u-blox LARA-R203 System Integration Manual page 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 .
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 boards.
Figure 42 and Figure 43 provide two examples of proper 50  coplanar waveguide designs. The first example of
an RF transmission line can be implemented for a 4-layer PCB stack-up herein described, and the second example
of an RF transmission line can be implemented for a 2-layer PCB stack-up herein described.
L1 Copper
FR-4 dielectric
L2 Copper
FR-4 dielectric
L3 Copper
FR-4 dielectric
L4 Copper
Figure 42: Example of a 50  coplanar waveguide transmission line design for the described 4-layer board layup
L1 Copper
FR-4 dielectric
L2 Copper
Figure 43: Example of a 50  coplanar waveguide transmission line design for the described 2-layer board layup
If the two examples do not match the application PCB layup, 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 examples of Figure 42 and Figure 43)

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 42, 1510 µm in Figure 43)
UBX-16010573 - R12
500 µm 380 µm 500 µm
400 µm 1200 µm 400 µm
LARA-R2 series - System Integration Manual
35 µm
270 µm
35 µm
760 µm
35 µm
270 µm
35 µm
35 µm
1510 µm
35 µm
Design-in
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