Multi-Tech Systems MT5600SMI-34 Other Design Considerations, PC Board Layout Guidelines, 121

Appendix B – Product Approvals, Regulatory Design Considerations, and Regulatory Compliance

Other Design Considerations

Good engineering practices must be adhered to when designing a printed circuit board (PCB) containing the SocketModem module. 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 SocketModem 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 2-layer design, provide an adequate ground grid in all unused space around and under components on both sides of the board and connect in such a manner as to avoid small islands. A grid is preferred over a solid plane to improve solderability. Typically, the grid is composed of 0.012- inch traces and 0.012- inch spaces on a 0.025-inch grid. Connect each grid to other grids on the same side at several points and grids on the opposite side through the board at several points.

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.

Tip and Ring signal traces are to be no closer than 0.062 inches to any other traces for U.S. applications. Tip and Ring signal traces are to be no closer than 0.078 inches (2.0 mm) from any other traces or ground circuits for European applications. 2.0 mm spacing must be used if the host board is to support both U.S. and European SocketModem applications.

If the SocketModem is mounted flush with the host PCB, the host PCB should be clear of all traces directly underneath the SocketModem oscillator section (under the chip). It is strongly recommended that the SocketModem be mounted at least 0.130 inch above the host PCB.

The recommended hole size for the SocketModem pins is 0.036 in. +/-0.003 in. in diameter. Spacers can be used to hold the SocketModem vertically in place during the wave solder process. A spacer should be placed on pin 32 and pin 64 of the SocketModem. A suggested part number for the spacer is BIVAR 938-0.130 for P1(0.310in) option SocketModems. The spacers can be left on permanently and will not effect operation.

Multi-Tech Systems, Inc. SocketModem MT5600SMI Developer’s Guide 121