PCB Tech - About the layout and routing of modern mixed-signal PCBs

The following will illustrate the technology of mixed-signal PCB layout and routing through the design of the OC48 interface card. OC48 stands for Optical Carrier Standard 48, which is basically oriented to 2.5Gb serial optical communication. It is one of the high-capacity optical communication standards in modern communication equipment. The OC48 interface card contains several typical mixed-signal PCB layout and wiring problems. The layout and wiring process will specify the sequence and steps to solve the mixed-signal PCB layout scheme.

The OC48 card contains an optical transceiver that realizes the bidirectional conversion of optical signals and analog electrical signals. Analog signal input or output digital signal processor, DSP converts these analog signals into digital logic levels, which can be connected with microprocessor, programmable gate array, DSP and microprocessor system interface circuit on OC48 card . The independent phase-locked loop, power filter and local reference voltage source are also integrated.

Among them, the microprocessor is a multi-power device, the main power is 2V, and the 3.3V I/O signal power is shared by other digital devices on the board. Independent digital clock source provides clock for OC48 I/O, microprocessor and system I/O.

After checking the layout and wiring requirements of different functional circuit blocks, a 12-layer board is initially recommended. The configuration of the microstrip and stripline layers can safely reduce the coupling of adjacent wiring layers and improve impedance control. A grounding layer is set between the first and second layers to isolate the wiring of sensitive analog reference sources, CPU cores and PLL filter power supplies from the microprocessor and DSP devices on the first layer. The power and ground planes always appear in pairs, the same as what is done on the OC48 card for the shared 3.3V power plane. This will reduce the impedance between the power supply and ground, thereby reducing the noise on the power signal.

Avoid running digital clock lines and high-frequency analog signal lines near the power plane, otherwise, the noise of the power signal will be coupled to the sensitive analog signal.

According to the needs of digital signal wiring, carefully consider the use of power and analog ground plane openings (split), especially at the input and output ends of mixed-signal devices. Passing through an opening in the adjacent signal layer will cause impedance discontinuity and poor transmission line loops. These will cause signal quality, timing and EMI issues.

Sometimes adding several ground layers, or using several outer layers for the local power layer or ground layer under a device, can eliminate the opening and avoid the above problems. Multiple ground layers are used on the OC48 interface card. Maintaining the stacking symmetry of the position of the opening layer and the wiring layer can avoid card deformation and simplify the manufacturing process. Since 1 ounce of copper clad laminates are highly resistant to large currents, 1 ounce of copper clad laminates should be used for the 3.3V power layer and the corresponding ground layer, and 0.5 ounces of copper clad laminates can be used for other layers. This can reduce transient high currents or spikes caused by Voltage fluctuations.

If you design a complex system from the ground plane up, you should use cards with a thickness of 0.093 inches and 0.100 inches to support the wiring layer and the ground isolation layer. The thickness of the card must also be adjusted according to the size of the via pad and the wiring feature of the hole, so that the aspect ratio of the hole diameter to the thickness of the finished card does not exceed the aspect ratio of the metallized hole provided by the manufacturer.

If you want to design a low-cost, high-yield commercial product with the least number of PCB wiring layers, you must carefully consider the wiring details of all special power supplies on the mixed-signal PCB before layout or wiring. Before starting layout and routing, let the target manufacturer review the preliminary layering plan. Basically, the layering should be based on the thickness of the finished product, the number of layers, the weight of the copper, the impedance (with tolerance) and the size of the smallest via pads and holes, and the manufacturer should provide a written layering recommendation.

The proposal should include all configuration examples of controlled impedance stripline and microstrip line. You need to consider the combination of your impedance prediction and the PCB manufacturer impedance, and then use these impedance predictions to verify the signal routing characteristics in the simulation tool used to develop CAD routing rules.