TOBY-L2 and MPCI-L2 series - System Integration Manual
UBX-13004618 - R07
Advance Information
Design-in
Page 91 of 158
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 45 and Figure 46 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 µm
500 µm
Figure 45: 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 µm
400 µm
Figure 46: 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 45 and Figure 46)
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 45, 1510 µm in Figure 46)
the dielectric constant of the dielectric material (e.g. dielectric constant of the FR-4 dielectric material in
Figure 45 and Figure 46)
