Input Impedance  RF Cafe Forums
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Measures

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


Joined: Tue Dec 15, 2009
10:22 am Posts: 9 
How can I obtain the input impedance of my circuit
from the measured Sparameters?
Is the expression
Zin=Zo*(1+S11)/(1S11) 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 Sparameters,
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


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=RjwC
. I have spent over 30 years dealing with practical
examples





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


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.
_________________ Sunshine Design Engineering
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