NINA-B50 series - Hardware integration manual
UBX-22021116 - R02
Design-in
Page 21 of 57
C1-Public
3.2.2
RF transmission line design
RF transmission lines connecting the
ANT
pad with the related antenna connector or antenna, must
be designed with a 50
impedance characteristic.
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
: track separated by dielectric material and sandwiched between two parallel ground
planes.
The parameters shown in the cross-sectional area of each trace design include:
•
Width (W)
–
shows the width of the copper layer on the top layer
•
Distance (S)
–
shows the distance between the top copper layer and the two adjacent GND planes.
•
Dielectric substrate thickness (H)
–
shows the distance between the GND reference on the bottom
plane and the copper layer on the top layer.
•
Thickness of the copper layer (T)
–
can
also be represented by “Base Copper Weight”, which is
commonly used as the parameter for PCB stack-up.
Dielectric constant
(
ε
r
) defines the ratio between the electric permeability of the material against the
electric permeability of free space.
☞
The width of a 50 Ω microstrip depends on mainly “
ε
r
”
and “H”
, which must be calculated for each
PCB layer stack-up.
Figure 7: 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 transmi
ssion 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.
