SARA-R4 series - System Integration Manual
UBX-16029218 - R06
Design-in
Page 52 of 102
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 23,
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 23,
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
Two examples of proper RF circuit design are reported in Figure 23, where the antenna detection circuit is not
implemented (if the antenna detection function is required by the application, follow the guidelines for circuit
and layout implementation reported in section 2.4.2):
In the first example described on the left, the
ANT
pin is directly connected to an SMA connector by means
of a proper 50
transmission line, designed with proper layout.
In the second example described on the right, the
ANT
pin is connected to an SMA connector by means of a
proper 50
transmission line, designed with proper layout, with an additional high pass filter (consisting of
a proper series capacitor and a proper shunt inductor, as for example the Murata GRM1555C1H150JA01
15 pF capacitor and the Murata LQG15HN39NJ02 39 nH inductor with Self-Resonant Frequency ~1 GHz) to
improve the ESD immunity at the antenna port
SARA module
SMA
connector
SARA module
SMA
connector
High-pass filter to
improve ESD immunity
Figure 23: Example of circuit and layout for antenna RF circuits on application board
