PCB Blog - How to use protel software to design high-speed PCB board

In order to continue to improve everyone's understanding of PCB board design software, this article will explain how to design high-speed PCB boards based on protel circuit design software. The significance of circuit design software is to design circuits. Without circuit design software, circuit design will become very troublesome. This article will explain how to design high-speed PCB boards based on protel circuit design software.

1. Questions

In high-speed circuit design, the inductance and capacitance on the circuit board can make the wired equivalent to a transmission line. Improper placement of termination components or incorrect routing of high-speed signals can cause problems with transmission line effects, resulting in incorrect data output from the system, improper circuit operation, or even no operation at all. Based on the transmission line model, it can be concluded that 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 board that can work reliably, the design must be fully and carefully considered to solve some unreliable problems that may occur during layout and wiring, shorten the product development cycle, and improve market competitiveness. Discuss some relevant principles of layout and wiring that need to be paid attention to in the process of using PROTEL design software to realize high-speed circuit printed circuit board design, and provide some practical and proven high-speed circuit layout and wiring techniques to improve high-speed circuit boards. Design reliability and validity. The results show that the design shortens the product development cycle and enhances market competitiveness.

2. Layout design of the high-frequency system

In the PCB board design of the circuit, the layout is an important link. The quality of the layout result will directly affect the effect of the wiring and the reliability of the system, which is time-consuming and difficult in the entire printed circuit board design. The complex environment of high-frequency PCB boards makes the layout design of high-frequency systems difficult to use the theoretical knowledge they have learned. It requires that the layout personnel must have rich experience in high-speed PCB board making, so as to avoid walking in the design process. Detours, and 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, divide the function of the entire circuit before the layout, separate the high-frequency circuit from the low-frequency circuit, and separate the analog circuit from the digital circuit. Avoid the transmission delay caused by the long wire, and improve the decoupling effect of the capacitor. In addition, pay attention to the relative position and orientation of pins and circuit components, and other tubes to reduce the mutual influence. All high-frequency components should be kept away from the chassis and other metal plates to reduce parasitic coupling. Secondly, attention should be paid to the thermal and electromagnetic effects between components during layout. These effects are particularly serious for high-frequency systems, and measures should be taken to keep them away or isolate, dissipate heat, and shield them. High-power rectifier tubes and adjustment tubes should be equipped with radiators and should be 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. There should be enough space in the layout to arrange the guard structure and prevent the introduction of various parasitic couplings. To prevent electromagnetic coupling between the coils on the printed circuit board, the two coils should be placed at right angles, in order to reduce the coupling coefficient. The method of vertical plate isolation can also be used. Directly use the leads of its components to solder on the circuit. The shorter the lead, the better. Do not use connectors and solder pads, because there are distributed capacitance and distributed inductance between adjacent solder pads. Avoid placing high-noise components around the crystal oscillator, RIN, analog voltage, and reference voltage signal traces. While ensuring the inherent quality and reliability, taking into account the overall aesthetics, and reasonable circuit board planning, 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 the same. This is not only beautiful but also easy to install and weld and easy to mass produce.

3. Wiring of high-frequency system

In high-frequency circuits, the distribution parameters of the 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 certain degree, so that the distributed capacitance, leakage magnetic flux, electromagnetic mutual inductance, and other interference caused by the noise of the circuit are suppressed. The application of PROTEL design tools is quite common in China. However, many designers only focus on the "through rate", and the improvements made to the PROTEL design tools to adapt to changes in device characteristics are not used in the design, which not only makes The waste of design tool resources serious, which makes it difficult to exert the excellent performance of many new devices. The following introduces some special functions that the PROTEL99 SE tool can provide.

1) The less the leads between the pins of the high-frequency circuit device are bent, the better. Use a full straight line. When bending is required, it can be bent with a 45° folded line or a circular arc, which can reduce the external emission and mutual coupling of high-frequency signals. . When routing with PROTEL, select 45-Degrees or Rounded in "RouTIng Corners" in "Rules" in the "Design" menu. You can also use the shift+space keys to quickly switch between lines.

2) The shorter the lead between the pins of the high-frequency circuit device, the better. The effective means of PROTEL 99 to meet the short wiring is to make wiring reservations for individual key high-speed networks before automatic wiring. Select shortest in "RouTIng Topology" in the "Design" menu "rules".

3) The less the alternation between the lead layers between the pins of the high-frequency circuit device, the better. That is, the fewer vias used in the component connection process, the better. One via can bring about 0.5pF of distributed capacitance, and reducing the number of vias can significantly increase the speed.

4) For high-frequency circuit wiring, pay attention to the "cross-interference" or crosstalk introduced by the parallel wiring of the signal line. If parallel distribution cannot be avoided, a large area of "ground" can be arranged on the opposite side of the parallel signal lines to greatly reduce interference. Parallel lines in the same layer are almost unavoidable, but in two adjacent layers, the direction of the lines must be taken to be perpendicular to each other, which is not difficult to achieve in PROTEL but easy to ignore. In the "Design" menu "rules" in "RouTIngLayers", select Horizontal for Top layer, and select Vertical for bottom layer. In addition, the "Polygonplane" function is provided in "place", that is, the polygon grid copper foil surface. If placed, the polygon will be taken as one surface of the entire printed circuit board, and this copper will be applied. It is connected to the GND of the circuit, which can improve the high-frequency anti-interference ability, and also has great benefits for heat dissipation and printing board strength.

5) Measures to be surrounded by ground wires are implemented for particularly important signal lines or local units. "Outline selected objects" is provided in "Tools", which can be used to automatically "ground" the selected important signal lines (such as oscillator circuit LT and X1).

6) Generally, the power line and ground line of the circuit is set wider than the signal line. You can use the "Classes" in the "Design" menu to classify the network into power network and signal network. Combined with the setting of wiring rules, it can be easily carried out. Line width switching of power lines and signal lines.

7) All kinds of traces cannot form a loop, and the ground wire cannot form a current loop. If a loop circuit is generated, it will cause great interference in the system. In this regard, a daisy-chain wiring method can be used, which can effectively avoid the formation of loops, branches, or tree stumps during wiring, but it also brings about the problem of not easy wiring.

8) According to the data and design of various chips, estimate the current passing through the power line and determine the required wire width. According to the empirical formula, it can be obtained: W (line width) ≥ L (mm/A) × I (A). According to the current size, try to increase the width of the power line to reduce the loop resistance. At the same time, the direction of the power line and the ground line is consistent with the direction of data transmission, which helps to enhance the anti-noise capability. When necessary, a high-frequency choke device made of copper wire wound ferrite can be added to the power line and ground line to block the conduction of high-frequency noise.

9) The wiring width of the same network should be kept the same. The change of the line width will lead to an uneven characteristic impedance of the line. When the transmission speed is high, the reflection will occur, which should be avoided as much as possible in the design. At the same time, increase the line width of the parallel lines. When the center distance of the lines does not exceed 3 times the line width, 70% of the electric field can be kept without mutual interference, which is called the 3W principle. In this way, the influence of distributed capacitance and distributed inductance caused by parallel lines can be overcome.

4. Design of power line 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 the power bus technology for wiring; the other is to use a separate power supply layer. In comparison, the production process of the latter is more complicated and the cost is more expensive. Therefore, the 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. The high-frequency transmission power is relatively large, and a large area of copper can be used to find a low-resistance ground plane nearby for multi-point grounding. Because the inductive reactance of the ground lead is proportional to the frequency and length, the common ground impedance will increase when the operating frequency is high, which will increase the electromagnetic interference generated by the common ground impedance, so the length of the ground wire is required to be as short as possible. Minimize the length of the signal lines and increase the area of the ground loop. One or several high-frequency decoupling capacitors are set at the power and ground terminals of the chip to provide a nearby high-frequency channel for the transient current of the integrated chip, so that the current will not pass through the power supply line with a large loop area, thereby greatly reducing radiated noise. It is necessary to choose a monolithic capacitive ceramic capacitor with a good high-frequency signal as a decoupling capacitor. Use large-capacity tantalum capacitors or polyester capacitors instead of electrolytic capacitors as energy storage capacitors for circuit charging. Because the distributed inductance of electrolytic capacitors is large, it is ineffective for high frequencies. When using electrolytic capacitors, use them in pairs with decoupling capacitors with good high-frequency PCB board characteristics.