User guide

UC864-E/G/WD/WDU Hardware User Guide

1vv0300766a Rev.4

- 03/02/09

10.1. Input Lines (Microphone)

10.1.1. Short Description

The Telit UC864-E/G/WD/WDU provides two audio paths in transmit section. Only

one of the two paths can be active at a time, selectable by AXE hardware line or by

AT command.

You must keep in mind the different audio characteristics of the transmit blocks when

designing.

The “MIC_MT” audio path must be used for handset function, while the “MIC_HF”

audio path is suited for hands-free function (car kit).

10.1.2. Input Line Characteristics

“Mic_MT” 1st differential microphone path

• line coupling AC*

• line type balanced

• coupling capacitor ≥ 100nF

• differential input impedance 20kΩ

• differential input voltage ≤ 1,03Vpp (365mVrms)

• microphone nominal sensitivity -45 dBVrms/Pa

• analog gain suggested +20dB

• echo canceller type handset

TIP:

Being the microphone circuitry more noise sensitive, its design and layout must be

done with particular care. Both microphone paths are balanced and the OEM circuitry

must be balanced designed to reduce the common mode noise typically generated

on the ground plane. However also an unbalanced circuitry can be also used for

particular OEM application needs.

TIP:

Due to the difference in the echo canceller type, the “Mic_MT” audio path is suited for

Handset applications, while the “Mic_HF”audio path is suited for hands-free function

(car kit). The Earphone applications should be made using the “Mic_HF” audio path

but DISABLING the echo canceller by software AT command. If the echo canceller is

left active with the Earphone, then some echo might be introduced by the echo

cancel algorithm.