u-blox LARA-R2 series System Integration Manual page 93

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 43, the measured
DC resistance is always at the limits of the measurement range (respectively open or short), and there is no
means 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 [2]) 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 primary / secondary antenna detection function is not required by the customer application, the
ANT_DET pin can be left not connected and the ANT1 / ANT2 pins can be directly connected to the
related antenna connector by means of a 50  transmission line as described in Figure 42.
2.4.2.2 Guidelines for ANT_DET layout design
The recommended layout for the primary antenna detection circuit to be provided on the application board to
achieve the primary antenna detection functionality, implementing the recommended schematic described in
Figure 43 and Table 34, is explained here:
 The ANT1 / ANT2 pins have to be connected to the antenna connector by means of a 50  transmission
line, implementing the design guidelines described in section 2.4.1 and the recommendations of the SMA
connector manufacturer.
 DC blocking capacitor at ANT1 / ANT2 pins (C2, C3) has to be placed in series to the 50  RF line.
 The ANT_DET pin has to be connected to the 50  transmission line by means of a sense line.
 Choke inductors in series at the ANT_DET pin (L1, L2) have to be placed so that one pad is on the 50 
transmission line and the other pad represents the start of the sense line to the ANT_DET pin.

The additional components (R1, C1 and D1) on the ANT_DET line have to be placed as ESD protection.
UBX-16010573 - R02
LARA-R2 series - System Integration Manual
Objective Specification Design-in
Page 93 of 148