Reference Circuit Design For Loudspeaker Interface; Audio Signal Design Considerations - Quectel SC200L Series Hardware Design

Smart Module Series

3.20.4. Reference Circuit Design for Loudspeaker Interface

C1
NM
F1
SPK_P EARP
F2
EA
SPK_N
RN
C4 C5 D1 D2
C2 C3
NM
NM
GND
Module
Figure 28: Reference Circuit Design for Loudspeaker Interface

3.20.5. Audio Signal Design Considerations

It is recommended to use the electret microphone with dual built-in capacitors (e.g. 10 pF and 33 pF) to
filter out RF interference, thus reducing TDD noise. The 33 pF capacitor is applied to filter out RF
interference when the module is transmitting at EGSM900. Without this capacitor, TDD noise could be
heard during voice calls. The 10 pF capacitor is used to filter out RF interference at DCS1800. Please
note that the resonant frequency point of a capacitor largely depends on the material and production
technique. Therefore, you should consult the capacitor vendors to choose the most suitable capacitor to
filter out the high-frequency noises.
The severity of the RF interference in the voice channel during GSM transmitting largely depends on the
application design. In some cases, EGSM900 TDD noise is more severe; while in other cases, DCS1800
TDD noise is more obvious. Therefore, a suitable capacitor should be selected based on test results.
Sometimes, even no RF filtering capacitor is required. .
To decrease radio or other signal interference, RF antennas should be placed away from audio interfaces
and audio traces. Additionally, keep power traces far away from the audio traces and do not route them in
parallel.
The differential audio traces must be routed according to differential signal layout rules.
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