Reference Circuit Design For Loudspeaker Interface; Audio Interfaces Design Considerations - Quectel SC20 Series Hardware Design

Smart Module Series

3.22.4. Reference Circuit Design for Loudspeaker Interface

Figure 29: Reference Circuit Design for Loudspeaker Interface

3.22.5. Audio Interfaces 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 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.
The capacitor which is used for filtering out RF noise should be close to the audio device or audio
interface. The trace should be as short as possible, and it is recommended to route the trace for
capacitors first and then for other points.
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.
SC20_Series_Hardware_Design 67 / 133