With the development of communication technology, handheld wireless radio frequency circuit technology is used more and more widely, such as: wireless pagers, mobile phones, wireless PDAs, etc. The performance indicators of the radio frequency circuit directly affect the quality of the entire product. One of the biggest features of these handheld products is miniaturization, and miniaturization means that the density of components is very high, which makes the mutual interference of components (including SMD, SMC, bare chips, etc.) very prominent. Improper handling of electromagnetic interference signals may cause the entire circuit system to fail to work properly. Therefore, how to prevent and suppress electromagnetic interference and improve electromagnetic compatibility has become a very important issue when designing RF circuit PCBs. The same circuit, different PCB design structure, its performance index will be very different.

　　1. Selection of plates

　　The substrates of printed circuit boards include two categories: organic and inorganic. The most important properties in the substrate are dielectric constant εr, dissipation factor (or dielectric loss) tanδ, thermal expansion coefficient CET and moisture absorption rate. Among them, εr affects the circuit impedance and the signal transmission rate. For high frequency circuits, the permittivity tolerance is a more critical factor to consider first, and a substrate with a small permittivity tolerance should be selected.

　　2. PCB design process

　　Since the use of Protel99SE software is different from that of Protel98 and other software, first, the process of PCB design using Protel99SE software is briefly discussed.

　　①Because Protel99SE adopts the project (PROJECT) database mode management, it is implicit under Windows99, so you should first set up a database file to manage the designed circuit schematic diagram and PCB layout.

　　②The design of the schematic diagram. In order to realize the network connection, the components used must exist in the component library between the principle design, otherwise, the required components should be made in SCHLIB and stored in the library file. Then, just call the required components from the component library and connect them according to the designed circuit diagram.

　　③After the schematic design is completed, a netlist can be formed for use in PCB design.

　　④PCB design. a. Determination of PCB shape and size. The shape and size of the PCB are determined according to the position of the designed PCB in the product, the size and shape of the space, and the cooperation with other components. in MECHANICAL

　　The LAYER layer uses the PLACETRACK command to draw the outline of the PCB. b. According to the requirements of SMT, make positioning holes, eyes, reference points, etc. on the PCB. c. The production of components. If you need to use some special components that do not exist in the component library, you need to make the components before the layout. The process of making components in Protel99SE is relatively simple. After selecting the "MAKELIBRARY" command in the "DESIGN" menu, you will enter the component making window, and then select the "NEWCOMPONENT" command in the "TOOL" menu to design components. . At this time, it is only necessary to draw the corresponding pads in a certain position with PLACEPAD and other commands on the TOPLAYER layer according to the shape and size of the actual components and edit them into the required pads (including the shape, size, inner diameter and angle of the pads). In addition, the corresponding pin name of the pad should be marked), and then use the PLACETRACK command to draw the largest shape of the component in the TOPOVERLAYER layer, and take a component name and store it in the component library. d. After the components are made, the layout and wiring are carried out. These two parts are discussed in detail below. e. Inspection must be carried out after the above process is completed. On the one hand, it includes the inspection of the circuit principle, and on the other hand, it must also check the mutual matching and assembly problems. The circuit principle can be checked manually, or it can be checked automatically by the network (the network formed by the schematic diagram can be compared with the network formed by the PCB). f. After checking, the files are archived and output. In Protel99SE, you must use the "EXPORT" command in the "FILE" option to store the file in the specified path and file (the "IMPORT" command is to transfer a file into Protel99SE). Note: After the "SAVECOPYAS..." command in the "FILE" option in Protel99SE is executed, the selected file name is not visible in Windows98, so the file cannot be seen in the resource manager. This is not exactly the same as the "SAVEAS..." function in Protel98.

　　3. Wiring

　　After the layout of the components is basically completed, the wiring can be started. The basic principle of wiring is: after the assembly density permits, try to use a low-density wiring design, and the thickness of the signal traces should be as consistent as possible, which is conducive to impedance matching.

　　For the RF circuit, the unreasonable design of the direction, width and line spacing of the signal lines may cause cross-interference between the signal signal transmission lines; in addition, the system power supply itself also has noise interference, so it must be integrated when designing the PCB RF circuit board. Consider, reasonable wiring.

　　When wiring, all traces should be kept away from the frame of the PCB board (about 2mm), so as to avoid disconnection or hidden danger of disconnection when the PCB board is made. The power line should be as wide as possible to reduce the loop resistance. At the same time, the direction of the power line and the ground line should be consistent with the direction of data transmission to improve the anti-interference ability; the signal line should be as short as possible, and minimize the excessive The number of holes; the connection between the components is as short as possible to reduce the distribution parameters and mutual electromagnetic interference; for incompatible signal lines should be far away from each other, and try to avoid parallel lines, and on both sides of the positive The signal lines should be perpendicular to each other; when wiring, the address side that needs a corner should be at an angle of 135°, and a right angle should be avoided.

　　When wiring, the lines directly connected to the pads should not be too wide, and the traces should be as far away from the disconnected components as possible to avoid short circuits; the vias should not be drawn on the components, and should be kept away from the disconnected components as much as possible to avoid production. There are phenomena such as virtual welding, continuous welding, and short circuit.

　　In the design of RF circuit PCB, the correct wiring of power lines and ground lines is particularly important, and reasonable design is the most important means to overcome electromagnetic interference. A considerable number of interference sources on the PCB are generated by the power supply and the ground wire, and the noise interference caused by the ground wire is the largest.

　　The main reason why the ground wire is prone to electromagnetic interference is that the ground wire has impedance. When there is current flowing through the ground wire, a voltage will be generated on the ground wire, resulting in a ground wire loop current and a ground wire loop interference. When multiple circuits share a ground wire, common impedance coupling is formed, resulting in so-called ground wire noise. Therefore, when routing the ground wire of the RF circuit PCB, you should do:

　　*First of all, the circuit is divided into blocks. The RF circuit can basically be divided into high-frequency amplification, frequency mixing, demodulation, local oscillator and other parts. It is necessary to provide a common potential reference point for each circuit module, that is, the ground wire of each module circuit. , so that the signal can be transmitted between different circuit modules. Then, it is summarized at the place where the RF circuit PCB is connected to the ground wire, that is, the total ground wire. Since there is only one reference point, there is no common impedance coupling and therefore no mutual interference problems.

　　* The digital area and the analog area should be isolated from the ground as much as possible, and the digital ground and the analog ground should be separated, and finally connected to the power ground.

　　*The ground wire inside each part of the circuit should also pay attention to the principle of single-point grounding, minimize the area of the signal loop, and connect it with the address of the corresponding filter circuit nearby.

　　*When space allows, it is better to isolate each module with ground wire to prevent the signal coupling effect between each other.

　　4. Layout of components

　　Since SMT generally uses infrared furnace heat flow welding to realize the welding of components, the layout of components affects the quality of solder joints, which in turn affects the yield of products. For RF circuit PCB design, electromagnetic compatibility requires that each circuit module does not generate electromagnetic radiation as much as possible, and has a certain anti-electromagnetic interference ability. Therefore, the layout of components also directly affects the interference and anti-interference ability of the circuit itself. , which is also directly related to the performance of the designed circuit. Therefore, in addition to considering the layout of ordinary PCB design when designing a RF circuit PCB, it is also necessary to consider how to reduce the mutual interference between various parts of the RF circuit, how to reduce the interference of the circuit itself to other circuits and the circuit itself. anti-interference ability. According to experience, the effect of the RF circuit depends not only on the performance indicators of the RF circuit board itself, but also on the interaction with the CPU processing board. Therefore, when designing a PCB, a reasonable layout is particularly important.

　　The general principle of layout: components should be arranged in the same direction as possible, and the phenomenon of poor soldering can be reduced or even avoided by selecting the direction in which the PCB enters the tin melting system; according to experience, there must be at least a 0.5mm spacing between components to meet the melting tin of components. Requirements, if the space of the PCB board allows, the spacing of components should be as wide as possible. For double-sided PCB boards, one side should generally be designed with SMD and SMC components, and the other side should be discrete components.

　　Layout should pay attention to:

　　*First of all, determine the position of the interface components with other PCB boards or systems on the PCB board, and you must pay attention to the coordination between the interface components (such as the direction of the components, etc.).

　　*Because the size of the handheld products is very small, and the components are very compactly arranged, so for the larger components, priority must be given to determine the corresponding position, and consider the problem of mutual cooperation.

　　* Carefully analyze the circuit structure, process the circuit in blocks (such as high-frequency amplifier circuit, mixing circuit and demodulation circuit, etc.), separate the strong electric signal and weak electric signal as much as possible, and separate the digital signal circuit and the analog signal circuit, The circuits that complete the same function should be arranged within a certain range as much as possible, thereby reducing the signal loop area; the filter network of each part of the circuit must be connected nearby, which can not only reduce radiation, but also reduce the probability of being interfered. According to the circuit anti-interference ability.

　　* According to the different sensitivity of the unit circuit to electromagnetic compatibility in use, it is grouped. For the components that are susceptible to interference in the circuit, the layout should also try to avoid interference sources (such as interference from the CPU on the data processing board, etc.).

　　5 Conclusion

　　The key to RF circuit PCB design is how to reduce radiation capability and how to improve anti-interference capability. Reasonable layout and wiring are the guarantee for designing RF circuit PCB. The method described in this paper is beneficial to improve the reliability of the RF circuit PCB design, solve the problem of electromagnetic interference, and then achieve the purpose of electromagnetic compatibility.

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