# input impedance of RF trace on PCB - RF Cafe Forums

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jinhao

Post subject: input impedance of RF trace on PCB Posted: Fri May 11, 2007 3:22 pm

Lieutenant

Joined: Tue Apr 24, 2007 1:10 pm

Posts: 2

Hi,

From PA2460 datasheet, we got

FR4,1.2mm; trace line: MLIN: W=12mil; L=616mil.

What can we expect to know?

what is the input impedance Zin of this line?

thickness between trace and ground?

1.2mm = what? 4-layer board thickness? or ...

dlelectric constant = 4.3?

I mean, we should either know Zin or exactly dimension / dielectric constant to calculate Zin. but we don't know any thing.

May someone help us?

Thank you very much.

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IR

Post subject: Posted: Sat May 12, 2007 7:53 am

Joined: Mon Jun 27, 2005 2:02 pm

Posts: 373

Location: Germany

Hello,

You can get the following from this information:

The data sheet of this PA IC shows that the board is a 2-layers board therefore the 1.2mm is the width of the substrate itself. (From the metal to the GND layer).

The lines that you mention are micrstrip transmission lines. You can easily calculate the impedance of the line under the following link:

http://mcalc.sourceforge.net/#calc

At frequency of 500MHz the impedance of this line (With the parameters you provided) is almost 125 ohm.

There is a freeware transmission line calculator available from Microwave Office call TXLine which you can download and use for calculating the impedance of the line based on physical parameters.

Hope this helps.

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rftroubleshooter

Post subject: Posted: Wed May 16, 2007 4:36 pm

Captain

Joined: Wed May 16, 2007 4:16 pm

Posts: 5

One thing to keep in mind is that the impedence of a microstrip trace is usually not an issue if the length is relative small in terms of wavelengths. I use an emperically derived rule of thumb where I don't really care about the strip impedence until the length is >1/16th wavelength.

What you really want to be careful with when designing source and load traces is to make sure that you are minimizing parasitic capacitance to ground. Having ground plane or grounded vias too close to the trace can cause shorts at RF.

Posted  11/12/2012