Rainbow Electronics GM862-GPS - Manual (Page 46)

GM862-GPS Hardware User Guide
1vv0300728 Rev. 8 - 20/09/07
Reproduction forbidden without Telit Communications S.p.A. written authorization - All Rights Reserved page 46 of 73
Hence in these conditions the signal level on the“Mic_HF” input pads of the GM862-GPS is 10 dB (3
times) lower than the nominal, as suggested.
8.2 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.
8.3 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;
• because the microphone type is very important, if you use an omni-directional one ( and this is the
typical application ) 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 .
“Mic_HF” Level = 0,33* 10 -3 * 3=1* 10 -3