Smart Module Series
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3.22.4. Reference Circuit Design for Loudspeaker Interface
EARP
EA
RN
F2
SPK_P
SPK_N
33 pF
33 pF
C1
C2
F1
Module
D1
D2
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
