NINA-B4 series - System integration manual
UBX-19052230 - R09
Design-in
Page 25 of 61
C1-Public
shows the design options for PCB transmission lines, where:
•
Microstrip
: trace coupled to a single ground plane, separated by dielectric material.
•
Coplanar microstrip
: trace coupled to ground plane and adjacent conductors, separated by
dielectric materials).
•
Stripline
: trace sandwiched between two parallel ground planes, separated by dielectric materials).
Figure 6: Transmission line trace design
Observe the following comments to design a proper 50
transmission line:
•
The designer shall provide enough clearance from adjacent traces and ground in the same layer.
The trace-to-ground clearance should be at least twice as wide as the trace width. The
transmission line should
be ‘guarded’
with ground planes on each side.
•
The characteristic impedance can be calculated as a first iteration by using tools provided by the
layout software. It is advisable to ask the PCB manufacturer for the final values that are usually
calculated during the PCB production process using dedicated software and the available stack-
ups. To measure the real impedance of the traces, it might also be possible to request that an
impedance coupon be attached to the side of the panel.
•
Despite the high losses anticipated at high frequencies, an FR-4 dielectric material can be
considered in the RF designs, providing that:
o
RF trace lengths are minimized to reduce dielectric losses.
o
If traces longer than a few centimeters are needed, coaxial connectors and cables are used to
reduce the anticipated losses.
o
To ensure good impedance control during the PCB manufacturing process, the PCB stack-ups
allow for wide 50
traces of at least 200
µ
m.
o
FR-4 material exhibits poor thickness stability with less control of impedance over the trace
length. Contact the PCB manufacturer for specific tolerance of controlled impedance traces.
•
The width and spacing of the transmission lines to GND must be uniform and routed as smoothly
as possible. Route RF lines in arcs or at 45° angles.
•
Add GND stitching vias around transmission lines.
•
Include sufficient vias to ensure that a low-impedance connection is made between the main
ground layer and the adjacent metal layer on the PCB stack-up.
•
To avoid crosstalk between RF traces and high-impedance or analog signals, route RF
transmission lines far away from noise sources (like switching supplies and digital lines) and
sensitive circuits.
•
Avoid stubs on the transmission lines; impedance matching components on the transmission line
should be placed with the connected pad over the trace.
•
Avoid unnecessary component on RF traces.