GSM/GPRS Module
M85 Hardware Design
M85_Hardware_Design Confidential / Released 46 / 92
Use AT command ―AT+QAUDCH‖ to select audio channel:
0--AIN1/AOUT1, the default value is 0.
1--AIN2/AOUT2
2--AIN2/AOUT3
For each channel, you 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 used to set the
side-tone gain level. For more details, please refer to the
document [1].
Table 10: AOUT3 Output Characteristics
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
GSM900MHz. Without placing this capacitor, TDD noise could be heard. 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, GSM900 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 audio interface or other audio
interfaces. 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
Item
Condition
Min.
Type
Max.
Unit
RMS power
8ohm load
VBAT=4.3v
THD+N=1%
800
mW
8ohm load
VBAT=3.7v
THD+N=1%
700
mW
Gain adjustment range
0
18
dB
Gain adjustment steps
3
dB
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