LISA-U2 series - System Integration Manual
UBX-13001118 - R19
Early Production Information
Design-In
Page 118 of 175
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 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 µm
400 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 AppCAD from Agilent or TXLine from Applied Wave Research, 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 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 µm in Figure 57, 1510 µm 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 µm in Figure 57, 400 µm 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
