Hayes Microcomputer Products RCV56HCF manual PC Board Layout Guidelines, General Principles

RCV56HCF PCI/CardBus Modem Designer’s Guide

4. DESIGN CONSIDERATIONS

Good engineering practices must be followed when designing a printed circuit board (PCB) containing the modem device. This is especially important considering the high data bit rate, high fax rate, record/play of analog speech and music audio, and full-duplex speakerphone operation. Suppression of noise is essential to the proper operation and performance of the modem and interfacing audio and DAA circuits.

Two aspects of noise in an OEM board design containing the modem device set must be considered: on-board/off-board generated noise that can affect analog signal levels and analog-to-digital conversion (ADC)/digital-to-analog conversion (DAC), and on-board generated noise that can radiate off-board. Both on-board and off-board generated noise that is coupled on-board can affect interfacing signal levels and quality, especially in low level analog signals. Of particular concern is noise in frequency ranges affecting modem and audio circuit performance.

On-board generated electromagnetic interference (EMI) noise that can be radiated or conducted off-board is a separate, but equally important, concern. This noise can affect the operation of surrounding equipment. Most local governing agencies have stringent certification requirements that must be met for use in specific environments. In order to minimize the contribution of the circuit design and PCB layout to EMI, the designer must understand the major sources of EMI and how to reduce them to acceptable levels.

Proper PC board layout (component placement and orientation, 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. In addition, design layout should meet requirements stated in the PCI Bus Specification, Section 4.4, Expansion Board Specification, as well as other applicable sections.

All the aspects of proper engineering practices are beyond the scope of this designer's 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. Seminars addressing noise suppression techniques are often offered by technical and professional associations as well as component suppliers.

The following guidelines are offered to specifically help achieve stated modem performance, minimize audible noise for audio circuit use, and to minimize EMI generation.

4.1 PC BOARD LAYOUT GUIDELINES

4.1.1 General Principles

1.Provide separate digital, analog, and DAA sections on the board.

2.Keep digital and analog components and their corresponding traces as separate as possible and confined to defined sections.

3.Keep high speed digital traces as short as possible.

4.Keep sensitive analog traces as short as possible.

5.Provide proper power supply distribution, grounding, and decoupling.

6.Provide separate digital ground, analog ground, and chassis ground (if appropriate) planes.

7.Provide wide traces for power and critical signals.

8.Position digital circuits near the host bus connection and position the DAA circuits near the telephone line connections.

4.1.2 Component Placement

1.From the system circuit schematic,

a)Identify the digital, analog, and DAA circuits and their components, as well as external signal and power connections.

b)Identify the digital, analog, mixed digital/analog components within their respective circuits.

c)Note the location of power and signals pins for each device (IC).

2.Roughly position digital, analog, and DAA circuits on separate sections of the board. Keep the digital and analog components and their corresponding traces as separate as possible and confined to their respective sections on the board. Typically, the digital circuits will cover one-half of the board, analog circuits will cover one-fourth of the board, and the DAA will cover one-fourth of the board. NOTE: While the DAA is primarily analog in nature, it also has many control and status signals routed through it. A DAA section is also governed by local government regulations covering subjects such as component spacing, high voltage suppression, and current limiting.

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