TOBY-L1 and MPCI-L1 series - System Integration Manual
UBX-13001482 - R04
Advance Information
Design-in
Page 52 of 90
Guidelines for RF transmission line design
Any RF transmission line, such as the ones from the
ANT1
and
ANT2
pads up to the related antenna connector
or up to the related internal antenna pad, must be designed so that the characteristic impedance is as close as
possible to 50
Ω
.
RF transmission lines can be designed as a micro strip (consists of a conducting strip separated from a ground
plane by a dielectric material) or a strip line (consists of a flat strip of metal which is sandwiched between two
parallel ground planes within a dielectric material). The micro strip, implemented as a coplanar waveguide, is the
most common configuration for printed circuit board.
Figure 28 and Figure 29 provide two examples of proper 50
Ω
coplanar waveguide designs. The first example of
RF transmission line can be implemented in case of 4-layer PCB stack-up herein described, and the second
example of RF transmission line can be implemented in case of 2-layer PCB stack-up herein described.
35 um
35 um
35 um
35 um
270 um
270 um
760 um
L1 Copper
L3 Copper
L2 Copper
L4 Copper
FR-4 dielectric
FR-4 dielectric
FR-4 dielectric
380 um 500 um
500 um
Figure 28: Example of 50
Ω
coplanar waveguide transmission line design for the described 4-layer board layup
35 um
35 um
1510 um
L2 Copper
L1 Copper
FR-4 dielectric
1200 um 400 um
400 um
Figure 29: Example of 50
Ω
coplanar waveguide transmission line design for the described 2-layer board layup
If the two examples do not match the application PCB stack-up the 50
Ω
characteristic impedance calculation
can be made using the HFSS commercial finite element method solver for electromagnetic structures from Ansys
Corporation, or using freeware tools like AppCAD from Agilent (www.agilent.com) or TXLine from Applied
Wave Research (www.mwoffice.com), taking care of the approximation formulas used by the tools for the
impedance computation.
To achieve a 50
Ω
characteristic impedance, the width of the transmission line must be chosen depending on:
•
the thickness of the transmission line itself (e.g. 35 µm in the example of Figure 28 and Figure 29)
•
the thickness of the dielectric material between the top layer (where the transmission line is routed) and the
inner closer layer implementing the ground plane (e.g. 270 µm in Figure 28, 1510 µm in Figure 29)
•
the dielectric constant of the dielectric material (e.g. dielectric constant of the FR-4 dielectric material in
Figure 28 and Figure 29)
•
the gap from the transmission line to the adjacent ground plane on the same layer of the transmission line
(e.g. 500 µm in Figure 28, 400 µm in Figure 29)
