User's Manual

SARA-R4 series - System Integration Manual

UBX-16029218 - R03 Design-in

Page 46 of 94

Figure 20 and Figure 21 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

270 µm

760 µm

L1 Copper

L3 Copper

L2 Copper

L4 Copper

FR-4 dielectric

380 µm 500 µm500 µm

Figure 20: Example of 50  coplanar waveguide transmission line design for the described 4-layer board layup

35 µm

1510 µm

L2 Copper

L1 Copper

FR-4 dielectric

1200 µm 400 µm400 µm

Figure 21: 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 20 and Figure 21)

 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 20, 1510 µm in Figure 21)

 the dielectric constant of the dielectric material (e.g. dielectric constant of the FR-4 dielectric material in

Figure 20 and Figure 21)

 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 20, 400 µm in Figure 21)

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.