RFX144V24-S23 and RFX96V24-S23 Modem Designer’s Guide
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14.2 PC BOARD LAYOUT CONSIDERATIONS
Good engineering practices must be adhered to when designing a printed circuit board (PCB) containing a MONOFAX
modem. 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 a MONOFAX modem must be considered: on-board/off-board
generated noise that can affect analog signal levels and analog-to-digital (A/D) conversion/digital-to-analog (D/A) 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 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 to allow 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 is required for the board design to achieve desired
modem performance and to attain EMI certification.
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.
14.2.1 General Board Layout Guidelines
The board design should adhere to the following general guidelines. Most of these guidelines are also applicable to
minimizing on-board noise EMI generation (see next section).
1. Place the MDP and XIA devices and all supporting analog circuitry, including the DAA, if required, on the same area of
PCB.
2. All power traces should be at least a 0.1 inch width.
3. Place the decoupling capacitors from the VDD pins to DGND near the VDD pins. Place the decoupling capacitors from
AVDD, VAA1, and VAA2 pins to AGND near the AVDD, VAA1, and VAA2 pins (see Figure 14-1).
4. All circuitry connected to the XTLI and XTLO pins should be kept short to prevent stray capacitance from affecting the
oscillator and to reduce EMI.
5. Keep the XTLO lead extremely short with no bends greater than 45° and containing no vias since the XTLO pin is
connected to a fast rise time, high current driver. All components associated with the oscillator function must be close to
the modem device.
6. Tie the AGND1 pin directly to the AGND2 pin at the modem package. Tie the AGND2 pin directly, by a dedicated path,
to the common ground point for analog and digital ground.
7. Supply an analog ground plane beneath all analog components. Connect the analog ground plane to the DGNDA and
AGND pins and to all analog ground points shown in Figure 14-1.
8. Supply a digital ground plane to cover the remaining allocated area. Connect the digital ground plane to the DGND pins
and to all digital ground points shown in Figure 14-1, plus the crystal-can ground.
9. Orient the MDP 100-pin PQFP package relative to the two ground planes so that the corner containing pin 1 is toward
the digital ground plane and the corner containing pin 51 is toward the analog ground plane.
10. Orient the XIA 28-pin PLCC package relative to the two ground planes so that the corner containing pin 5 is toward the
digital ground plane and the corner containing pin 19 is toward the analog ground plane.
11. As a general rule, route digital signals on the component side of the PCB and route all analog signals on the solder side.
The sides may be reversed to match a particular OEM requirement.
12. Routing of the modem signals should provide maximum isolation between noise sources and sensitive inputs. When
layout requirements necessitate routing these signals together, they should be separated by neutral signals. Refer to
Table 14-3 for the noise characteristics of each modem pin.
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