JODY-W2 - System integration manual
UBX-18068879 - R14
Design-in
Page 21 of 84
C1 - Public
RF transmission line design
RF transmission lines such as the ones from the
ANT0
and
ANT1
pins up to the related antenna
connectors must be designed so that the characteristic impedance is as close as possible to 50
.
shows the design options for implementing a transmission line, namely:
•
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. This in the most common transmission line implementation.
•
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 “
ε
r
”
and “H”
, which must be calculated for each PCB
layer stack-up
.
Figure 6: Transmission line trace design
Follow these recommendations to design a 50
transmission line correctly:
•
Designers must provide enough clearance from surrounding traces and ground in the same layer.
In general, the trace to ground clearance should be at least twice that of the trace width. The
transmission line should also be “guarded” by the ground plane area on each side.
•
In the first iteration, calculate the characteristic impedance using tools provided by the layout
software. Ask the PCB manufacturer to provide the final values usually calculated using dedicated
software and production stack-ups. It is sometimes possible to request an impedance test coupon
on side of the panel to measure the real impedance of the traces.
