# Input impedance - RF Cafe Forums

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Measures
 Post subject: Input impedance Posted: Mon Mar 01, 2010 11:35 am
 Captain

Joined: Tue Dec 15, 2009 10:22 am
Posts: 10
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

alperuslu
 Post subject: Re: Input impedance Posted: Fri Mar 05, 2010 7:34 pm
 Lieutenant

Joined: Fri Mar 05, 2010 9:02 am
Posts: 2
They are general expressions

PeterMartin
 Post subject: Re: Input impedance Posted: Wed Apr 14, 2010 2:08 am
 Lieutenant

Joined: Sun Apr 04, 2010 7:13 pm
Posts: 2
 The spread sheet program "matching network calculator" which is available for free on this page:http://www.rfshop.co.uk/blog/downloads can do what you need.For example, it will convert the reflection coefficient Mag =0.35 Angle = 78 degs , in a 50 ohm system, into an impedance of :Z=44.91+j35.04 ohms Does that sound like the right answer?Incidentally, the program uses the same equation you mention. Maybe your problem was in the manipulation of complex numbers?

Posted  11/12/2012