Here you'll find insights into PCB design, tech trends, assembly issues, and trending topics
in the general news media as they relate to printed circuit board technology.
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Many people are very vague about the concept of the critical length of the lines on the PCB. Even many people don't even know the concept. If you design a high-speed circuit board but don't know the concept, you can be sure that the final board may not work stably. And you are confused, you can't start debugging.
The rise time of the signal is critical to understanding the signal integrity problem. Most of the problems in high-speed pcb design are related to it, and you must pay enough attention to him.
Today's computer system DDR3 memory technology has been widely used, and the data transmission rate has been repeatedly improved, and has now reached 1866Mbps. Under such high-speed bus conditions, to ensure the reliability of data transmission quality and meet the timing requirements of parallel buses, it poses great challenges to design implementation.
To maintain signal integrity on printed circuit boards, a unique approach to inter-layer interconnects (vias) that allows the trace impedance to be accurately matched should be used.
With the increase of package density and the increase of operating frequency, the signal integrity problem in MCM circuit design can not be ignored. Taking the detector circuit as an example, this paper firstly uses APD software to realize the layout and layout design of the circuit, and then combines the signal integrity analysis to adjust the circuit layout and wiring structure repeatedly. The final Spectra Quest software simulation results show that the improved circuit layout and routing Meet signal integrity requirements while maintaining high simulation accuracy.
Due to the time relationship, this paper only explains the operation steps. The application needs to have a deeper operational capability and signal integrity theory foundation for protel99se and a certain understanding of the constrained drive design method of modern EDA software. As for why I would consider protel99se for signal integrity analysis, because of the company's reasons, the company originally wanted to cadence, but since I came to the company, the board with experience has been able to pass, there is nothing to change the design. The problem of the tool, the company is not familiar with cadence in order to take care of other people, so I don't think about it anymore. Although I know that there is no need for cadence when the requirements are high, I have no choice but to repeatedly replace the design tools. I have to make an idea on protel99se. This is The origin of this article.
More and more manufacturers are requesting the DC trace impedance on the PCB. The reasons for specifying and controlling the DC trace impedance are given below from the designer's point of view.
As the current demand for speed and bandwidth for communications and computer systems continues to rise, system designers are facing a severe test. The parallel bus structure tested in time series is close to the limit of its capability, and the bus width is now over 64 bits, which makes the circuit layout extremely complicated. In addition, the large number of signals in a wide parallel bus are also very cumbersome to synchronize, especially as these signals are subject to random factors such as noise and crosstalk.
In order to perform circuit simulation, it is necessary to first build a model of the component, that is, for the various components supported by the circuit simulation program, there must be a corresponding mathematical model to describe them in the simulation program, that is, a calculation formula that can be calculated by a computer. To express them.
IBIS is the abbreviation of I/OBufferInformationSpecification. It is a fast and accurate method for I/OBUFFER based on I/V curve. It is an international standard that reflects the electrical characteristics of chip driver and receiver. It provides a standard file. The format is used to record parameters such as drive source output impedance, rise/fall time, and input load. It is ideal for calculation and simulation in high-speed circuit design such as oscillation and crosstalk.
Common PCB board level signal integrity analysis model
Knowing the basic principles of the IBIS model, it is easy to perform signal integrity analysis on the designed circuit. Due to the high precision and transparency of the IBIS model, it was supported by major EDA vendors as soon as it was introduced. Now all kinds of EDA tools have the function of system simulation using IBIS model, and some of them integrate with PCB design tools. In the design process, the simulation of signals can be directly performed online, which is very convenient to use.
With the advancement of technology, the signal blocking time of high-speed integrated circuits has reached several hundred ps, and the clock frequency can reach several hundred MHz. Such a high edge rate causes a large number of interconnect lines on the printed circuit board to be produced in low-speed circuits. The transmission line effect causes distortion of the signal, which seriously affects the correct transmission of the signal. If the board design is not considered, the circuit with the correct logic function will not work properly when debugging. In order to solve this problem, signal integrity analysis must be carried out when designing high-speed circuits. The system is thoroughly simulated by virtual template, and the influence of circuit layout and routing on signal integrity is accurately analyzed, and the circuit design is guided. In this way, many problems that can be found in debugging can be solved during the design, which greatly improves the design success rate and shortens the design cycle.
As the packing density increases and the operating frequency increases, the signal integrity problem in MCM circuit design cannot be ignored. Taking the detector circuit as an example, this paper firstly uses APD software to realize the layout and layout design of the circuit, and then combines the signal integrity analysis to adjust the circuit layout and wiring structure repeatedly. The final Spectra Quest software simulation results show that the improved circuit layout and routing Meet signal integrity requirements while maintaining high simulation accuracy.
With the development of integrated circuit process technology, the operation speed of multi-chip components is getting higher and higher, and the processing of high-speed signals has become the key to the success of MCM circuit design. When the rising or falling edge of the clock signal is small, the transmission line effect is caused, that is, signal integrity problems occur.
Combine an actual DSP high-speed image data acquisition system to illustrate the generation of signal integrity problems and specific solutions.
At present, the increasingly fine semiconductor process makes the transistor size smaller and smaller, so the signal transition of the device is faster and faster, the fast slope transient of the high-speed digital system and the extremely high operating frequency, and the large circuit density. This has led to signal integrity issues and electromagnetic compatibility issues in the field of high-speed digital circuit system design. Destroying signal integrity directly leads to signal distortion, timing errors, and incorrect data, address, and control signals, which can cause misoperations and even system crashes. Therefore, signal integrity issues have increasingly attracted the attention of high-speed digital circuit designers.
When the impedances don't match, what are the ways to match it? First, consider using a transformer for impedance conversion, as in the example in the TV above. Second, consider using series/parallel capacitors. Or inductive methods, which are often used when debugging RF circuits. Third, consider the use of series/parallel resistors. Some drivers have lower impedance and can be connected in series with a suitable resistor to match the transmission line, such as high-speed signal lines, sometimes A series of resistors of several tens of ohms will be connected in series. The input impedance of some receivers is relatively high. The method of parallel resistors can be used to match the transmission line. For example, 485 bus receivers often have a matching resistor of 120 ohms in parallel at the data line terminal.
The so-called input impedance mainly considers the power consumed by the circuit itself (can be understood as meaningless loss). For a voltage-driven circuit, the larger the impedance, the smaller the current, the smaller the P=I*I*R, and the smaller the current. In terms of the driving circuit, the smaller the impedance, the smaller the P=I*I*R, and the smaller the power consumption, so that for the latter circuit, more power can be output.
1. Signal Integrity: refers to the quality of the signal in the circuit system. If the signal can be transmitted from the source to the receiver without distortion in the required time, we call the signal complete.
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