LARA-R2 series - System Integration Manual
UBX-16010573 - R12
Design-in
Page 91 of 157
the dielectric constant of the dielectric material (e.g. dielectric constant of the FR-4 dielectric material in Figure
42 and Figure 43)
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 42, 400 µm in Figure 43)
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.
Additionally to the 50
impedance, the following guidelines are recommended for the transmission 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.
Add GND keep-out (i.e. clearance, a void area) on buried metal layers below any pad of component present
on the RF transmission line, if top-layer to buried layer dielectric thickness is below 200 µm, to reduce parasitic
capacitance to ground.
The transmission line width and spacing to GND must be uniform and routed as smoothly as possible: avoid
abrupt changes of width and spacing to GND.
Add GND vias around transmission line, as described in Figure 44.
Ensure solid metal connection of the adjacent metal layer on the PCB stack-up to the main ground layer,
providing enough on the adjacent metal layer, as described in Figure 44.
Route RF transmission lines far from any noise source (as switching supplies and digital lines) and from any
sensitive circuit (as analog audio lines).
Avoid stubs on the transmission line.
Avoid signal routing in parallel to the transmission line or crossing the transmission line on buried metal layer.
Do not route the microstrip line below discrete components or other mechanics placed on the top layer.
An example of proper RF circuit design is illustated in Figure 44. In this case, the
ANT1
and
ANT2
pins are directly
connected to SMA connectors by means of proper 50
transmission lines, designed with proper layout.
LARA
SMA
SMA
Figure 44: Example of the circuit and layout for antenna RF circuits on the application board
