Integration Manual

TSD-19090601 - R15 Contents Page 82 of 143
Additionally to the 50  impedance, the following guidelines are recommended for 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 a component
present on the RF transmission lines, if the top-layer to buried layer dielectric thickness is below 200 µm,
to reduce parasitic capacitance to ground.
 The transmission lines 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 stitching vias around the transmission lines, as shown in Figure 22.
 Ensure a solid metal connection of the adjacent metal layer on the PCB stack-up to the main ground
layer, providing enough vias on the adjacent metal layer, as shown in Figure 22.
 Route RF transmission lines far from any noise source (as switching supplies and digital lines) and from
any sensitive circuit (as USB).
 Avoid stubs on the transmission lines.
 Avoid signal routing in parallel to the transmission lines or crossing the transmission lines on a buried
metal layer.
 Do not route the microstrip lines below discrete components or other mechanics placed on the top
layer.
An example of a suitable RF circuit design is illustrated in Figure 22. In this case, the ANT1 and ANT2 pins
are directly connected to SMA connectors by means of suitable 50  transmission lines, designed with a
suitable layout.
SMA Connector
Primary Antenna
SMA Connector
Secondary Antenna
TOBY-L3
Figure 22: Example of the circuit and layout for antenna RF circuits on the application board