Altec ISDN Socket Modem AL5068S-3V - Manual (Page 20)

AL5068S Designer’s Guide
No. AL5068S-E00-105 Altec Electronic AG Seite 20 / 25
4.1.2 Electromagnetic Interference (EMI) Considerations
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. 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.
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.
4.1.3 Other Considerations
The pins of all Socket Modems are grouped according to function. The ISDN interface, Host
interface, and LED interface pins are all conveniently arranged, easing the host board layout
design.
Altec Electronic has tested the AL5068S Socket Modems for compliance with CTR3. The
certificates apply only to designs that route the ISDN signals (pins 1 to 4) directly to the RJ45
jack.