Post subject: input impedance of RF trace on PCB Posted:
Fri May 11, 2007 3:22 pm
Apr 24, 2007 1:10 pm
From PA2460 datasheet,
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?
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.
Thank you very much.
subject: Posted: Sat May 12, 2007 7:53 am
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).
The lines 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 pm
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 wavelength.
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.