SARA-G3 series - System Integration Manual
UBX-13000995 - R06
Objective Specification
Design-in
Page 143 of 218
2.6.1.2
Guidelines for microphone and loudspeaker connection circuit design (hands-free mode)
shows an application circuit for the analog audio interface in hands-free mode,
connecting a 2.2 k
electret microphone and an 8
or 4
loudspeaker:
External microphone can be connected to the uplink path of the module, since the module provides
supply and reference as well as differential signal input for the external microphone
Using a 8
or 4
loudspeaker for the hands-free mode, an external audio amplifier must be
provided on the application board to amplify the low power audio signal provided by the downlink path
of the module, so that the external audio amplifier will drive the 8
or 4
loudspeaker, since
differential analog audio output of the module is able to directly drive loads with resistance rating
greater than 14
, follow the general guidelines for the design of an analog
audio circuit for hands-free mode:
Provide proper supply to the used electret microphone, providing a proper connection from the
MIC_BIAS supply output to the microphone. It is suggested to implement a bridge:
o
The electret microphone, with its nominal intrinsic resistance value, represents one resistor of
the bridge.
o
To achieve good supply noise rejection, the ratio of the two resistance in one leg (R2/R3)
should be equal to the ratio of the two resistance in the other leg (R4/MIC), i.e. R2 has
to be equal to R4 (e.g. 2.2 k
) and R3 must be equal to the microphone nominal
intrinsic resistance value (e.g. 2.2 k
).
Provide a series resistor at the MIC_BIAS supply output and then mount a good bypass capacitor to
provide additional supply noise filtering, as the R1 series resistor (2.2 k
) and the C1 bypass
capacitor (10 µF).
Do not place a bypass capacitor directly at the MIC_BIAS supply output, since proper internal bypass
capacitor is already provided to guarantee stable operation of the internal regulator.
Connect the reference of the microphone circuit to the MIC_GND pin of the module as a sense line.
Provide a proper series capacitor at both MIC_P and MIC_N analog uplink inputs for DC blocking (C2
and C3 100 nF Murata GRM155R71C104K capacitors in
). This provides a high-
pass filter for the microphone DC bias with proper cut-off frequency according to the value of the
resistors of the microphone supply circuit. Then connect the signal lines to the microphone.
Provide proper parts on each line connected to the external microphone as noise and EMI
improvements, to minimize RF coupling and TDMA noise, according to the custom application
requirements.
o
Mount an 82 nH series inductor with self resonance frequency ~1 GHz (e.g. Murata
LQG15HS82NJ02) on each microphone line (L1 and L2 inductors in
o
Mount a 27 pF bypass capacitor (e.g. Murata GRM1555C1H270J) from each microphone
line to solid ground plane (C4 and C5 capacitors in
Use a microphone designed for GSM applications, which typically have internal built-in bypass
capacitor.
Provide a 47 nF series capacitor at both
SPK_P
and
SPK_N
analog downlink outputs for DC blocking
(C8 and C9 Murata GRM155R71C473K capacitors in
). Then connect the lines to
the differential input of a proper external audio amplifier, differential output which must be connected to
Formatted:
English (U.S.)
Formatted:
English (U.S.)
Formatted:
English (U.S.)
