Reference Circuit Design For Loudspeaker Interface; Design Considerations For Audio Interfaces; Usb_Boot Control Interface - Quectel SC262R Series Hardware Design

Smart Module Series

3.22.4. Reference Circuit Design for Loudspeaker Interface

F1
SPK_P
EARP
F2
EA
SPK_N
RN
D1 D2
C1 C2
33 pF
33 pF
Module
Figure 28: Reference Circuit Design for Loudspeaker Interface

3.22.5. Design Considerations for Audio Interfaces

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 its material and manufacturing
technique. Therefore, you should consult the capacitor vendors to choose the most suitable capacitor to
filter out the high-frequency noises.
The severity of 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, you should select a suitable capacitor according to the test results.
Sometimes, even no RF filtering capacitor is required.
In order to decrease radio or other signal interference, place RF antennas away from audio interfaces and
audio traces. Additionally, keep power traces far away from the audio traces and do not route them in
parallel.
Route the differential audio traces according to the differential signal layout rule.

3.23. USB_BOOT Control Interface

USB_BOOT is an emergency download interface. You can force the module to enter emergency
download mode by pulling it up to LDO5_1V8 during power-on. This is an emergency option when failures
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