Design of high speed PCB based on PROTEL

- Oct 26, 2017-

With the complexity and integration of electronic system design, the clock speed and device rise time are faster and faster. The design of high speed circuit becomes an important part of the design process. In high speed circuit design, the inductance and capacitance of the circuit board will make the wire equivalent to a transmission line. The incorrect layout of the terminating element or the wrong wiring of the high-speed signal can cause the transmission line effect problem, which makes the system output incorrect data, the circuit work is not normal, and even does not work completely. Based on the transmission line model, it can be concluded that the transmission line can bring bad effects such as signal reflection, crosstalk, electromagnetic interference, power and ground noise to the circuit design.

In the PCB design of the circuit, layout is an important link, the layout results will directly affect the effect of wiring and system reliability, which is the most time-consuming and difficult in the whole PCB design. The complex environment of high frequency PCB high frequency makes the layout design system is very difficult to learn the theoretical knowledge of the people, it requires the board must have rich experience in high speed PCB system, so as to detours in the process of design, improve the reliability of the circuit and effectiveness. In the process of layout, the mechanical structure, heat dissipation, electromagnetic interference, the convenience and beauty of the future cabling should be considered comprehensively.

First of all, in the layout before the function of the whole circuit, separate separate, high frequency circuit and low frequency analog circuits and digital circuits, each functional circuit chip as the center as close as possible to the layout of the line as short as possible, to avoid long wires caused by the transmission delay, improve the effect of decoupling capacitor. In addition, attention should be paid to the relative position and direction between the pins and the circuit components and other pipes, so as to reduce the mutual influence. All high frequency components should be far away from the housing and other metal plates to reduce parasitic coupling.

Secondly, attention should be paid to the thermal and electromagnetic effects between the components, which are especially serious for the high frequency system. The measures of isolation or isolation, heat dissipation and shielding should be taken. High power rectifiers and regulating tubes should be equipped with radiators and keep away from transformers. Electrolytic capacitor, heat element should stay away from the heating element, otherwise the electrolyte will be dried, resulting in resistance increases, poor performance, influence the stability of the circuit. There should be enough space in the layout to arrange the protective structure and prevent the introduction of various parasitic coupling. To prevent electromagnetic coupling between coils on a printed circuit board, the two coils should be placed at right angles to reduce the coupling coefficient. The method of vertical plate isolation can also be adopted. It is better to solder directly to the circuit with the lead of its components. The shorter the lead, the less the connector and the solder piece are used, because there is distributed capacitance and distributed inductance between the adjacent solder pieces. To avoid placing high noise components around the crystal oscillator, RIN, analog voltage and reference voltage signal.

Finally, to ensure the inherent quality and reliability, take into account the overall beauty, reasonable circuit board planning, components should be parallel or vertical surface, and with the main plate edge parallel or vertical. The components should be uniformly distributed on the panel surface and the density is uniform. In this way, it is not only beautiful, but also easy to assemble and weld. It is easy to produce in batches.

In order to solve the pressure drop caused by the power supply noise and line impedance introduced by the high-frequency circuit, the reliability of the power supply system in the high frequency circuit must be taken into full consideration.  Generally there are two solutions: one is the use of power bus technology for wiring; two is the use of separate power supply layer. Comparatively speaking, the manufacturing process of the latter is more complicated, and the cost is also expensive. Therefore, the network power bus technology can be used to make the wiring, so that each component belongs to different circuits, the current on each bus on the network tends to balance, reducing the voltage drop caused by the line impedance.

High frequency transmitting power, large area copper can be used to search for low resistance grounding multi-point grounding. Because the ground lead impedance and frequency and length is proportional to the working frequency is high, will increase the total impedance, which will increase the total impedance of the electromagnetic interference, so the requirements of wire length as short as possible. Try to reduce the length of the signal line and increase the area of the ground loop.

Set up one or several high frequency decoupling capacitors in the chip power supply and a ground terminal, as chip transient current provides high frequency channel nearby, so the current for power supply line through a larger area of the loop, thereby greatly reducing the noise radiated. To choose the monolithic capacitor ceramic capacitors of high frequency signal as well as decoupling capacitors. A large capacity tantalum capacitor or a polyester capacitor instead of an electrolytic capacitor is used as a charging capacitor for the circuit. Because the distribution inductance of electrolytic capacitor is large, it is invalid to high frequency. When using electrolytic capacitors, the decoupling capacitors with high frequency characteristics should be used in pairs.

Due to the lack of high-speed analysis and simulation guidance, the traditional PCB design can not guarantee the quality of the signal, and most of the problems must be detected after the plate test. This greatly reduces the efficiency of the design, improve the cost, in the fierce market competition is obviously unfavorable. In view of the high speed PCB design industry, put forward a new design idea, a design method of "top-down", through policy analysis and Optimization in many aspects, to avoid the most possible problems, save a lot of time, to meet the project budget, the printed circuit board to produce high quality, avoid tedious high consumption test error etc..

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