Integration Manual

Table Of Contents
TOBY-L2 and MPCI-L2 series - System Integration Manual
UBX-13004618 - R26 Design-in
Page 92 of 162
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 .
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 46 and Figure 47 provide two examples of proper 50 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 µm
35 µm
35 µm
35 µm
270 µm
270 µm
760 µm
L1 Copper
L3 Copper
L2 Copper
L4 Copper
FR-4 dielectric
FR-4 dielectric
FR-4 dielectric
380 µm 500 µm500 µm
Figure 46: Example of 50 coplanar waveguide transmission line design for the described 4-layer board layup
35 µm
35 µm
1510 µm
L2 Copper
L1 Copper
FR-4 dielectric
1200 µm 400 µm400 µm
Figure 47: Example of 50 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 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 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 46 and Figure 47)
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 46, 1510 µm in Figure 47)
the dielectric constant of the dielectric material (e.g. dielectric constant of the FR-4 dielectric material in Figure
46 and Figure 47)