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Electronic Design

Electronic Design

Electronic Design

Electronic Design

RF circuit PCB design

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 characteristics of these handheld products is miniaturization, and miniaturization means that the density of components is very large, 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 normally. Therefore, how to prevent and suppress electromagnetic interference and improve electromagnetic compatibility has become a very important topic when designing radio frequency circuit PCBs. The same circuit, different PCB design structure, its performance indicators will be very different. In this discussion, when using Protel99 SE software to design the radio frequency circuit PCB of handheld products, how to achieve the circuit performance index to the greatest extent to meet the electromagnetic compatibility requirements.

    1 Selection of plates

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

pcb board

    2 PCB design process

    Since the use of Protel99 SE software is different from Protel 98 and other software, firstly, we will briefly discuss the process of PCB design using Protel99 SE software.

    ①Since Protel99 SE adopts the project (PROJECT) database mode management, it is implicit under Windows 99, so you should first create a database file to manage the designed circuit schematic and PCB layout.

    ② The design of the schematic diagram. In order to realize the network connection, the components used must exist in the component library during 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 of the space, the shape and the cooperation with other components. Use the PLACE TRACK command to draw the appearance of the PCB on the MECHANICAL LAYER layer. b. According to the requirements of SMT, make positioning holes, sights, reference points, etc. on the PCB. c. The production of components.

    3 Layout of components

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

    The general principle of the layout: components should be arranged in the same direction as much as possible, and poor soldering can be reduced or avoided by choosing the direction of the PCB entering the soldering system; according to experience, there must be at least 0.5mm spacing between components to meet the soldering of components It is required that if the space of the PCB board allows, the spacing of the components should be as wide as possible. For double-sided boards, one side should be SMD and SMC components, and the other side should be discrete components.

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

    4 Wiring

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

    For the radio frequency circuit, the unreasonable design of the signal line's direction, width, and line spacing may cause cross interference between the signal signal transmission lines; in addition, the system power supply itself also has noise interference, so comprehensive consideration must be taken when designing the radio frequency circuit PCB , Reasonable wiring.

    When wiring, all traces should be far away from the border of the PCB board (about 2mm), so as to avoid wire breakage or hidden dangers when the PCB board is made. The power cord should be as wide as possible to reduce loop resistance. At the same time, the direction of the power cord and ground wire should be consistent with the direction of data transmission to improve anti-interference ability; The number of holes; the shorter the wiring between the components, the better, in order to reduce the distribution parameters and mutual electromagnetic interference; for incompatible signal lines should be kept away from each other, and try to avoid parallel wiring, and on both sides 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° to avoid turning at right angles.

    When wiring, the line directly connected to the pad should not be too wide. The trace should be as far away as possible from unconnected components to avoid short circuits; vias should not be drawn on the components, and should be as far away as possible from unconnected components to avoid production Phenomenon such as virtual welding, continuous welding, short circuit and so on.

    In the PCB design of radio frequency circuits, the correct wiring of power lines and ground lines is particularly important. A reasonable design is the most important means to overcome electromagnetic interference. Quite a lot of interference sources on the PCB are generated by the power supply and the ground wire, of which the noise interference caused by the ground wire is the largest

        5 Conclusion

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