Rf Transmission Line Design - Ublox JODY-W1 Series System Integration Manual

2.2.1

RF transmission line design

RF transmission lines such as the ones from the ANT0, ANT1 and ANT2 pins up to the related
antenna connectors must be designed so that the characteristic impedance is as close as possible to
50  (max. ±10%).
Figure 5
account when implementing a transmission line on a PCB:
• Microstrip (track coupled to a single ground plane, separated by dielectric material),
• Coplanar microstrip (track coupled to ground plane and side conductors, separated by dielectric
material).
• Stripline (track sandwiched between two parallel ground planes, separated by dielectric material).
The coplanar microstrip is the most common configuration for a printed circuit board (PCB).
Figure 5: Transmission line trace design
To properly design a 50  transmission line, the following remarks should be considered:
• 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, and the transmission line should 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.
• FR-4 dielectric material, although its high losses at high frequencies can be considered in RF
designs providing that:
RF trace length must be minimized to reduce dielectric losses.
o
If traces longer than few centimeters are needed, it is recommended to use a coaxial connector
o
and cable to reduce losses.
Stack-up should allow for wide 50  traces and at least 200 µm trace width is recommended
o
to assure good impedance control over the PCB manufacturing process.
FR-4 material exhibits poor thickness stability and thus less control of impedance over the
o
trace length. Contact the PCB manufacturer for specific tolerance of controlled impedance
traces.
UBX-16012621 - R18
C1-Public
shows the design options and the main parameters to be taken into
Design-in
JODY-W1 series - System integration manual
Page 22 of 72