ANNA-B4 series - System integration manual
UBX-21000517 - R03
Contents
Page 24 of 74
C1-Public
RF transmission line design
RF transmission lines, such as those that connect from the RF
pin to the antenna or antenna
connector, must be designed with a characteristic impedance of 50
.
shows the design options and the most important parameters for designing a transmission
line on a PCB:
•
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.
A coplanar microstrip is the most common configuration for a printed circuit board (PCB).
Figure 9: 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.
•
Although FR-4 dielectric material can result in high losses at high frequencies, it can still be an
appropriate choice for RF designs. In which case, aim to:
o
Minimize RF trace lengths to reduce dielectric losses.
o
If traces longer than few centimeters are needed, use a coaxial connector and cable to reduce
losses.
o
For good impedance control over the PCB manufacturing process, design the stack-up with
wide 50
traces with width of at least 200 µm.
o
To make the trace less lossy and to have better impedance control the trace shall be made
wide. This is achieved by increasing the distance to reference GND by clearing copper in the
inner layers closest under the trace.