SARA-G3 and SARA-U2 series - System Integration Manual
UBX-13000995 - R26
Design-in
Page 121 of 217
Additionally to the 50
impedance, the following guidelines are recommended for the transmission line 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 line, if top-layer to buried layer dielectric thickness is below 200 µm, to reduce
parasitic capacitance to ground.
The transmission line 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 vias around transmission line, as described in Figure 59.
Ensure solid metal connection of the adjacent metal layer on the PCB stack-up to main ground layer,
providing enough on the adjacent metal layer, as described in Figure 59.
Route RF transmission line far from any noise source (as switching supplies and digital lines) and from any
sensitive circuit (as analog audio lines).
Avoid stubs on the transmission line.
Avoid signal routing in parallel to transmission line or crossing the transmission line on buried metal layer.
Do not route microstrip line below discrete component or other mechanics placed on top layer.
Two examples of proper RF circuit design are reported in Figure 59, 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) to improve the ESD immunity at the antenna port of
SARA-U260, SARA-U270 and SARA-U280 modules as described in section 2.13 (a series 0
jumper can be
mounted for SARA-G3 modules instead of the high pass filter as no further precaution to ESD immunity test
is needed).
SARA module
SMA
connector
SARA module
SMA
connector
High-pass filter for
SARA-U2 ANT port
ESD immunity increase
Figure 59: Suggested circuit and layout for antenna RF circuit on application board, if antenna detection is not required
