User's Manual

Telit GM862-QUAD/-QUAD-PY

Hardware User Guide

1vv0300692, Rev. ISSUE#0, - 04/10/2005

5 Microphone

The microphone circuitry is the more noise sensitive and its design and layout must be done with

particular care.

5.1 Microphone line Characteristic and

requirements

The Telit GM862-QUAD provides two audio paths for the microphone and the earpiece: the

internal and the external audio paths. Only one of the two paths can be active at a time and it is

selectable by hardware line AXE or by AT command. The audio characteristics of the two paths are

slightly different and this should be kept in mind when designing. The internal audio path should be

used for handset function, while the external audio path is suited for hands free function (car kit).

Both microphone paths from the Telit GM862-QUAD are balanced and the OEM circuitry should

be designed balanced to reduce the common mode noise typically generated on the ground plane,

however for particular OEM application needs also an unbalanced circuitry can be used.

The microphone input lines characteristics are:

internal microphone path

• line coupling: AC ( 100nF cond.)

• line type: balanced

• differential input resistance: 25kΩ

• line nominal sensitivity: 50 mV

rms

• max input voltage: 360 mV

rms

• microphone nominal sensitivity/analog gain suggested: -45 dB

Vrms/Pa

/ +24dB

• echo canceller type: handset

external microphone path

• line coupling: AC ( 100nF cond.)

• line type: balanced

• differential input resistance: 25kΩ

• line nominal sensitivity: 3 mV

rms

• max input voltage: 22 mV

rms

• microphone nominal sensitivity/analog gain suggested: -45 dB

Vrms/Pa

/ +10dB

• echo canceller type: car kit hands free