Reference Circuit Design For Loudspeaker Interface; Audio Interfaces Design Considerations - Quectel SC600Y-EM Series Hardware Design

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3.23.4. Reference Circuit Design for Loudspeaker Interface

SPK_P

EARP

SPK_N

Module

Figure 31: Reference Circuit Design for Loudspeaker Interface

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 can 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.

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33pF

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