LISA-U1 series - System Integration Manual
3G.G2-HW-10002-3
Preliminary
System description
Page 71 of 125
1.11.1.5
Hands-free mode
The hands-free profile is configured when the uplink audio path is set to “Hands-free microphone” and the
downlink audio path is set to “Loudspeaker” (refer to
u-blox
AT commands manual
[2]: AT+USPM command:
<main_uplink>, <main_downlink> parameters):
Hands-free microphone signal must be connected to the module differential input
MIC_P / MIC_N
High power loudspeaker must be connected to the output of an external audio amplifier, for which the
input must be connected to the module differential output
SPK_P / SPK_N
The module differential analog audio output is not able to drive an 8 speaker: an external audio amplifier
must be provided on the application board to amplify the low power audio signal provided by the module
differential output
SPK_P / SPK_N
.
Hands-free functionality is implemented using appropriate digital signal processing algorithms for voice-band
handling (echo canceller and automatic gain control), managed via software (refer to
u-blox
AT commands
manual
[2], AT+UHFP command).
Figure 39 shows an example of an application circuit connecting a 2.2 k electret microphone and an 8
speaker to the LISA-U120 and LISA-U130 modules, with an external low noise LDO voltage regulator to provide
a proper supply for the microphone and with an external audio amplifier to amplify the low power audio signal
provided by the module differential output.
Mount an 82 nH series inductor (e.g. Murata LQG15HS82NJ02) on each microphone line and a 27 pF
bypass capacitor (e.g. Murata GRM1555C1H270J) on all audio lines to minimize RF coupling and TDMA
noise.
The physical width of the audio outputs lines on the application board must be wide enough to
minimize series resistance.
C1
C2
C3
L1
39
MIC_N
53
SPK_P
40
MIC_P
54
SPK_N
D1
Microphone
Connector
D2
IN
OUT
GND
Low Noise
LDO Regulator
U1
R4
R1
C6
R3
R2
C5
2V5
Sense lines connected to GND in one star point
C4
SPK
L2
MIC
Speaker
Connector
OUT+
IN+
GND
VMAIN
U2
OUT-
IN-
C8
C9
R5
R6
VDD
C11
C10
LISA-U120/U130
Audio
Amplifier
J1
J2
VMAIN
C7
Figure 40: Hands-free mode application circuit
