SARA-N2 / N3 series - System integration manual
UBX-17005143 - R13
Design-in
Page 48 of 95
C1-Public
If the distance between the transmission line and the adjacent GND area (on the same layer) does not
exceed 5 times the track width of the micro strip, use the “Coplanar Waveguide” model for the 50
calculation.
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
µ
m, 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
•
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
•
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 the
, 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
•
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 of the
modules
SARA module
SMA
connector
SARA module
SMA
connector
High-pass filter
for ANT port
ESD immunity increase
Figure 27: Suggested circuit and layout for antenna RF circuit on application board, if antenna detection is not required
