: Input Impedance Posted: Mon Mar 01, 2010 6:57 am
 Captain

Joined: Tue Dec 15, 2009 10:22 am
Posts: 9
How can I obtain the input impedance of my circuit from the measured S-parameters?

Is the expression Zin=Zo*(1+S11)/(1-S11) a general expression, or it is only valid in the cases in which GammaL is zero? I don't obtain the input impedance using this expression.
Is GammaL zero occurs only when the load impedance is equal to the impedance of the network? Or does it occur whenever the load impedance is equal to the reference impedance?

Does anybody know which is the expression than ADS uses to obtain the Zin from the S11 parameter? I know this expression works perfectly, but I would like to obtain Zin by myself from the measured S-parameters, not using ADS (I cannot use touchstone files).

Thank you very much

david1hitch
 Post subject: Re: Input Impedance Posted: Mon Mar 15, 2010 10:35 pm
 Captain

Joined: Mon Jan 04, 2010 1:12 am
Posts: 5
The input impedance of any network is determined by the combination of the real and imaginary elements combined , real being the resistive element and the imaginary being the reactive elements . Your results should be in the for z=R+jwl or z=R-jwC . I have spent over 30 years dealing with practical examples

SunshineDesign
 Post subject: Re: Input Impedance Posted: Wed Apr 07, 2010 3:28 pm
 Colonel

Joined: Sat Nov 08, 2008 11:35 pm
Posts: 30
Location: Ramona, CA
David didn't answer your question, of course impedance is a complex value. But the question was is the formula he posted the general expression for Input impedance from S11 and the answer is yes. Do remember to convert the S11 from dB to linear terms, where it is a voltage ratio. This can be used to convert the S11 measurement of the Zo =50 or 75 ohms of the measurement instrument plane to the Zload of the device.
GammaL zero occurs when the load impedance is equal to the reference impedance i.e. when both impedances are exactly matched.

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