ANNA-B112 - System integration manual
UBX-18009821 - R09
Design-in
Page 42 of 66
C1-Public
Figure 26: Transmission line trace design
To properly design a 50
Ω
transmission line, the following points should be considered:
•
The designer should provide enough clearance from surrounding traces and ground in the same
layer; in general, a trace to ground clearance of at least two times the trace width should be
considered. The transmission line should be ‘guarded’ by ground plane area on each side.
•
The characteristic impedance can be calculated as first iteration using tools provided by the layout
software. It is advisable to ask the PCB manufacturer to provide the final values that are usually
calculated using dedicated software and available stack-ups from production. It could also be
possible to request an impedance coupon on panel’s side to measure the real impedance of the
traces.
•
FR-4 dielectric material has high losses at high frequencies, but can be considered in RF designs
provided that the RF trace length is minimized to reduce dielectric losses.
•
If traces longer than few centimeters are needed, a coaxial cable and connector are recommended
to reduce losses.
•
Stack-up should allow for thick 50
Ω
traces and at least 200
µ
m trace width is recommended to
assure good impedance control over the PCB manufacturing process.
•
FR-4 material exhibits poor thickness stability and thus less control of impedance over the trace
length. Contact the PCB manufacturer for specific tolerance of controlled impedance traces.
•
The transmission lines’ width and spacing to GND must be uniform and routed as smoothly as
possible: route RF lines in 45 ° angle or in arcs.
•
Add GND stitching vias around transmission lines.
•
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.
•
Route RF transmission lines far from any noise source (such as switching supplies and digital lines)
and any sensitive circuit to avoid crosstalk between RF traces and Hi-impedance or analog signals.
•
Avoid stubs on the transmission lines; any component on the transmission line should be placed
with the connected pad over the trace. Also avoid any unnecessary component on RF traces.
RF connector design
If an external antenna is required, the designer should consider using a proper RF connector. It is the
responsibility of the designer to verify the compatibility between plugs and receptacles used in the
design.
