Ublox LISA-U2 Series, System Integration Manual

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LISA-U2 series - System integration manual
UBX-13001118 - R27 Design-In Page 138 of 183
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2.4.4 Antenna detection functionality
The internal antenna detect circuit is based on ADC measurement at ANT : the RF port is DC coupled
to the ADC unit in the baseband chip which injects a DC current (10 µA for 128 µs) on ANT and
measures the resulting DC voltage to evaluate the resistance from the ANT pad to GND.
The antenna detection is forced by the +UANTR AT command
:
see the u-blox AT commands
manual [2] for more details on how to access this feature.
To achieve antenna detection functionality, use an RF antenna with a built-in resistor from the ANT
signal to GND, or implement an equivalent solution with a circuit between the antenna cable
connection and the radiating element as shown in
Figure 67 .
Application Board Antenna Assembly
Diagnostic Circuit
LISA-U2 series
ADC
Current
Source
RF
Choke
DC
Blocking
Front-End
RF Module
RF
Choke
DC
Blocking
Radiating
Element
Zo=50
Ω
Resistor for
Diagnostic
Coaxial Antenna Cable
ANT
Figure 67: Antenna detection circuit and antenna with diagnostic resistor
Examples of components for the antenna detection diagnostic circuit are listed in the following table:
Description Part Number - Manufacturer
DC Blocking Capacitor Murata GRM1555C1H220JA01 or equivalent
RF Choke Inductor Murata LQG15HS68NJ02, LQG15HH68NJ02 or equivalent (Self Resonance Frequency ~1GHz)
Resistor for Diagnostic 15 k  5%, various Manufacturers
Table 50: Example of components for the antenna detection diagnostic circuit
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 67 ,
the measured DC resistance will always be at the limits of the measurement range (respectively open
or short), and there will be no means to distinguish between a defect on the antenna path with similar
characteristics (respectively: removal of a linear antenna or RF cable shorted to GND for a PIFA
antenna).
Furthermore, any other DC signal injected to the RF connection from the ANT connector to the
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
Ω
to
30 k
Ω
to assure good antenna detection functionality and to 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 the load resistor.