SARA-R4/N4 series - System Integration Manual
UBX-16029218 - R11
Design-in
Page 89 of 157
R2
15 k
Resistor for Diagnostics
Various Manufacturers
Table 25: Suggested parts for antenna detection circuit on application PCB and diagnostic circuit on antennas assembly
The antenna detection circuit and diagnostic circuit suggested in
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 needed at the
ANT_DET
pin as ESD protection.
Additional high pass filter (C3 and L2 in
) is provided at the
ANT
pin 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:
