User's Manual

SARA-G3 and SARA-U2 series - System Integration Manual
UBX-13000995 - R18 Advance Information Design-in
Page 113 of 206
Figure 56 and Figure 57 provide two examples of proper 50 : coplanar waveguide designs. The first
transmission line can be implemented in case of 4-layer PCB stack-up herein described, the second 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 um500 um
Figure 56: Example of 50
:
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 um400 um
Figure 57: 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 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:
x the thickness of the transmission line itself (e.g. 35 μm in the example of Figure 56 and Figure 57)
x 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 56, 1510 μm in Figure 57)
x the dielectric constant of the dielectric material (e.g. dielectric constant of the FR-4 dielectric material in
Figure 56 and Figure 57)
x 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 56, 400 μm in Figure 57)
If the distance between the transmission line and the adjacent GND area (on the same layer) does not exceed 5
times the track width of the micro strip, use the “Coplanar Waveguide” model for the 50 : calculation.