LEXI-R422 - System integration manual
UBX-23007449 - R02
Design-in
Page 62 of 108
C1-Public
The antenna detection circuit and diagnostic circuit suggested in
explained:
•
When antenna detection is forced by the +UANTR AT command, the
ANT_DET
pin generates a DC
current measuring the resistance (R2) from the antenna connector (J1) provided on the
application board to GND.
•
DC blocking capacitors are needed at the
ANT
pin (C2) and at the antenna radiating element (C4)
to decouple the DC current generated by the
ANT_DET
pin.
•
Choke inductors with a Self-Resonance Frequency (SRF) in the range of 1 GHz are needed in series
at the
ANT_DET
pin (L1) and in series at the diagnostic resistor (L3), to avoid a reduction of the
RF performance of the system, improving the RF isolation of the load resistor.
•
Resistor on the
ANT_DET
path (R1) is needed for accurate measurements through the +UANTR
AT command; it also acts as an ESD protection.
•
Additional components (C1 and D1 in
) are provided as
ANT_DET
pin as ESD protection.
•
Additional high pass filter (C3 and L2 in
) is provided as ESD immunity improvement.
•
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
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 (for example from 13 k
to
17 k
if a 15 k
diagnostic resistor is used) indicate that the antenna is correctly connected.
•
Values close to the measurement range maximum limit (approximately 50 k
) or an open-circuit
“over
range” report (s
) 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 unclean connection, a damaged antenna or incorrect value of the antenna load resistor for
diagnostics.
•
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
