NORA-W36 series - System integration manual
UBX-22021120 - R01
Design-in
Page 14 of 48
C1-Public
3.2.1
RF transmission line design (NORA-W361)
RF transmission lines, such as the ones from the
ANT
pad up to the related internal antenna pad,
must be designed so that the characteristic impedance is as close as possible to 50
.
Design options and the most important parameters for implementing a transmission line on a PCB
are described below:
•
Microstrip: track separated with dielectric material and coupled to a single ground plane.
•
Coplanar microstrip: track separated with dielectric material and coupled to both the ground plane
and side conductor.
•
Stripline: track separated by dielectric material and sandwiched between two parallel ground
planes.
Figure 3: Transmission line trace design
Follow these recommendations to design a 50
transmission line correctly:
•
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 also 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, although it has high losses at high frequencies, can be considered in RF
designs provided that:
o
RF trace length must be minimized to reduce dielectric losses.
o
If traces longer than a few centimeters are needed, it is recommended to use a coaxial
connector and cable to reduce losses
o
Stack-up should allow for thick 50
traces and at least 200 µm trace width is recommended
to assure good impedance control over the PCB manufacturing process.
o
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 the GND must be uniform and routed as smoothly as
possible: route RF lines in arcs (preferred) or 45° angles.
•
Add GND stitching vias around transmission lines.
