NORA-W30 series - System integration manual
UBX-22021119 - R02
Design-in
Page 19 of 52
C1-Public
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Coplanar microstrip: track separated with dielectric material and coupled to both the ground plane
and side conductor.
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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.
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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.
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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.
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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.
•
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 (e.g., switching supplies and digital lines)
and from any sensitive circuit to avoid crosstalk between RF traces and high 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 components on RF traces.
