UMTS/HSPA Module Series
WCDMA UGxx Audio Design Note
WCDMA_UGxx_Audio_Design_Note Confidential / Released 18 / 56
4.3. TDD Noise
It is important to avoid or reduce TDD noise in audio circuit design and layout, some suggestions are
listed below:
1. Different capacitors have their own self-resonant frequencies due to the different fabrication
processes and materials. Generally, we choose 33pF capacitor (0603 package) to filter GSM900
interference and 10pF capacitor (0603 package) to filter DCS1800 interference on the power and
analog audio signal lines. It is strongly recommended to add an electrets-microphone within two
capacitors (10pF and 33pF) in handset and hands-free application. These two capacitors could
largely suppress coupling TDD noise from RF interference.
2. Capacitors should be placed close to audio components or audio interface, and layout must be short.
3. Flood ground area should be as large as possible to reduce the ground impedance and improve
ground integrality.
4. Reduce power voltage ripple, especially the power supply in audio circuits. A wide layout is used from
power source (like adapter interface, battery connector, or LDO output pin) to audio power supply.
Good antenna matching is also important to reduce power ripple.
5. The filtering capacitors and ESD protection devices should be connected to main digital ground.
Separate analog ground and digital ground, and connect them at a single point on the PCB by one 0
ohm resistor to reduce digital interference and background noise.
6. Antenna must be stay away from audio components and layout of audio circuit. Antenna should be
kept at least 5cm away from microphone.
7. Layout of power supply must be stay away from audio and cannot be parallel.
4.4. Mechanical Design
It is important to consider how to suppress echo in the equipment with hands-free function or in an
application of which microphone and speaker are very close to each other.
The mechanical structure design has significant impact on echo issue. If it is not properly designed, the
echo suppressing arithmetic in software could not make up for echo issue caused by bad mechanical
structure, and even force to redesign.
Echo issue could be generated by several paths, as shown in Figure 11.
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