Technical information

Chapter 2: Hardware Interface Description
November 15, 2008 G24-L Module Hardware Description 37
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Audio Design
The audio quality delivered by G24-L is highly affected by the application audio design,
particularly when using the analog audio interface. Therefore, special care must be taken when
designing the G24-L audio interface. Improper design and implementation of the audio interface
will result in poor audio quality.
Poor audio quality is a result of electrical interferences, or noises, from circuits surrounding the
audio interface. There are several possible sources for the audio noise:
• Transients and losses on the power supply
• EMI from antenna radiations
• Digital logic switching noise
Most of the audio noise originates from the GSM transmit burst current surges (217 Hz TDMA
buzz), which appear on the main power supply lines and antenna, but also indirectly penetrate the
internal application's supplies and signals. The noises are transferred into the G24's audio circuits
through the microphone input signals and then are amplified by the G24's internal audio
amplifiers.
To minimize the audio noise and improve the audio performance the microphone and speaker
signals must be designed with sufficient protection from surrounding noises.
The following guidelines should be followed to achieve best audio performance:
• Reference the microphone input circuits to the G24-L AGND interface signal.
• If using single-ended audio outputs, they should be referenced to the G24-L AGND interface
signal.
• Keep the audio circuits away from the antenna.
• Use RF filtering capacitors on the audio signals, as described in Table 2-3.
• The audio signals should not be routed adjacent to digital signals.
• Isolate the audio signals by a surrounding ground plane or shields.
• Filter internal supplies and signals that may indirectly affect the audio circuits, from noises
and voltage drops.
Table 2-15:
Microphone and Headset Microphone Port Gains
AT Command
Value
0dBm0 Input
Level
(mVRMS)
Maximum Input
Level (mV
RMS
)
0 87 350
... ... ...
3 63 250
... ... ...
16 15 55
... ... ...
31 4 11