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input impedance of RF trace on PCB - RF Cafe Forums
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Post subject: input impedance of RF trace on PCB Posted:
Fri May 11, 2007 3:22 pm
Joined: Tue Apr
24, 2007 1:10 pm
From PA2460 datasheet, we
FR4,1.2mm; trace line: MLIN: W=12mil; L=616mil.
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?
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.
Post subject: Posted: Sat May 12, 2007 7:53
Joined: Mon Jun 27, 2005 2:02 pm
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).
that you mention are micrstrip transmission lines. You can easily calculate
the impedance of the line under the following link:
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.
Post subject: Posted: Wed May 16, 2007 4:36
Joined: Wed May 16, 2007 4:16 pm
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
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.