Telit GM862-QUAD/-QUAD-PY
Hardware User Guide
1vv0300692, Rev. ISSUE#0, - 04/10/2005
Reproduction forbidden without DAI Telecom written authorization – All Right reserved – Right of modification reserved
page 26 of 69
5.2 General Design Rules
There are several configurations for the audio path, but the most effective difference is between
balanced and unbalanced microphone.
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.
5.3 Microphone Biasing
The electret microphones usually need a biasing voltage to work properly. Refer to your
microphone provider for the characteristics required.
NOTE: The microphones have a hot wire were the positive biasing must be connected, usually it is
indicated by a + sign or a red point. If the polarity of the bias is reversed, then the microphone will not
work properly. For this reason be sure to respect the mic. biasing polarity.
