SARA-G3 and SARA-U2 series - System Integration Manual
UBX-13000995 - R08
Objective Specification
Design-in
Page 106 of 188
Additional high pass filter (C3 and L2 in Figure 52) is provided at the
ANT
pin as ESD immunity improvement
for SARA-U2 modules (a series 0
jumper can be mounted for SARA-G340 and SARA-G350 modules
instead of the high pass filter, as no further precaution to ESD immunity test is needed).
The
ANT
pin must be connected to the antenna connector by means of a transmission line with nominal
characteristics impedance as close as possible to 50
The DC impedance at RF port for some antennas may be a DC open (e.g. linear monopole) or a DC short to
reference GND (e.g. PIFA antenna). For those antennas, without the diagnostic circuit of Figure 52, the measured
DC resistance is always at the limits of the measurement range (respectively open or short), and there is no mean
to distinguish between a defect on antenna path with similar characteristics (respectively: removal of linear
antenna or RF cable shorted to GND for PIFA antenna).
Furthermore, any other DC signal injected to the RF connection from ANT connector to radiating element will
alter the measurement and produce invalid results for antenna detection.
It is recommended to use an antenna with a built-in diagnostic resistor in the range from 5 k
to 30 k
to assure good antenna detection functionality and avoid a reduction of module RF performance. The
choke inductor should exhibit a parallel Self Resonance Frequency (SRF) in the range of 1 GHz to improve
the RF isolation of load resistor.
For example:
Consider an antenna with built-in DC load resistor of 15 k
. Using the +UANTR AT command, the module
reports the resistance value evaluated from the antenna connector provided on the application board to GND:
Reported values close to the used diagnostic resistor nominal value (i.e. values from 13 k
to 17 k
if a
15 k
diagnostic resistor is used) indicate that the antenna is properly connected
Values close to the measurement range maximum limit (approximately 50 k
) or an open-circuit
“over range” report (see
u-blox AT Commands Manual
[3]) means that that the antenna is not connected or
the RF cable is broken
Reported values below the measurement range minimum limit (1 k
) highlights a short to GND at antenna
or along the RF cable
Measurement inside the valid measurement range and outside the expected range may indicate an improper
connection, damaged antenna or wrong value of antenna load resistor for diagnostic
Reported value could differ from the real resistance value of the diagnostic resistor mounted inside the
antenna assembly due to antenna cable length, antenna cable capacity and the used measurement method
If the antenna detection function is not required by the customer application, the
ANT_DET
pin can be
left not connected and the
ANT
pin can be directly connected to the antenna connector by means of a
50
transmission line as described in Figure 51.
