SARA-G3 and SARA-U2 series - System Integration Manual
UBX-13000995 - R26
Design-in
Page 126 of 217
Additional high pass filter (C3 and L2 in Figure 60) is provided at the
ANT
pin as an ESD immunity
improvement for SARA-U260, SARA-U270 and SARA-U280 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 60, 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 the
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 the
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 the wrong value resistance of the antenna load resistor for diagnostics.
The 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 measurement method used.
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 59.
