1. Traditional PCB board design method
In the traditional design process, the PCB design consists of circuit design, layout design, PCB fabrication, measurement and debugging steps. In the circuit design stage, due to the lack of effective analysis methods and means for signal transmission characteristics on the actual PCB board, the circuit design can only be based on component manufacturers and expert advice and past design experience. So for a new design project, it is often difficult to make the correct choice of signal topology and component parameters based on the specific situation.
In the PCB layout design phase, it is also difficult to make real-time analysis and evaluation of the signal performance changes caused by the PCB layout and signal wiring, so the layout design is more dependent on the designer's experience. In the PCB board production stage, since the PCB board and component manufacturers do not have the same process, the parameters of the PCB board and components generally have a large tolerance range, making the performance of the PCB board more difficult to control.
In the traditional PCB design process, the performance of the PCB can only be judged by instrumental measurements after the production is completed. The problems found during the PCB board debugging phase must be modified in the next PCB board design. But even more difficult is that some problems are often difficult to quantify into the parameters of the previous circuit design and layout design, so for more complex PCB boards, it is generally necessary to repeat the above process to meet the design requirements.
It can be seen that with the traditional PCB design method, the product development cycle is longer, and the cost of research and development is correspondingly higher.
2. PCB design method based on signal integrity analysis
Compared to traditional PCB design methods, design methods based on signal integrity analysis have the following characteristics:
Before the PCB board design, the signal integrity model of high-speed digital signal transmission is first established.
According to the SI model, a series of pre-analysis is carried out on the signal integrity problem. According to the results of the simulation calculation, the appropriate component type, parameters and circuit topology are selected as the basis for the circuit design.
In the design process of the circuit, the design scheme is sent to the SI model for signal integrity analysis, and the tolerance range of the components and PCB parameters, the possible topological structure and parameter changes in the PCB layout design, and other factors are calculated and analyzed. Solution space.
After the circuit design is completed, each high-speed digital signal should have a continuous, achievable solution space. That is, when the PCB and component parameters change within a certain range, the layout of components on the PCB, and the way the signal wires are routed on the PCB have certain flexibility, the signal integrity requirements can still be guaranteed. .
Before the PCB layout design begins, the boundary value of each signal solution space obtained is used as the constraint condition of the layout design, which serves as the design basis for PCB layout and layout.
During the PCB layout design process, the partially completed or fully completed design is sent back to the SI model for signal integrity analysis after design to confirm that the actual layout design meets the expected signal integrity requirements. If the simulation results do not meet the requirements, you need to modify the layout design or even the circuit design, which can reduce the risk of product failure due to improper design.
After the PCB design is completed, PCB board production can be performed. The tolerance range of the PCB board manufacturing parameters should be within the solution space of the signal integrity analysis.
After the PCB board is manufactured, the instrument is used for measurement and debugging to verify the correctness of the SI model and the SI analysis, and use this as a basis for the correction model.
On the basis of the correctness of the SI model and the analysis method, the PCB board can be finalized without requiring or requiring only a small number of repeated modifications to the design and production, thereby shortening the product development cycle and reducing development costs.
3. Combination of design methods with existing EDA software
At present, there is no integrated EDA software in the PCB design industry to complete the above design method, so it must be realized through a combination of some common software tools.
Use common SPICE software (such as PSPICE, HSPICE, etc.) to establish SPICE models for discrete, passive devices and transmission lines on the PCB, and debug verification.
Add the SPICE/IBIS models of the components and transmission lines that have been obtained to the general signal integrity analysis software, such as SPECCTRAQuest, HyperLynx, Tau, IS_Analyzer, etc., to establish the SI analysis model of the signal on the PCB, and complete the signal. Sexual analysis calculations.
Use the database function that comes with the SI analysis software, or use other common database software to further organize and analyze the results of the simulation operation, and search for the ideal solution space.
The boundary value of the solution space is used as the basis for the PCB circuit design and the constraint of the layout design. The EDA software of the general PCB design, such as OrCAD, Protel, PADS, PowerPCB, Allegro and Mentor, is used to complete the PCB circuit design and layout design.
After the PCB layout design is completed, the parameters of the actual design line (such as topology, length, spacing, etc.) can be automatically or manually extracted through the above layout design software, and sent back to the previous signal integrity analysis software for wiring. SI analysis to verify that the actual design meets the requirements of the solution space.
After the PCB board is manufactured, the correctness of each model and simulation calculation can also be verified by measurement of the experimental instrument.
This design method has a strong practical significance for the design and development of high-speed digital PCB boards, which not only can effectively improve the performance of product design, but also can greatly shorten the product development cycle and reduce development costs. It is foreseeable that as signal integrity analysis models and computational analysis algorithms continue to improve and improve, PCB design methods based on signal integrity computer analysis will be increasingly used in electronic product design.
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