LISA-U2 series - System Integration Manual
UBX-13001118 - R19
Early Production Information
Design-In
Page 134 of 175
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
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 built-in resistor from
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 reported 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 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
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 load resistor.