OC48 Card Layout
1. The high-speed analog signal between the optical transceiver and the DSP is highly susceptible to external noise. Similarly, all specialized power supply and reference voltage circuits also introduce significant coupling between the analog and digital power transmission circuits of the card. At times, the design of a high-density board is constrained by the chassis shape. Additionally, due to the fixed location of the external optical cable access card and the relatively large size of PCB components in the optical transceiver, the transceiver’s position on the card remains largely fixed. Likewise, the position of the system I/O connectors and signal distribution is predetermined. These are fundamental tasks that must be addressed prior to layout.
2. Similar to most successful high-density analog layout and routing strategies, the layout must satisfy routing requirements while maintaining a balance with layout and routing demands. For the analog section of a mixed-signal PCB featuring a local CPU core operating at 2V, the “route before place” method is not recommended. In the case of the OC48 card, the DSP’s analog circuitry, encompassing analog reference voltages and analog power supply bypass capacitors, should be interconnected first. Once wiring is completed, position the entire DSP unit, including analog components and wiring, in close proximity to the optical transceiver. This ensures that the wiring distance from the high-speed analog differential signal to the DSP is minimized, with fewer bends and vias. Symmetrical layout and routing of differentials help mitigate common mode noise. Nonetheless, determining the optimal layout plan before routing remains challenging.
Consult the chip distributor for PCB layout design guidelines. Before proceeding with the layout, thorough communication with the distributor’s application engineer is essential. Many distributors impose strict time constraints on delivering high-quality layout recommendations tailored for “tier-one customers.” In signal integrity (SI) design, ensuring the integrity of new device signals is crucial. Begin the layout and routing of the OC48 card with integrated DSP and microprocessor, following distributor guidelines and specific pin requirements for each power and ground pin in the package.
Once the high-frequency analog section’s location and wiring are finalized, position the remaining digital circuits based on the grouping method depicted in the block diagram. Pay meticulous attention to designing:
1. The location of PLL power filter circuits sensitive to analog signals in the CPU.
2. Local CPU core voltage regulators.
3. Reference voltage circuits for the digital microprocessor.
Apply digital wiring and manufacturing guidelines to the digital circuit design. The high-speed digital bus and clock signal design necessitate specific wiring topologies for the processor bus, balanced Ts, and timing-matched clock signals. Consider integrating additional termination resistors, which has been suggested recently.
Adjustments are naturally made during the layout phase to resolve issues. However, before initiating wiring, verifying the digital part’s timing against the layout plan is crucial. A comprehensive DFM/DFT layout review at this stage ensures the board meets customer requirements.
For digital wiring of the OC48 card:
Start digital wiring from SMD escape patterns for digital devices and the mixed-signal DSP’s digital part. Utilize the shortest and widest printed lines permissible by the assembly process. Longer power traces increase inductance, exacerbating power supply noise and unwanted coupling between analog and digital circuits.
Optimize layout and routing schemes using digital bypass capacitors. Adjust bypass capacitor placement as needed to facilitate installation around the digital and mixed-signal device sections. Route bypass capacitors using the “shortest and widest trace” method.
When power supply branches cross continuous planes (e.g., the 3.3V power plane on the OC48 interface card), separate power supply pins from bypass capacitors to minimize inductance and ESR. Emphasize power supply branch wiring on mixed-signal PCBs like the OC48 interface card, placing additional bypass capacitors in a matrix layout across the entire board, even near passive components.
Define ATE test contacts during logic design for the OC48 card to ensure 100% node coverage with ATE testing probes sized at 0.070 inches. Reserve breakout via positions to prevent power plane interruptions.
If using a power and ground plane split solution, select a layer bias parallel to the adjacent wiring layer near the split. Define restricted wiring areas around the split to prevent wiring from entering. Ensure any wiring through the split area connects to a continuous ground layer adjacent to reduce reflection paths.
For digital signal layouts, position bypass capacitors across open power planes but avoid bridging between digital and analog power planes to prevent noise coupling.
Advanced automatic routing tools support high-density multi-layer digital circuit routing. Initially use 0.050-inch large-size via spacing at SMD exits, adjusting spacing in subsequent stages to maximize layout efficiency and minimize via counts. Prioritize the OC48 processor bus with an improved star topology during automatic routing.
After completing the OC48 card layout, conduct signal integrity verification and timing simulations. Simulation results should confirm that PCB wiring meets expected requirements and enhances second-layer bus timing metrics. Conclude the layout process with design rule checks, final manufacturing reviews, mask creation, and reviews for issuance to the manufacturer.
1. The high-speed analog signal between the optical transceiver and the DSP is highly susceptible to external noise. Similarly, all specialized power supply and reference voltage circuits also introduce significant coupling between the analog and digital power transmission circuits of the card. At times, the design of a high-density board is constrained by the chassis shape. Additionally, due to the fixed location of the external optical cable access card and the relatively large size of PCB components in the optical transceiver, the transceiver’s position on the card remains largely fixed. Likewise, the position of the system I/O connectors and signal distribution is predetermined. These are fundamental tasks that must be addressed prior to layout.
2. Similar to most successful high-density analog layout and routing strategies, the layout must satisfy routing requirements while maintaining a balance with layout and routing demands. For the analog section of a mixed-signal PCB featuring a local CPU core operating at 2V, the “route before place” method is not recommended. In the case of the OC48 card, the DSP’s analog circuitry, encompassing analog reference voltages and analog power supply bypass capacitors, should be interconnected first. Once wiring is completed, position the entire DSP unit, including analog components and wiring, in close proximity to the optical transceiver. This ensures that the wiring distance from the high-speed analog differential signal to the DSP is minimized, with fewer bends and vias. Symmetrical layout and routing of differentials help mitigate common mode noise. Nonetheless, determining the optimal layout plan before routing remains challenging.
Consult the chip distributor for PCB layout design guidelines. Before proceeding with the layout, thorough communication with the distributor’s application engineer is essential. Many distributors impose strict time constraints on delivering high-quality layout recommendations tailored for “tier-one customers.” In signal integrity (SI) design, ensuring the integrity of new device signals is crucial. Begin the layout and routing of the OC48 card with integrated DSP and microprocessor, following distributor guidelines and specific pin requirements for each power and ground pin in the package.
Once the high-frequency analog section’s location and wiring are finalized, position the remaining digital circuits based on the grouping method depicted in the block diagram. Pay meticulous attention to designing:
1. The location of PLL power filter circuits sensitive to analog signals in the CPU.
2. Local CPU core voltage regulators.
3. Reference voltage circuits for the digital microprocessor.
Apply digital wiring and manufacturing guidelines to the digital circuit design. The high-speed digital bus and clock signal design necessitate specific wiring topologies for the processor bus, balanced Ts, and timing-matched clock signals. Consider integrating additional termination resistors, which has been suggested recently.
Adjustments are naturally made during the layout phase to resolve issues. However, before initiating wiring, verifying the digital part’s timing against the layout plan is crucial. A comprehensive DFM/DFT layout review at this stage ensures the board meets customer requirements.
For digital wiring of the OC48 card:
Start digital wiring from SMD escape patterns for digital devices and the mixed-signal DSP’s digital part. Utilize the shortest and widest printed lines permissible by the assembly process. Longer power traces increase inductance, exacerbating power supply noise and unwanted coupling between analog and digital circuits.
Optimize layout and routing schemes using digital bypass capacitors. Adjust bypass capacitor placement as needed to facilitate installation around the digital and mixed-signal device sections. Route bypass capacitors using the “shortest and widest trace” method.
When power supply branches cross continuous planes (e.g., the 3.3V power plane on the OC48 interface card), separate power supply pins from bypass capacitors to minimize inductance and ESR. Emphasize power supply branch wiring on mixed-signal PCBs like the OC48 interface card, placing additional bypass capacitors in a matrix layout across the entire board, even near passive components.
Define ATE test contacts during logic design for the OC48 card to ensure 100% node coverage with ATE testing probes sized at 0.070 inches. Reserve breakout via positions to prevent power plane interruptions.
If using a power and ground plane split solution, select a layer bias parallel to the adjacent wiring layer near the split. Define restricted wiring areas around the split to prevent wiring from entering. Ensure any wiring through the split area connects to a continuous ground layer adjacent to reduce reflection paths.
For digital signal layouts, position bypass capacitors across open power planes but avoid bridging between digital and analog power planes to prevent noise coupling.
Advanced automatic routing tools support high-density multi-layer digital circuit routing. Initially use 0.050-inch large-size via spacing at SMD exits, adjusting spacing in subsequent stages to maximize layout efficiency and minimize via counts. Prioritize the OC48 processor bus with an improved star topology during automatic routing.
After completing the OC48 card layout, conduct signal integrity verification and timing simulations. Simulation results should confirm that PCB wiring meets expected requirements and enhances second-layer bus timing metrics. Conclude the layout process with design rule checks, final manufacturing reviews, mask creation, and reviews for issuance to the manufacturer.