Speaker Interface Reference Circuit; Audio Signal Design Considerations - Quectel SC200L-EM Hardware Design

3.20.4 Speaker interface reference circuit

3.20.5 Audio signal design considerations

It is recommended to use the electret microphone with dual built-in capacitors (e.g. 10pF and 33pF) for
filtering out RF interference, thus reducing TDD noise. The 33pF capacitor is applied for filtering out RF
interference when the module is transmitting at EGSM900. Without placing this capacitor, TDD noise
could be heard. The 10pF capacitor here is used for filtering out RF interference at DCS1800. Please note
that the resonant frequency point of a capacitor largely depends on the material and production technique.
Therefore, customers would have to discuss with their capacitor vendors to choose the most suitable
capacitor for filtering out high-frequency noises.
The severity degree 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 the test results. Sometimes, even no RF filtering capacitor is required.
In order to decrease radio or other signal interference, RF antennas should be placed away from
audio interfaces and audio traces. Power traces cannot be parallel with and also should be far away from
the audio traces.
The differential audio traces must be routed according to the differential signal layout rule.
SC200L_Hardware_Design
Figure 25: Speaker interface reference circuit
SC200L Hardware Design
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