Electronic Design - High-speed PCB circuit board design based on PROTEL

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Discuss some layout and wiring related principles that need to be paid attention to in the process of using PROTEL design software to realize high-speed circuit PCB design, and provide some practical and proven high-speed circuit layout and wiring techniques to improve the reliability of high-speed circuit board design Sex and effectiveness. The results show that the design shortens the product development cycle and enhances market competitiveness.

1 Questions raised

With the large-scale increase in the design complexity and integration of electronic systems, clock speeds and device rise times are getting faster and faster, and high-speed circuit design has become an important part of the design process. In high-speed circuit design, the inductance and capacitance on the circuit board will make the wire equivalent to a transmission line. Incorrect layout of termination components or incorrect wiring of high-speed signals will cause transmission line effect problems, which will cause the system to output incorrect data, and the circuit does not work properly or even does not work at all. Based on the transmission line model, to sum up, the transmission line will bring adverse effects such as signal reflection, crosstalk, electromagnetic interference, power supply and ground noise to the circuit design.

In order to design a high-speed PCB circuit board that can work reliably, it is necessary to fully and carefully consider the design, solve some unreliable problems that may arise during layout, shorten the product development cycle, and improve market competitiveness.

2 Layout design of high frequency system

In the PCB design of the circuit, the layout is an important link, and the result of the layout will directly affect the effect of the wiring and the reliability of the system, which is the most time-consuming and difficult in the entire printed circuit board design. The complex environment of high-frequency PCB makes the layout design of the high-frequency system difficult to use the learned theoretical knowledge. It requires that the person who lays out must have rich experience in high-speed PCB manufacturing, so as to avoid detours in the design process. Improve the reliability and effectiveness of circuit work. In the process of layout, comprehensive consideration should be given to the mechanical structure, heat dissipation, electromagnetic interference, convenience of future wiring, and aesthetics.

First of all, before layout, the entire circuit is divided into functions. The high-frequency circuit is separated from the low-frequency circuit, and the analog circuit and the digital circuit are separated. Each functional circuit is placed as close as possible to the center of the chip. Avoid transmission delay caused by excessively long wires, and improve the decoupling effect of capacitors. In addition, pay attention to the relative positions and directions between the pins and circuit components and other tubes to reduce their mutual influence. All high-frequency components should be far away from the chassis and other metal plates to reduce parasitic coupling.

Second, attention should be paid to the thermal and electromagnetic effects between components during PCB layout. These effects are particularly serious for high-frequency systems, and measures such as distance or isolation, heat dissipation, and shielding should be taken. The high-power rectifier tube and adjustment tube should be equipped with a radiator and kept away from the transformer. Heat-resistant components such as electrolytic capacitors should be kept away from heating components, otherwise the electrolyte will be dried, resulting in increased resistance and poor performance, which will affect the stability of the circuit.

Finally, while ensuring the inherent quality and reliability, while taking into account the overall beauty, reasonable circuit board planning should be carried out. The components should be parallel or perpendicular to the board surface, and parallel or perpendicular to the main board edge. The distribution of components on the board surface should be as uniform as possible and the density should be consistent. In this way, it is not only beautiful, but also easy to assemble and weld, and it is easy to mass produce.

3 Wiring of high frequency system

In high-frequency circuits, the distribution parameters of resistance, capacitance, inductance and mutual inductance of the connecting wires cannot be ignored. From the perspective of anti-interference, reasonable wiring is to try to reduce the line resistance, distributed capacitance, and stray inductance in the circuit., The resulting stray magnetic field is reduced to a minimum, so that the distributed capacitance, leakage magnetic flux, electromagnetic mutual inductance and other interference caused by noise are suppressed.

The following introduces some special functions that PROTEL99 SE tool can provide.

(1) The lead between the pins of the high-frequency circuit device should be bent as little as possible. It is best to use a full straight line. When bending is required, a 45° fold line or a circular arc can be used to reduce the external emission and mutual interference of high-frequency signals. The coupling between. When using PROTEL wiring, you can select 45-Degrees or Rounded in the "Routing Corners" in the "Design" menu "rules". You can also use the shift + space keys to quickly switch between the lines.

(2) The shorter the lead between the pins of the high-frequency circuit device, the better.

The most effective way for PROTEL 99 to meet the shortest wiring is to make a wiring appointment for individual key high-speed networks before automatic wiring. Select shortest in "Routing Topology" in "Design" menu "rules".

(3) Alternation of lead layers between pins of high-frequency circuit devices is as small as possible. That is, the fewer vias used in the component connection process, the better.

(4) For high-frequency circuit wiring, pay attention to the "cross interference" introduced by the parallel wiring of the signal line, that is, crosstalk. If parallel distribution is unavoidable, a large area of "ground" can be arranged on the opposite side of the parallel signal line

4 Design of power cord and ground wire

In order to solve the voltage 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 fully considered. There are generally two solutions: one is to use power bus technology for wiring; the other is to use a separate power supply layer. In comparison, the latter's manufacturing process is more complicated and the cost is more expensive. Therefore, network-type power bus technology can be used for wiring, so that each component belongs to a different loop, and the current on each bus on the network tends to be balanced, reducing the voltage drop caused by the line impedance.

5 Other high-speed circuit design techniques

Crosstalk refers to the undesirable voltage noise interference caused by electromagnetic coupling to adjacent transmission lines when the signal propagates on the transmission line. Coupling is divided into capacitive coupling and inductive coupling. Excessive crosstalk may cause false triggering of the circuit and cause the system to fail to work normally. According to some characteristics of crosstalk, several main methods to reduce crosstalk can be summarized:

(1) Increase the line spacing, reduce the parallel length, and use the jog method for wiring if necessary.

(2) When high-speed signal lines meet the conditions, adding termination matching can reduce or eliminate reflections, thereby reducing crosstalk.

(3) For microstrip transmission lines and strip transmission lines, restricting the trace height to within the range of the ground plane can significantly reduce crosstalk.

The use of differential lines to transmit digital signals is an effective measure to control factors that destroy signal integrity in high-speed digital circuits. The differential line on the printed circuit board is equivalent to a differential microwave integrated transmission line pair working in quasi-TEM mode. Among them, the differential line on the top or bottom of the PCB is equivalent to the coupled microstrip line and is located in the inner layer of the multilayer PCB The differential line is equivalent to a broadside coupled strip line.