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The reason why PCB board design puts forward higher requirements is that with the rapid development of the modern electronics industry, digital and high-frequency circuits are developing in the direction of high speed, low consumption, small size, Forte capacité anti - interférence. The Protel 99SE design system fully utilizes the advantages of the Windows XP and Windows(2)000 platforms, L'environnement de conception du module PCB rend la conception plus efficace pour répondre aux exigences de conception. For designers engaged in high-frequency circuits, La vitesse de câblage des PCB n'est plus une exigence simple, but requires designers to have solid theoretical knowledge and rich experience in PCB board design based on the working characteristics of the circuit. Et la conception de l'environnement de travail réel, Attendez.., only in this way can we make an ideal PCB board. Cet article traite principalement de la disposition et du câblage des circuits à haute fréquence dans le processus de conception des PCB., taking Protel 99SE software as an example, to discuss the countermeasures and design skills of high-frequency circuits in the process of PCB board design.
1. High frequency PCB board layout
Layout operation is very important in the entire PCB board design. Layout is the basis of wiring operation. To achieve a perfect component layout, designers need to think about the layout of components from the perspective of circuit working characteristics and routing. Protel 99SE has the function of automatic layout and has two functions of clustering and statistical layout, but it cannot fully meet the working requirements of high-frequency circuits. Les concepteurs doivent également tenir compte de la fabrication, mechanical structure, Dissipation de chaleur, EMI of PCB (Electromagnetic interference), Fiabilité, signal integrity and other aspects of the layout are considered comprehensively. Ce n'est qu'ainsi que la vie sera meilleure., stability, EMC (electromagnetic compatibility) of the PCB board be effectively improved, and the layout can be made more perfect. For the layout of high-frequency circuits, designers should first consider the layout of those components that are closely matched with the structure and fixed in position (such as power sockets, Feu indicateur, connectors and switches, Attendez..), and then lay out special components on the circuit. (such as heating elements, Transformateurs, chips, etc.), and lay out some small devices. At the same time, Tenir compte des exigences en matière de câblage, the placement of high-frequency components should be as compact as possible, and the wiring of signal lines should be as short as possible, so as to reduce the Interférence croisée of signal lines as much as possible.
1. Mechanical structure
Power sockets, indicator lights, connectors and switches all belong to this category of components, which are all positioning plug-ins related to mechanical dimensions. Usually, the interface between the power supply and the PCB board is placed at the edge of the PCB board, and the distance from the edge of the PCB board is generally not less than 2mm; the indicator light-emitting diode should be placed accurately as needed; switches and some fine-tuning components, Comme un inducteur réglable, adjustable resistors, etc. should be placed close to the edge of the PCB board for easy adjustment and connection; components that need to be replaced frequently must be placed in a location with fewer components for easy replacement. Components with a mass exceeding 15g should be fixed with brackets, and large and heavy components should not be placed directly on the PCB.
2. Heat dissipation
High-power tubes, Transformateurs, rectifier tubes and other heating devices generate a lot of heat when working at high frequencies. Ventilation and heat dissipation should be fully considered in the layout. Such components should be placed on the edge of the PCB board or at the ventilated place. L'élément chauffant doit être placé sur la partie supérieure de la plaque., and the heating element should not be placed on the bottom layer of the double-sided board. High-power rectifier tubes and adjustment tubes should be equipped with radiators and should be kept away from the transformer. Components that are afraid of heat, Comme un condensateur électrolytique, should also be kept away from heating devices, Sinon, l'électrolyte séchera, resulting in increased resistance and poor performance, Cela affecte la stabilité du circuit.
3. Layout of special components
Due to the 50Hz leakage magnetic field generated inside the power supply equipment, when it is cross-connected with some parts of the low frequency amplifier, Il interfère avec les amplificateurs à basse fréquence. Alors..., Ils doivent être isolés ou protégés. Selon le schéma, tous les niveaux de l'amplificateur peuvent être disposés en ligne droite. The advantage of this arrangement is that the ground current of each level is closed and flowed at this level, which does not affect the work of other circuits. The input stage and output stage should be as far away as possible to reduce the parasitic coupling interference between them. Tenir compte de la relation de transmission du signal entre les circuits fonctionnels de chaque Unit é, the low-frequency circuit and the high-frequency circuit should also be separated, Les circuits analogiques et numériques doivent être séparés. The integrated circuit should be placed in the center of the PCB board, so as to facilitate the wiring connection between each pin and other devices. Devices such as inductors and transformers have magnetic coupling and should be placed orthogonally to each other to reduce magnetic coupling. En outre, they all have strong magnetic fields, Un grand espace ou un bouclier magnétique approprié doit être prévu autour de celui - ci afin de réduire au minimum l'impact sur d'autres circuits..
4. Electromagnetic interference
Our commonly used methods to eliminate electromagnetic interference include reducing loops, filtering, shielding, Réduire au minimum la vitesse des équipements à haute fréquence, and increasing the dielectric constant of the PCB board. Par exemple:, decoupling capacitors of integrated circuits should be placed as close as possible. Generally, 0.1uF capacitors are used for operating frequencies below 10MHz, and 0.01uF capacitors are used for operating frequencies above 10MHz. Différence de potentiel élevée entre certains composants ou fils, and the distance should be increased to avoid discharge. Les composants haute tension doivent être disposés de manière à être inaccessibles à la main pendant la mise en service.. Components that are easy to interfere with each other should not be too close, and the input and output components should be as far away as possible to avoid feedback interference. In order to reduce the distribution parameters of high-frequency components, generally placed nearby (irregularly arranged) general circuits (low-frequency circuits) should be arranged according to the rules, Facile à installer et à souder.
2. High frequency PCB board wiring
High-frequency circuits tend to have high integration and high wiring density. The use of multi-layer boards is not only necessary for wiring, but also an effective means to reduce interference. The PCB board system of Protel 99SE can provide 32 signal layers, 16 couches mécaniques et résistances. , solder paste layer and more than 70 working layers for users to choose. Reasonable selection of the number of layers can greatly reduce the size of the PCB board, can make full use of the middle layer to set the shield, can better achieve the nearest grounding, can effectively reduce the parasitic inductance, Peut efficacement réduire la longueur de transmission du signal, Réduit considérablement l'interférence entre les signaux. cross-interference, etc. All of these are beneficial to the reliability of high-frequency circuits. Some data show that the noise of four-layer boards is 20dB lower than that of double-sided boards when using the same material, Mais plus le nombre de couches est élevé,, the more complicated the manufacturing process and the higher the cost.
1. General principles of wiring
The wires between the pins of high-frequency circuit devices should be as short as possible, with as few bends as possible. The wires should be all straight, and sharp bends and sharp corners should be avoided as much as possible. Turns are required, Les arcs ou les lignes pointillées doivent être utilisés pour la transition. Cette exigence n'est utilisée que pour augmenter la résistance de fixation de la feuille d'aluminium dans les circuits à basse fréquence., but meeting this requirement in the high-frequency circuit can reduce the external emission and mutual coupling of the high-frequency signal. In the high-frequency circuit wiring, horizontal and vertical wiring are alternately performed on adjacent layers. Parallel wiring in the same layer cannot be avoided, but a large area of ground wire can be laid on the reverse side of the PCB to reduce interference. For commonly used double-sided boards, many Layers can use an intervening power plane to do this.
2. Power and ground wiring
In order to prevent the ground resistance interference caused by the local current in the multi-level circuit, the circuits at all levels should be grounded at one point (or as concentrated as possible). Lorsque le circuit haute fréquence est supérieur à 30 MHz, a large area of A small area should also be grounded. The devices and wires that are susceptible to interference can be surrounded by ground wires. Various signal traces cannot form loops, Le fil de terre ne doit pas former de boucle de courant. Power wires and ground wires should be close to each other to minimize the enclosed area to reduce electromagnetic interference. Generally, when wiring, the wire width is between 12-80mil, the power wire is generally 20mil-40mil, Les fils de mise à la terre sont généralement supérieurs à 40 mm. If possible, Les fils doivent être aussi larges que possible. When the analog ground wire, Digital Ground Wire, etc. Connexion au sol commun, a high-frequency choke link is used. Dans l'assemblage pratique de l'étranglement à haute fréquence, a high-frequency ferrite bead with a wire in the center hole is often used. Généralement non représenté sur le schéma de circuit, and the resulting network table does not contain such components, Sa présence est ignorée pendant le routage. In view of this reality, Dans un schéma, il peut être considéré comme un inducteur. Define a separate component package for it in the PCB component library, Avant le câblage, déplacez - le manuellement à un endroit approprié près du connecteur de mise à la terre commun..
3. Wiring of integrated chips
A high-frequency decoupling capacitor should be set near each integrated circuit block. Comme le logiciel Protel 99se ne tient pas compte de la relation de position entre le condensateur de découplage et le circuit intégré de découplage lors du placement automatique des composants, let the software place it so that the two If the distance between the two is too far, Mauvais effet de découplage. At this time, La position des deux composants doit être préalablement interposée en déplaçant manuellement les composants pour les fermer..
4. Copper coating
The main purpose of copper coating is to improve the anti-interference ability of the circuit, and at the same time, Ceci est très bénéfique pour la dissipation de chaleur des PCB et la résistance des PCB. The copper coating can also play a shielding role. Cependant,, large-area strip copper foil cannot be used, Parce que le PCB est utilisé trop longtemps, a large amount of heat will be generated, Et les bandes de cuivre se dilatent et tombent facilement. Copper foil, Et connectez le réseau électrique au réseau au sol du circuit, so that the grid will have a better shielding effect. La taille de la grille est déterminée par la fréquence d'interférence à protéger.
3. Conclusion
The design process of high-frequency circuit PCB board is a complex process. In addition to the design strategies discussed above, Il comprend également l'intégrité du signal, including signal crosstalk, Et comment réduire le bruit. Therefore, designers need to have a comprehensive plan when designing With consideration, Utiliser différentes méthodes et techniques à chaque étape du cycle de conception pour assurer l'exactitude de la conception, so as to design a reasonable Haute fréquencePCB board with excellent performance.
