Altec GSM SOCKET AL7020S - Manual (Page 22)

AL7020S, AL7024S Designer’s Guide
Doc. No. DG_7024E Altec Electronic AG Seite 22 / 32
3.8.2 Microphone circuit
The AL7020S has an analog input, referenced to the microphone ground (pin 63).
The AL7020S contains an analog amplifier with a default gain of 22dB, which can be
adjusted in seven steps between 0 and 31dB using the AT+MMICG command. For
more details about this command, refer to the AT Commands document (AT_7020A).
When the default gain of 22dB is used, then the level considerations should be as
follows:
Pin 62 is the headset microphone input. The input for this line can come directly from
a headset microphone. The maximum input to the AL7020S headset microphone
before there is signal distortion in the network is 12 mvRMS.
The Microphone ist connected single-ended to pin 62, with analog ground at pin 63.
At pin 62 there is also a phantom feeding-voltage of 2.2 Volts.
Recommended characteristics for the microphone:
• 2V – 0.5mA
• KOhms
• Sensitivity -40 to –50dB
• SNR > 50dB
• Frequency response compatible with the GSM specifications
3.8.3 Audio Circuit Design Considerations
In order to design an audio circuit that produces clear audio, without being affected by
the GSM transmission, the following guidelines should be considered when designing
the circuit:
• The audio input to the AL7020S should be referenced to the MIC GND line of the
Module.
• The microphone bias of the AL7020S for the microphone is 2.2 V. If a microphone
with a different voltage is used, the bias should be as clean as possible, and
referenced to MIC GND. Because most GSM buzz (217Hz TDMA noise) is
generated from the microphone path, if a different bias is created, it should be
filtered to supply DC only.
• The MIC GND and GND are connected inside the module and should not be
connected to the application board.
• Keep the lines of the microphone inputs as short as possible. To filter the lines
from RF emission, 39pF capacitors can be used.
• The Module GND should be connected on all pins.
• If possible, the RF cable ground from the AL7020S should be connected to the
GND of the Module.
• The digital lines of the application should never be referenced to the MIC
GND(63).