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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 SocketModem-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 SocketModem
module power pins as possible.
4.
Eliminate ground loops, which are unexpected current return paths to the power source and
ground.
5.
Decouple the telephone line cables at the telephone line jacks. Typically, use a combination
of series inductors, common mode chokes, and shunt capacitors. Methods to decouple
telephone lines are similar to decoupling power lines; however, telephone line decoupling
may be more difficult and deserves additional attention. A commonly used design aid is to
place footprints for these components and populate as necessary during performance/EMI
testing and certification.
6.
Decouple the power cord at the power cord interface with decoupling capacitors. Methods to
decouple power lines are similar to decoupling telephone lines.
7.
Locate high frequency circuits in a separate area to minimize capacitive coupling to other
circuits.
8.
Locate cables and connectors so as to avoid coupling from high frequency circuits.
9.
Lay out the highest frequency signal traces next to the ground grid.
10.
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.
11.
Minimize the number of through-hole connections on traces carrying high frequency signals.
12.
Avoid right angle turns on high frequency traces. Forty-five degree corners are good;
however, radius turns are better.
13.
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.
14.
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.
Multi-Tech Systems, Inc. strives to follow all of these recommendations. Input protection circuitry has
been incorporated into the Multi-Tech devices to minimize the effect of this static buildup, proper
precautions should be taken to avoid exposure to electrostatic discharge during handling.
Multi-Tech uses and recommends that others use anti-static boxes that create a faraday cage
(packaging designed to exclude electromagnetic fields). Multi-Tech recommends that you use our
packaging when returning a product and when you ship your products to your customers.
Multi-Tech Systems, Inc. SocketModem GSM Hardware Guide for Developers
Chapter 3 – Design Considerations
15
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