User's Manual

Part 1 − Universal Socket Connectivity Chapter 1 – Universal Socket Connectivity
Multi-Tech Systems, Inc. Universal Socket Hardware Guide for Developers (S000342K) 16
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 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.
Phone Line Warning Statement for the Developer
Board
Use extreme caution when the phone line is installed due to live energized components. In fact, do not touch any
components on the board while the phone line is installed.
In addition, the phone line should be detached when making modifications to or servicing the developer board.