User's Manual

SARA-R4 series - System Integration Manual
UBX-16029218 - R03 Design-in
Page 47 of 94
Additionally to the 50  impedance, the following guidelines are recommended for transmission lines design:
 Minimize the transmission line length: the insertion loss should be minimized as much as possible, in the
order of a few tenths of a dB,
 Add GND keep-out (i.e. clearance, a void area) on buried metal layers below any pad of component present
on the RF transmission lines, if top-layer to buried layer dielectric thickness is below 200 µm, to reduce
parasitic capacitance to ground,
 The transmission lines width and spacing to GND must be uniform and routed as smoothly as possible: avoid
abrupt changes of width and spacing to GND,
 Add GND stitching vias around transmission lines, as described in Figure 22,
 Ensure solid metal connection of the adjacent metal layer on the PCB stack-up to main ground layer,
providing enough vias on the adjacent metal layer, as described in Figure 22,
 Route RF transmission lines far from any noise source (as switching supplies and digital lines) and from any
sensitive circuit (as USB),
 Avoid stubs on the transmission lines,
 Avoid signal routing in parallel to transmission lines or crossing the transmission lines on buried metal layer,
 Do not route microstrip lines below discrete component or other mechanics placed on top layer
An example of proper RF circuit design is reported in Figure 22. In this case, the ANT pin is directly connected to
SMA connectors by means of proper 50  transmission lines, designed with proper layout.
Figure 22: Example of circuit and layout for antenna RF circuits on application board
Guidelines for RF termination design
The RF termination must provide a characteristic impedance of 50  as well as the RF transmission line up to the
RF termination, to match the characteristic impedance of the ANT port.
However, real antennas do not have perfect 50  load on all the supported frequency bands. Therefore, to
reduce as much as possible performance degradation due to antennas mismatch, the RF termination must
provide optimal return loss (or V.S.W.R.) figure over all the operating frequencies, as summarized in Table 7.
SARA module
SMA
connector