Chapter 1 – RJModem Hardware
Multi-Tech Systems, Inc. RJModem Developer's Guide (S000363I)
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Design Considerations
Good engineering practices must be adhered to when designing a printed circuit board (PCB) containing the
RJModem. Suppression of noise is essential to the proper operation and performance of the modem itself and for
surrounding equipment.
Two aspects of noise in an OEM board design containing the RJ Modem must be considered: on-board/off-board
generated noise that can affect digital signal processing. Both on-board and off-board generated noise that is coupled
on-board can affect interface signal levels and quality. Of particular concern is noise in frequency ranges affecting
modem performance.
On-board generated electromagnetic interference (EMI) noise that can be radiated or conducted off-board is a
separate, but equally important, concern. This type of noise can affect the operation of surrounding equipment. Most
local government agencies have stringent certification requirements that must be met for use in specific
environments.
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.
The aspects of proper engineering practices are beyond the scope of this designer guide. The designer should
consult noise suppression techniques described in technical publications and journals, electronics and electrical
engineering text books, and component supplier application notes.
PC Board Layout Guidelines
In a 4-layer design, provide adequate ground plane covering the entire board. In 4-layer designs, power and ground
are typically on the inner layers. All power and ground traces should be 0.05 inches wide.
The recommended hole size for the RJ Modem pins is 0.036 in. +/-0.003 in. (check with CAD on this) in diameter.
All creepages and clearances for the RJModem have been designed to meet requirements of safety standards
EN60950. The requirements are based on a working voltage of 250V.
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 EMI by the RJModem to the design, 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 RJModem 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.
6.
Locate high frequency circuits in a separate area to minimize capacitive coupling to other circuits.
7.
Locate cables and connectors so as to avoid coupling from high frequency circuits.
8.
Lay out the highest frequency signal traces next to the ground grid.
9.
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.
10.
Minimize the number of through-hole connections on traces carrying high frequency signals.
11.
Avoid right angle turns on high frequency traces. Forty-five degree corners are good; however, radius turns
are better.
12.
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.
13.
Distribute high frequency signals continuously on a single trace rather than several traces radiating from one
point.
