Decrease Tdd Noise And Other Noise; Microphone Interfaces Design - Quectel M35 User Manual

3.8.1. Decrease TDD Noise and Other Noise

The 33pF capacitor is applied for filtering out 900MHz RF interference when the module is transmitting at
EGSM900MHz. Without placing this capacitor, TDD noise could be heard. Moreover, the 10pF capacitor
here is for filtering out 1800MHz RF interference. However, the resonant frequency point of a capacitor
largely depends on the material and production technique. Therefore, you would have to discuss with its
capacitor vendor to choose the most suitable capacitor for filtering out GSM850MHz, EGSM900MHz,
DCS1800MHz and PCS1900MHz separately.
The severity degree of the RF interference in the voice channel during GSM transmitting period 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 can have a choice based on test results.
Sometimes, even no RF filtering capacitor is required.
The capacitor which is used for filtering out RF noise should be close to audio interface. Audio alignment
should be as short as possible.
In order to decrease radio or other signal interference, the position of RF antenna should be kept away
from audio interface and audio alignment. Power alignment and audio alignment should not be parallel,
and power alignment should be far away from audio alignment.
The differential audio traces have to be placed according to the differential signal layout rule.

3.8.2. Microphone Interfaces Design

AIN1 and AIN2 channels come with internal bias supply for external electret microphone. A reference
circuit is shown in the following figure.
MICP
Module
MICN
M35_User_Manual
Close to Module
GND
33pF
10pF
Differential
0603
0603
10pF
33pF
0603
0603
10pF
33pF
0603
0603
GND
Figure 24: Reference Design for AIN1&AIN2
Confidential / Released
Close to
Microphone
GND
10pF
layout
0603
10pF
0603
10pF
0603
GND
GSM/GPRS Module Series
M35 User Manual
GND
GND
33pF
ESD
0603
33pF
0603
Electret
Microphone
33pF
0603
ESD
GND
GND
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