Altec ISDN Socket Modem AL5068S-3V - Manual (Page 19)

AL5068S Designer’s Guide
No. AL5068S-E00-105 Altec Electronic AG Seite 19 / 25
4. DESIGN CONSIDERATIONS
Good engineering practices must be adhered to when designing a printed circuit board
(PCB) containing the AL5068S Socket Modem 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 AL5068S Socket Modem
module 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 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 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 certification.
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. Technical and professional associations as well as component
suppliers often offer seminars addressing noise suppression techniques.
4.1 PC Board Layout Guidelines
4.1.1 General
1. In a 4-layer design, provide an adequate ground plane covering the entire board.
Socket Modem DGND and AGND pins are tied together on the Socket Modem.
2. As a general rule, route digital signals on the component side of the PCB and the
analog signals on the solder side. The sides may be reversed to match particular OEM
requirements. Route the digital traces perpendicular to the analog traces to minimize
signal cross coupling.
3. Route the modem signals to provide maximum isolation between noise sources and
noise sensitive inputs. When layout requirements necessitate routing these signals
together, they should be separated by neutral signals.
4. All power and ground traces should be at least 0.05 in. wide.
5. ISDN S0 signal traces are to be no closer than 2.5mm (0.1") from any other traces for
European applications.
6. If the Socket Modem is mounted flush with the host PCB, the host PCB should be clear
of all traces directly underneath the Socket Modem oscillator section. It is strongly
suggested that the Socket Modem is mounted at least 0.130 inch above the host board.
(See section 4.4)