Users Manual

GSM/GPRS Module Series
M66 Hardware Design
M66_Hardware_Design Confidential / Released 44 / 82
AIN can be used for input of microphone and line. An electret microphone is usually used. AIN are
differential input channels.
AOUT1 is used for output of the receiver. This channel is typically used for a receiver built into a handset.
AOUT1 channel is a differential channel.
AOUT2 is typically used with earphone. It is a single-ended and mono channel. SPK2P and AGND can
establish a pseudo differential mode.
All of these two audio channels support voice and ringtone output, and so on, and can be switched by
AT+QAUDCH command. For more details, please refer to the document [1].
Use AT command AT+QAUDCH to select audio channel:
 0--AIN/AOUT1, the default value is 0.
 1--AIN/AOUT2, this channel is always used for earphone.
For each channel, you can use AT+QMIC to adjust the input gain level of microphone. You 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].
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
EGSM900MHz. 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,
EGSM900MHz, 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, EGSM900 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.
AIN/AOUT2
MICP 3 Microphone positive input
MICN 4 Microphone negative input
SPK2P 2 Channel 2 Audio positive output
AGND 1 Form a pseudo-differential pair with SPK2P