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 36 of 69
6.2 General Design rules
There are several configurations for the audio output path, but the various design requirements can
be grouped into three different categories:
•
handset earphone (low power, typically a handset)
•
hands free earphone (low power, typically a headset)
•
car kit speakerphone (high power, typically a speaker)
The three groups have different power requirements, usually the first two applications need only
few mW of power, which can be directly drained from the GM862-QUAD pins, provided a suited
speaker is used. This direct connect design is the cheaper and simpler solution and will be suited for
the most of the earphone design requirements. There's no need to decouple the output ear lines if a
suited earpiece is connected. For the last group, the speakerphone, a power amplifier is required to
raise the output power up to 5-10W required in a car cabin application.
All the designs shall comply with the following guidelines:
•
Where possible use a bridged earphone circuitry, to achieve the maximum power output from
the device.
•
Keep the earphone traces on the PCB and wires as short as possible.
•
If your application requires a single ended earpiece and you want a direct connection, then
leave one of the two output lines open and use only the other referred to ground. Remember
that in this case the power output is 4 times lower than the bridged circuit and may not be
enough to ensure a good voice volume.
•
Make sure that the earphone traces in the PCB don't cross or run parallel to noisy traces
(especially the power line)
•
The cable to the speaker shall be a twisted pair with both the lines floating for the bridged
output type, shielded with the shield to ground for the single ended output type.
6.2.1 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 39
μ
H .
6.3 Handset earphone design
As seen previously, a 16
Ω
speaker can be directly connected to the output pins and
EAR_MT- of the GM862-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
