User's Manual

TOBY-R2 series - System Integration Manual
UBX-16010572 - R04 Design-in
Page 89 of 147
 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 transmission lines 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 lines, if 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 transmission lines, as described in Figure 44,
 Ensure solid metal connection of the adjacent metal layer on the PCB stack-up to main ground layer,
providing enough vias 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 USB),
 Avoid stubs on the transmission lines,
 Avoid signal routing in parallel to transmission lines or crossing the transmission lines on buried metal layer,
 Do not route microstrip lines below discrete component or other mechanics placed on top layer
An example of proper RF circuit design is reported 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.
SMA Connector
Primary Antenna
SMA Connector
Secondary Antenna
TOBY
Figure 44: Example of circuit and layout for antenna RF circuits on application board