User's Manual

Telit GE863-QUAD / GE863-PY

Hardware User guide

1vv0300697, Rev. ISSUE#0, - 21/11/05

7.2.1 Noise Filtering

The I/O of the PCB should have a noise filter close to the connector, to filter the high frequency

GSM noise. The filter can be a Π of 2 capacitor and a inductance, with the one capacitor of 39pF

- 0603 case , and the other capacitor of 1nF - 0603; the inductance shall have a value of 39μH .

7.3 Handset earphone design

As seen previously, a 16Ω speaker can be directly connected to the output pads EAR_MT+ and

EAR_MT- of the GE863-QUAD.

This solution is often the more cost effective, reducing the components count to a minimum.

There are several limitations to the use of this solution: speaker direct connect imposes the

speaker characteristics to be almost exactly the suggested ones, otherwise the power output may

be reduced (if speaker impedance is bigger than 16Ω) or the GE863-QUAD ear port may be

damaged (if speaker impedance is less than 15Ω).

The other limitation of the speaker direct connection is the power output capability of the

GE863-QUAD which is limited and for some particular applications may not be enough.

For these reasons, when the power output of the GE863-QUAD is not enough or if the speaker

characteristics are different from the suggested, then it is preferable to use an amplifier to

increase the power and current output capabilities.

Again the output from the GE863-QUAD is bridged and both lines should be used, where

possible, as inputs to the power amplifier. This ensures a higher common mode rejection ratio,

reducing the GSM current busts noise on the speaker output.

In this case the EAR_MT lines from the GE863-QUAD should be AC coupled with a capacitor

of 100nF.

It is always desirable to have a mute control on the amplifier, in order to turn it off while the

device is not sending signal to the output, in this manner the amplifier background noise which

may be audible during idle conditions is cut off.

A principle schematic may be: