Altec GSM SOCKET AL7020S User Manual
Page 25
AL7020S, AL7024S Designer’s Guide
Doc. No. DG_7024E
Altec Electronic AG
Seite 25 / 32
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Good engineering practices must be adhered to when designing a printed circuit
board (PCB) containing the GSM Socket Modem module. Suppression of noise is
essential to the proper operation and performance of the modem itself and for
surrounding equipment.
Two aspects of noise in an OEM board design containing the GSM Socket Modem
module must be considered: on-board/off-board generated noise that can affect
analog signal levels and analog-to-digital conversion (ADC)/digital-to-analog
conversion (DAC), and on-board generated noise that can radiate off-board. Both on-
board and off-board generated noise that is coupled on-board can affect interfacing
signal levels and quality, especially in low level analog signals. Of particular concern
is noise in frequency ranges affecting modem performance.
On-board generated electromagnetic interference (EMI) noise that can be radiated or
conducted off-board is a separate, but equally important, concern. This noise can
affect the operation of surrounding equipment. Most local governing agencies have
stringent certification requirements that must be met for use in specific environments.
Proper PC board layout (component placement, signal routing, trace thickness and
geometry, etc.), component selection (composition, value, and tolerance), interface
connections, and shielding are required for the board design to achieve desired
modem performance and to attain EMI certification.
5.1 Electromagnetic Interference (EMI) Considerations
The EMC tests have to be performed as soon as possible on the application to
detect any possible problem. When designing, special attention should be paid to:
• Possible spurious emission radiated by the application to the RF receiver in the
receiver band.
• Metallic case or plastic casing with conductive paint are recommended
The following guidelines are offered to specifically help minimize EMI generation.
Some of these guidelines are the same as, or similar to, the general guidelines but
are mentioned again to reinforce their importance. In order to minimize the
contribution of the Socket Modem-based design to EMI, the designer must
understand the major sources of EMI and how to reduce them to acceptable levels.
1. Keep traces carrying high frequency signals as short as possible.
2. Provide a good ground plane or grid. In some cases, a multilayer board may be
required with full layers for ground and power distribution.
3. Decouple power from ground with decoupling capacitors as close to the Socket
Modem module power pins as possible.
4. Eliminate ground loops, which are unexpected current return paths to the power
source and ground.
5. Locate high frequency circuits in a separate area to minimize capacitive coupling
to other circuits. Distribute high frequency signals continuously on a single trace
rather than several traces radiating from one point.