Multitech MT9234SMI-HV-92 Specifications download pdf (Page 17)

CHAPTER 3 – UNIVERSAL DESIGN CONSIDERATIONS

Universal Socket Developer Guide

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. To minimize the contribution of the SocketModem-based design to

EMI, you must understand the major sources of EMI and how to reduce them to acceptable levels.

 Keep traces carrying high frequency signals as short as possible.

 Provide a good ground plane or grid. In some cases, a multilayer board may be required with full layers for

ground and power distribution.

 Decouple power from ground with decoupling capacitors as close to the SocketModem power pins as

possible.

 Eliminate ground loops, which are unexpected current return paths to the power source and ground.

 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.

 Decouple the power cord at the power cord interface with decoupling capacitors. Methods to decouple

power lines are similar to decoupling telephone lines.

 Locate high frequency circuits in a separate area to minimize capacitive coupling to other circuits.

 Locate cables and connectors to avoid coupling from high frequency circuits.

 Lay out the highest frequency signal traces next to the ground grid.

 If using a multilayer board design, make no cuts in the ground or power planes and be sure the ground plane

covers all traces.

 Minimize the number of through-hole connections on traces carrying high frequency signals.

 Avoid right angle turns on high frequency traces. Forty-five degree corners are good; however, radius turns

are better.

 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.

 Distribute high frequency signals continuously on a single trace rather than several traces radiating from one

point.

Electrostatic Discharge Control

Handle all electronic devices with certain precautions to avoid damage due to the static charge accumulation.

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 static buildup, take proper precautions to avoid

exposure to electrostatic discharge during handling.

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Multitech MT9234SMI-HV-92 Specifications Page 17