PCB Blog - Special Countermeasures for High-frequency Circuits in PCB board Design

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, and high anti-interference. The Protel 99SE design system fully utilizes the advantages of the Windows XP and Windows2000 platforms, and its super-strong design environment for the PCB board module enables the design work to more effectively achieve its design requirements. For designers engaged in high-frequency circuits, it is no longer a simple requirement for the routing rate of PCB boards, but requires designers to have solid theoretical knowledge and rich experience in PCB board design based on the working characteristics of the circuit. And the actual working environment and other aspects to consider its design, only in this way can we make an ideal PCB board. This article aims at the layout and wiring of high-frequency circuits in the process of PCB board design, 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. Designers also need to consider the manufacturability, mechanical structure, heat dissipation, EMI of PCB (Electromagnetic interference), reliability, signal integrity and other aspects of the layout are considered comprehensively. Only in this way can the life, 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, indicator lights, connectors and switches, etc.), and then lay out special components on the circuit. (such as heating elements, transformers, chips, etc.), and lay out some small devices. At the same time, to take into account the requirements of wiring, 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 cross-interference 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, such as adjustable Inductors, 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, transformers, 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. The heating element should be placed on the upper part of the board, 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, such as electrolytic capacitors, should also be kept away from heating devices, otherwise the electrolyte will be dried, resulting in increased resistance and poor performance, which will affect the stability of the 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, it will interfere with the low frequency amplifier. Therefore, they must be isolated or shielded. All levels of the amplifier can be arranged in a straight line according to the schematic diagram. 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. Considering the signal transmission relationship between the functional circuits of each unit, the low-frequency circuit and the high-frequency circuit should also be separated, and the analog circuit and the digital circuit should be separated. 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. In addition, they all have strong magnetic fields, and there should be an appropriate large space or magnetic shielding around them to reduce the impact on other circuits.

4. Electromagnetic interference
Our commonly used methods to eliminate electromagnetic interference include reducing loops, filtering, shielding, reducing the speed of high-frequency devices as much as possible, and increasing the dielectric constant of the PCB board. For example, 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. There is a high potential difference between some components or wires, and the distance should be increased to avoid discharge. Components with high voltage should be arranged in places that are not easily accessible by hand during debugging. 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, which is convenient for installation and welding.

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 mechanical layers and solder mask layers., 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, can effectively shorten the transmission length of the signal, and greatly reduce the interference between the signals. 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, but the higher the number of layers, 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, and arcs or broken lines should be used for transition. This requirement is only used to improve the fixing strength of the steel foil in the low-frequency circuit, 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). When the high-frequency circuit is above 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, and ground wires cannot form current loops. 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, and the ground wire is generally more than 40mil. If possible, the wire should be as wide as possible. When the analog ground wire, digital ground wire, etc. are connected to the public ground wire, a high-frequency choke link is used. In the actual assembly of the high-frequency choke link, a high-frequency ferrite bead with a wire in the center hole is often used. It is generally not expressed on the circuit schematic diagram, and the resulting network table does not contain such components, and its existence will be ignored during wiring. In view of this reality, it can be regarded as an inductor in the schematic diagram. Define a separate component package for it in the PCB component library, and manually move it to a suitable position close to the common ground junction before wiring.

3. Wiring of integrated chips
A high-frequency decoupling capacitor should be set near each integrated circuit block. Since the Protel 99SE software does not consider the positional relationship between the decoupling capacitor and the decoupled integrated circuit when placing components automatically, let the software place it so that the two If the distance between the two is too far, the decoupling effect is not good. At this time, the position of the two must be intervened in advance by manually moving the components to make them close.

4. Copper coating
The main purpose of copper coating is to improve the anti-interference ability of the circuit, and at the same time, it is very beneficial to the heat dissipation of the PCB board and the strength of the PCB board. The copper coating can also play a shielding role. However, large-area strip copper foil cannot be used, because when the PCB board is used for too long, a large amount of heat will be generated, and the strip copper foil is prone to expansion and falling off. Copper foil, and connect the grid to the grounding network of the circuit, so that the grid will have a better shielding effect. The size of the grid is determined by the frequency of interference to be shielded.

3. Conclusion
The design process of high-frequency circuit PCB board is a complex process. In addition to the design strategies discussed above, it also includes signal integrity, including signal crosstalk, and how to suppress noise. Therefore, designers need to have a comprehensive plan when designing With consideration, different methods and techniques are used at each stage of the design cycle to ensure the accuracy of the design, so as to design a reasonable high frequencyPCB board with excellent performance.