Telit Wireless Solutions GE863-QUAD, GE863-PY manual General Design Rules, Other considerations

GE863-QUAD

GE863-PY

1vv0300715 Rev. 1 - 19/09/06

7.2 General Design Rules

There are several configurations for the audio paths, but the most effective difference is between balanced and unbalanced microphone configuration.

It is highly recommended to keep the whole microphone path balanced even if this means having 2 wires connecting the microphone instead of one needed (plus ground) in the unbalanced case. The balanced circuitry is more suited because of its good common mode noise rejection, reducing the 216 Hz burst noise produced during the GSM transmissions.

•Where possible use balanced microphone circuitry

•Keep the microphone traces on the PCB and wires as short as possible.

•If your application requires an unbalanced microphone, then keep the lines on the PCB balanced and "unbalance" the path close to the microphone wire connector if possible.

•For the microphone biasing voltage use a dedicated voltage regulator and a capacitor multiply circuit.

•Make sure that the microphone traces in the PCB don't cross or run parallel to noisy traces (especially the power line)

•If possible put all around to the microphone lines a ground trace connected to the ground plane by several vias. This is done in order to simulate a shielded trace on the PCB.

•The biasing circuit and eventually the buffer can be designed in the same manner for the internal and external microphones.

7.3 Other considerations

If your application is a hands-free/car kit scenario , but you need to put microphone and speaker inside the same box :

•try to have the maximum possible distance between them, at least 7cm ;

•becauses the microphone type is very important, if you use an omni-directional one ( and this is the typical applicaton ) please seal it on the rear side (no back cavity) in order not to collect unwanted signals ;

•try to make divergent the main axes of the two devices .

Reproduction forbidden without Telit Communications S.p.A. written authorization - All Right reserved page 35 of 79