# measuring input impedance of a RFID tag in ADS

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#### elgiovo

##### Junior Member level 1
Input impedance

I have to measure the input impedance variation (with frequency) of a RFID tag (a simple circuit) from the antenna's terminals (call them in1 and in2). How should I do this in ADS? I tried with a termination between the inputs and with a HB simulation, but I get weird results.

Thanks for help.

#### elgiovo

##### Junior Member level 1
Input impedance

Nobody? Impossible.
Maybe the wrong section?

#### GGAPBE96

##### Member level 2
Re: Input impedance

Why don't you calculate "Zin = (1+S11)*Z0/(1-S11)"?
(Z0 represents the characteristic Impedance)

#### elgiovo

##### Junior Member level 1
Input impedance

For Z0 you mean 50 ohms? In my circuit, anyway, I don't make use of transmission lines, it is just made of lumped components.

And what about S11? It is a two-terminal network, so I believe one should use S12 or S21 instead.

#### GGAPBE96

##### Member level 2
Re: Input impedance

Z0 is the characteristic impedance of feed line.
You can use the equation for any 2-port passive circuit because the reflection coefficients are directly linked the impedance.

If you want to know the detail of the circuit,you assume the equivalent circuit and optimize it.
In this case,Sij (i,j=1,2) is needed.

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