User's Manual

GSM/GPRS/GNSS Module Series
MC60 Hardware Design
MC60_Hardware_Design Confidential / Released 58 / 99
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 used for filtering out 1800MHz RF interference. However, 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 EGSM900MHz,
DCS1800MHz.
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 the 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 must be routed according to the differential signal layout rule.
3.8.2. Microphone Interfaces Design
AIN channels come with internal bias supply for external electret microphone. A reference circuit is shown
in the following figure.
Figure 30: Reference Design for AIN