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Conception de PCB à grande vitesse basée sur Protel
Blogue PCB
Conception de PCB à grande vitesse basée sur Protel

Conception de PCB à grande vitesse basée sur Protel

2022-02-24
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Author:pcb

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. Fiabilité et validité. 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. Dans la conception de circuits à grande vitesse, 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, Mauvais fonctionnement du circuit, 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, La conception doit être soigneusement examinée afin de résoudre certains problèmes de fiabilité qui peuvent survenir lors de la mise en page et du câblage., shorten the product development cycle, Améliorer la compétitivité du marché.

PCB board

2. Layout design of high frequency system
In the PCB board design of the circuit, the layout is an important link, Les résultats du câblage influeront directement sur l'effet du câblage et la fiabilité du système., 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. Contournement, improve the reliability and effectiveness of circuit work. Pendant la mise en page, comprehensive consideration should be given to the mechanical structure, heat dissipation, electromagnetic interference, Commodité du câblage futur, Esthétique. 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, Amélioration de l'effet de découplage du condensateur. 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. Deuxièmement,, 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. Les parties résistantes à la chaleur, telles que les condensateurs électrolytiques, doivent être éloignées des Parties chauffantes., Sinon, l'électrolyte séchera, resulting in increased resistance and poor performance, Cela affecte la stabilité du circuit. There should be enough space in the layout to arrange the guard structure and prevent the introduction of various parasitic couplings. Empêcher le couplage électromagnétique entre les bobines sur la carte de circuit imprimé, Les deux bobines doivent être placées à angle droit pour réduire le facteur de couplage.. 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, C'est mieux.. Do not use connectors and solder pads, En raison de la capacité distribuée et de l'inductance distribuée entre les Pads adjacents. Évitez de placer des éléments bruyants autour de l'oscillateur à cristaux, RIN, Tension analogique, and reference voltage signal traces. While ensuring the inherent quality and reliability, taking into account the overall aesthetics, Planification rationnelle des circuits imprimés, 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, La densité devrait être la même. 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. Du point de vue de l'anti - interférence, reasonable wiring is to try to reduce the line resistance, Capacité distribuée et inductance errante dans le circuit. , the resulting stray magnetic field is reduced to a certain degree, Capacité de distribution, 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. Cependant,, many designers only focus on the "layout rate", Les améliorations apportées aux outils de conception de Protel pour tenir compte des changements dans les caractéristiques de l'équipement n'ont pas non plus été utilisées dans la conception., Cela entraîne non seulement un gaspillage important des ressources des outils de conception, 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, C'est mieux.. 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. Lors du routage avec 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. Dans les règles du menu conception, sélectionnez la topologie de câblage la plus courte.
(3) The less the alternation between the lead layers between the pins of the high-frequency circuit device, the better. Oui., 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, Notez l '« interférence croisée » introduite par le câblage parallèle du fil de signalisation.. 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, Mais dans deux couches adjacentes, 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", Sélectionnez horizontal pour le haut et vertical pour le bas. 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, Il est également très bon pour la dissipation de chaleur et la résistance des panneaux imprimés.
(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, La ligne d'alimentation et la ligne au sol du circuit sont réglées à la largeur de la ligne de signal. 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, Vous pouvez utiliser la méthode de routage de la chaîne Daisy, Peut efficacement éviter la formation de boucles, 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, Estimer le courant à travers le cordon d'alimentation et déterminer la largeur requise du conducteur. 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. En même temps, the direction of the power line and the ground line is consistent with the direction of data transmission, Cela aide à améliorer la résistance au bruit. When necessary, Des dispositifs d'étranglement à haute fréquence en Ferrite enroulé de cuivre peuvent être ajoutés aux lignes électriques et au sol pour bloquer la conduction du bruit à haute fréquence..
(9) The wiring width of the same network should be kept the same. Le changement de largeur de ligne peut entraîner une impédance caractéristique inégale de la ligne. 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. Lorsque la distance centrale de la ligne ne dépasse pas 3 fois la largeur de la ligne, 70% of the electric field can be kept without mutual interference, C'est ce qu'on appelle le principe 3W.. 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, La fiabilité du système d'alimentation en circuit à haute fréquence doit être pleinement prise en considération.. Il existe généralement deux solutions: l'une consiste à utiliser la technologie Power bus pour le câblage; L'autre est d'utiliser une couche d'alimentation séparée. In comparison, the production process of the latter is more complicated and the cost is more expensive. Alors..., a network-type power bus technology can be used for wiring, De sorte que chaque composant appartient à une boucle différente, and the currents on each bus on the network tend to be balanced, Réduire la chute de tension causée par l'impédance de la ligne. The high-frequency transmission power is relatively large, Une grande surface de cuivre peut être utilisée pour trouver un plan de mise à la terre à faible résistance à proximité pour la mise à la terre multipoint. 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. Un ou plusieurs condensateurs de découplage à haute fréquence sont fournis aux bornes d'alimentation et de mise à la terre de la puce pour fournir un canal à haute fréquence adjacent au courant transitoire de la puce intégrée., De cette façon, le courant ne passe pas à travers le cordon d'alimentation de la grande zone de boucle, Réduit considérablement le bruit rayonné. It is necessary to choose a monolithic capacitive ceramic capacitor with a good high-frequency signal as a decoupling capacitor. Utilisation de condensateurs de tantale ou de polyester de grande capacité au lieu de condensateurs électrolytiques comme condensateurs de stockage d'énergie pour charger les circuits. Because the distributed inductance of electrolytic capacitors is large, Il n'est pas efficace pour les hautes fréquences. 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. C'est un chiffre approximatif.. Generally, La bande de base du câble coaxial est de 50 ), the frequency band is 75 Ω, and the twisted pair is 100 Ω. It is only an integer, Pour faciliter l'appariement. According to the specific circuit analysis, parallel AC termination is adopted, and the resistor and capacitor network is used as the termination impedance. La r ésistance terminale R doit être inférieure ou égale à l'impédance de la ligne de transmission Z0, Le condensateur c doit être supérieur à 100 PF. It is recommended to use a 0.Condensateur céramique multicouche 1uf. 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, Réduire la longueur parallèle, and use jog wiring if necessary.
(2) When high-speed signal lines meet the conditions, adding termination matching can reduce or eliminate reflections, Pour réduire les échanges.
(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, Peut jouer un rôle d'isolement, réduire les échanges.
Due to the lack of high-speed analysis and simulation guidance in traditional PCB board design, La qualité du signal n'est pas garantie, and most problems cannot be found until after the plate-making test. Cela réduit considérablement l'efficacité de la conception et augmente les coûts, which is obviously disadvantageous in the fierce market competition. Alors..., for the design of high-speed PCB boards, Les initiés de l'industrie proposent un nouveau concept de conception, which has become a "top-down" design method. Analyse et optimisation des politiques, most of the possible problems have been avoided and saved A lot of time, Veiller à ce que le budget du projet soit respecté, producing high-quality printed boards, Éviter les erreurs de test fastidieuses et coûteuses, etc. La transmission de signaux numériques par des lignes différentielles est une mesure efficace pour contrôler les facteurs de défaillance de l'intégrité des signaux dans les circuits numériques à grande vitesse.. The differential line on the printed circuit board is equivalent to the differential microwave integrated transmission line pair working in the quasi-TEM mode, Où la ligne différentielle située sur la couche supérieure ou la couche inférieure du PCB est équivalente à la ligne Microstrip couplée, which is located on the multi-layer PCB board. Ligne différentielle de la couche interne, équivalent à une bande de couplage à large bord. When the digital signal is transmitted on the differential line, C'est une méthode de transmission en mode impair, that is, La différence de phase entre les signaux positifs et négatifs est de 180°, and the noise is coupled on a pair of differential lines in a common-mode manner. La tension ou le courant du circuit est soustrait, so that the signal can be obtained to eliminate common mode noise. L'amplitude de basse tension ou la sortie d'entraînement de courant de la paire différentielle permet une intégration à grande vitesse et une faible consommation d'énergie.

6. Conclusion
With the continuous development of electronic technology, it is imperative to understand the theory of signal integrity, Ensuite, guider et vérifier la conception des PCB à grande vitesse. Certaines expériences résumées dans cet article peuvent aider les concepteurs de PCB à grande vitesse à raccourcir le cycle de développement, avoid unnecessary detours, Économiser les ressources humaines et matérielles. Designers must continue to research and explore in actual work, Accumuler de l'expérience, and combine new technologies to design high-speed PCB board with excellent performance.