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LISA-U series - System Integration Manual
UBX-13001118 - R17
Advance information
Design-In
Page 145 of 190
Application Board
Antenna Assembly
Diagnostic Circuit
LISA-U 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 81: 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 53: 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 81, 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.
For example:
Consider a GSM antenna with built-in DC load resistor of 15 k
. Using the +UANTR AT command, the module
reports the resistance value evaluated from
ANT
connector to GND:
Reported values close to the used diagnostic resistor nominal value (i.e. values from 13 k
Ω
to 17 k
Ω
if a
15 k
Ω
diagnostic resistor is used) indicate that the antenna is properly connected
Values close to the measurement range maximum limit (approximately 50 k
Ω
) or an open-circuit
“over range” report (see
u-blox AT Commands Manual
[3]) means that that the antenna is not connected or
the RF cable is broken
Reported values below the measurement range minimum limit (1 k
Ω
) will highlight a short to GND at
antenna or along the RF cable