Integration Manual

LISA-U2 series - System Integration Manual
UBX-13001118 - R25 Design-In Page 122 of 182
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 57: 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 58: 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 Avago / Broadcom
AppCAD (https://www.broadcom.com/appcad), 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 µ m in the example of Figure 57 and Figure 58)
 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 µ m in Figure 57,
1510 µ m in Figure 58)
 the dielectric constant of the dielectric material (e.g. dielectric constant of the FR-4 dielectric
material in Figure 57 and Figure 58)
 the gap from the transmission line to the adjacent ground plane on the same layer of the
transmission line (e.g. 500 µ m in Figure 57, 400 µ m in Figure 58)
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 microstrip, use the “Coplanar Waveguide” model for the 50 
calculation.
Additionally to the 50  impedance, the following guidelines are recommended for the RF line design:
 Minimize the transmission line length; the insertion loss should be minimized as much as possible,
in the order of a few tenths of a dB
 The transmission line should not have abrupt change to thickness and spacing to GND, but must
be uniform and routed as smoothly as possible