Decrease Tdd Noise And Other Noise; Microphone Interfaces Configuration - Quectel M12 Hardware Design

M12 Hardware Design

AIN1 and AIN2, which may be used for both microphone and line inputs. An electret
microphone is usually used. AIN1 and AIN2 are both differential input channels.

AOUT1 and AOUT2, which may be used for both receiver and speaker outputs. AOUT1
channel is typically used for a receiver, while AOUT2 channel is typically used for headset or
speaker. AOUT1 channel is a differential channel and AOUT2 is a single-ended channel.
SPK2P and AGND can establish a pseudo differential mode. Both AOUT1 and AOUT2
support voice and audio output, and so on.
 These two audio channels can be swapped by "AT+QAUDCH" command. For more details,
please refer to document [1].
Use AT command "AT+QAUDCH" to select audio channel:
0--AIN1/AOUT1 (normal audio channel), the default value is 0.
1--AIN2/AOUT2 (auxiliary audio channel).
For each channel, customer can use AT+QMIC to adjust the input gain level of microphone.
Customer can also use "AT+CLVL" to adjust the output gain level of receiver and speaker.
"AT+QSIDET" is to set the side-tone gain level. For more details, please refer to document [1].

3.9.1. 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 GSM900MHz and DCS1800MHz 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, GSM900 TDD noise is more severe;
while in other cases, DCS1800 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 rule.

3.9.2. Microphone interfaces configuration

AIN1/AIN2 channels come with internal bias supply for external electret microphone. A reference
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