UMTS/HSDPA Module
UC15 Hardware Design
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3.10.1. Decrease TDD noise and other noise
The 33pF capacitor is applied for filtering out 900MHz RF interference when the module is transmitting at
GSM900MHz. TDD noise could be heard without this capacitor. 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, customer would have to discuss with its
capacitor vendor to choose the most suitable capacitor for filtering out GSM850MHz, GSM900MHz,
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, GSM850/GSM900 TDD noise is more severe; while in
other cases, DCS1800/PCS1900 TDD noise is more obvious. Therefore, customer 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 rules.
3.10.2. Microphone interfaces configuration
MIC1/MIC2 channels come with internal bias supply for external electret microphone. A reference circuit
is shown in the following figure
.