Multitech QuickCarrier USB MT100UCC Series, Руководство разработчика

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Chapter 2 – QuickCarrier USB GPRS (MT100UCC-G2)
QuickCarrier Developer’s Guide 9
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.
Other aspects of proper noise-suppression engineering practices are beyond the scope of this developer’s
guide. The developer should consult noise suppression techniques described other sources, such as
technical publications, electronics and electrical engineering text books, and component supplier
application notes.
Mounting Hardware
Use #4 or M2/M3 hardware to mount the QuickCarrier modem to the board.
Electromagnetic Interference (EMI) Considerations
The following guidelines are offered specifically to 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 QuickCarrier-based design to EMI, the designer must
understand the major sources of EMI and how to reduce them to acceptable levels.
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Keep traces carrying high frequency signals as short as possible.
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Provide a good ground plane or grid. In some cases, a multilayer board may be required with full layers
for ground and power distribution.
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Decouple power from ground with decoupling capacitors as close to the QuickCarrier power pins as
possible.
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Eliminate ground loops, which are unexpected current return paths to the power source and ground.
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Locate high frequency circuits in a separate area to minimize capacitive coupling to other circuits.
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Locate cables and connectors so as to avoid coupling from high frequency circuits.
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Lay out the highest frequency signal traces next to the ground grid.
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If a multilayer board design is used, make no cuts in the ground or power planes and be sure the
ground plane covers all traces.
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Minimize the number of through-hole connections on traces carrying high frequency signals.
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Avoid right angle turns on high frequency traces. Forty-five degree corners are good; however, radius
turns are better.
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On 2-layer boards with no ground grid, provide a shadow ground trace on the opposite side of the
board to traces carrying high frequency signals. This will be effective as a high frequency ground return
if it is three times the width of the signal traces.
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Distribute high frequency signals continuously on a single trace rather than several traces radiating
from one point.
Electrostatic Discharge Control
All electronic devices should be handled with certain precautions to avoid damage due to the accumulation
of static charge.
See the ANSI/ESD Association Standard (ANSI/ESD S20.20-1999) – a document “for the Development of an
Electrostatic Discharge Control for Protection of Electrical and Electronic Parts, Assemblies and Equipment.”
This document covers ESD Control Program Administrative Requirements, ESD Training, ESD Control
Program Plan Technical Requirements (grounding/bonding systems, personnel grooming, protected areas,
packaging, marking, equipment, and handling), and Sensitivity Testing.