PCB Blog - Design of high-speed PCB board based on PROTEL

In the process of high-speed printed circuit printeds design, some principles related to layout and wiring that need to be paid attention to when using PROTEL design software, provide some practical and proven high-speed circuit layout and wiring techniques, improve high-speed circuit board design. reliability and validity. The results show that the design shortens the product development cycle and enhances the market competitiveness.

1. The question is raised
With the massive increase in the complexity and integration of electronic system designs, 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 can make the wire 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.

2. Layout design of high frequency system
In the PCB board design of the circuit, the layout is an important link, and 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 the high-frequency PCB board makes it difficult to use the theoretical knowledge learned for the layout design of the high-frequency system. It requires that the layout person must have rich experience in high-speed PCB board making, so as to avoid the design process. Detours, 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 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. 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 the 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 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, 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. In this way, it is not only beautiful, but also easy to install and weld, and it is 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 noise of the circuit are suppressed. The application of PROTEL design tools has been quite common in China. However, many designers only focus on the "layout 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 is serious, which makes it difficult to exert the excellent performance of many new devices. The following introduces some special functions that 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 it needs to be bent, it can be bent with a 45° folded line or a circular arc, which can reduce the external emission of high-frequency signals and the mutual interference. coupling. When routing with PROTEL, select 45-Degrees or Rounded in "Routing Corners" in "Design" menu "rules". You can also use the shift+space key 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 wiring shortening is to make wiring reservations for individual key high-speed networks before automatic wiring. Select shortest in "Routing Topology" in "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. A 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" that is introduced by the parallel wiring of signal lines. 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 Toplayer and Vertical for BottomLayer. 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 selectedobjects" 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 are set wider than the signal line. You can use "Classes" in the "Design" menu to classify the network into power network and signal network. It is convenient to combine the setting of wiring rules. Switch the line width 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 the uneven characteristic impedance of the line. When the transmission speed is high, reflection will occur, which should be avoided as much as possible in the design. At the same time, increase the line width of 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 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 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, a network-type power bus technology can be used for wiring, so that each component belongs to a different loop, and the currents on each bus on the network tend to be balanced, reducing the voltage drop problem caused by 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 characteristics.

5. Other high-speed circuit design technologies
Impedance matching refers to a working state in which the load impedance and the internal impedance of the excitation source are matched to each other to obtain the power output. When wiring high-speed PCB boards, in order to prevent signal reflection, the impedance of the line is required to be 50 Ω. This is an approximate figure. Generally, the baseband of coaxial cable is 50 Ω, the frequency band is 75 Ω, and the twisted pair is 100 Ω. It is only an integer, for the convenience of matching. According to the specific circuit analysis, parallel AC termination is adopted, and the resistor and capacitor network is used as the termination impedance. The termination resistor R should be less than or equal to the transmission line impedance Z0, and the capacitor C must be greater than 100 pF. It is recommended to use a 0.1UF multilayer ceramic capacitor. The capacitor has the function of blocking low frequency and passing high frequency, so the resistor R is not the DC load of the driving source, so this termination method does not have any DC power consumption. Crosstalk refers to undesired voltage noise interference on adjacent transmission lines due to electromagnetic coupling when a signal propagates on a transmission line. Coupling is divided into capacitive coupling and inductive coupling. Excessive crosstalk may cause false triggering of the circuit, resulting in the system not working properly. 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 jog 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, the crosstalk can be significantly reduced by limiting the height of the traces to be higher than the ground plane range.
(4) Under the condition that the wiring space allows, insert a ground wire between the two lines with serious crosstalk, which can play the role of isolation and reduce the crosstalk.
Due to the lack of high-speed analysis and simulation guidance in traditional PCB board design, the quality of the signal cannot be guaranteed, and most problems cannot be found until after the plate-making test. This greatly reduces the design efficiency and increases the cost, which is obviously disadvantageous in the fierce market competition. Therefore, for the design of high-speed PCB boards, people in the industry have proposed a new design idea, which has become a "top-down" design method. After various policy analysis and optimization, most of the possible problems have been avoided and saved A lot of time, ensuring that project budgets are met, producing high-quality printed boards, avoiding tedious and costly testing errors, etc. The use of differential lines to transmit digital signals is an effective measure to control the factors that destroy signal integrity in high-speed digital circuits. The differential line on the printed circuit board is equivalent to the differential microwave integrated transmission line pair working in the quasi-TEM mode, wherein the differential line located on the top or bottom layer of the PCB board is equivalent to the coupled microstrip line, which is located on the multi-layer PCB board. A differential line on the inner layer, equivalent to a broadside coupled stripline. When the digital signal is transmitted on the differential line, it is an odd-mode transmission method, that is, the phase difference between the positive and negative signals is 180°, and the noise is coupled on a pair of differential lines in a common-mode manner. The voltage or current of the circuit is subtracted, so that the signal can be obtained to eliminate common mode noise. The low-voltage amplitude or current-driven output of the differential line pair realizes the requirements of high-speed integration and low power consumption.

6. Conclusion
With the continuous development of electronic technology, it is imperative to understand the theory of signal integrity, and then guide and verify the design of high-speed PCB boards. Some experiences summarized in this article can help high-speed circuit PCB board designers shorten the development cycle, avoid unnecessary detours, and save manpower and material resources. Designers must continue to research and explore in actual work, accumulate experience, and combine new technologies to design high-speed PCB board with excellent performance.