# How to measure and obtain input impedance by R//C model

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#### rf1008

##### Full Member level 2
Hi all,

Maybe you've found one input impedance of some RF or microwave device by the model of R//C,for example, the SAW filter is the most popular device in handset phone system and the input differential impedance of SAW filter is often the R//C or R//L model,such as 150Ω//82nH.

My question is that how i can measure and obtain that R//C or R//L model impedance with VNA or somewhat equipment.As we know, we can get impedance at every frequency point with VNA, but R//C or R//L impedance is independent with frequency.

thanks.

#### BigBoss

A VNA gives you serial or parallel equivalent R or X ( C or L ) values at certain frequency.When you divide X value ( 2xPIxF or 1/2xPIxF ) you'll get L or C values respectively.
For instance, let say you have read 0.55+j*0.77 Ohm ( normalized ) impedance on your VNA @ 2GHz.It's a serial R+j*XL impedance.
Z=50*(0.55+j0.77)=27.5+j*38.5 @2GHz
So R=27.5 Ohm, L=3.06nH

If you need parallel equivalent, transform it to serial to parallel...

I hope I understood your question well...

#### rf1008

##### Full Member level 2
A VNA gives you serial or parallel equivalent R or X ( C or L ) values at certain frequency.When you divide X value ( 2xPIxF or 1/2xPIxF ) you'll get L or C values respectively.
For instance, let say you have read 0.55+j*0.77 Ohm ( normalized ) impedance on your VNA @ 2GHz.It's a serial R+j*XL impedance.
Z=50*(0.55+j0.77)=27.5+j*38.5 @2GHz
So R=27.5 Ohm, L=3.06nH

If you need parallel equivalent, transform it to serial to parallel...

I hope I understood your question well...
Hi Sir,
Thank you very much for the detailed explanation and calculation.

My original concern is from Impinj's M4 datasheet(please see it in attachment),the chip load model is 1650 Ω || 1.21 pF which is frequency independent, and I can transfer it to serial R+jX impedance by some mathematic formulas, and get the impedance in the attachment,13-j*151Ω@866MHz,11-j*143Ω@915MHz,10-j*137Ω@956MHz. These serial R+jX impedance are frequency dependent.

If we measure one device,we can get serial impedance of R+jX@Freq. directly from VNA,i doubt how to get the parallel impedance of R//C or R//L directly from VNA. Even though the parallel impedance of R//C or R//L could be calculated from serial impedance by some mathematic formulas.

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