In order to ensure the performance of the circuit, electromagnetic compatibility should be considered when designing the RF circuit PCB. Therefore, the wiring principle of the components should be discussed to achieve the purpose of electromagnetic compatibility.
With the development of communication technology, handheld wireless RF circuit technology is used more and more widely, such as: wireless pager, mobile phone, wireless PDA, etc., the performance indicators of the RF 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 large, which makes the mutual interference of components (including SMD, SMC, die, etc.) very prominent. If the electromagnetic interference signal is not handled properly, the whole circuit system may not work properly. Therefore, how to prevent and suppress electromagnetic interference and improve electromagnetic compatibility has become a very important issue in designing the RF circuit PCB. The same circuit, different PCB design structure, its performance indicators will vary greatly. In this discussion, Protel99 SE software is used to design the RF circuit PCB of the handheld product. If the performance of the circuit is maximized, the electromagnetic compatibility requirements are met.
1 choice of boardThe substrate of the printed circuit board includes two types of organic and inorganic materials. The most important properties in the substrate are the dielectric constant εr, the dissipation factor (or dielectric loss) tan δ, the coefficient of thermal expansion CET, and the moisture absorption rate. Where εr affects circuit impedance and signal transmission rate. For high-frequency circuits, the dielectric constant tolerance is the more important factor to consider. The substrate with a small dielectric constant tolerance should be selected.
2 PCB design processSince the use of Protel99 SE software is different from software such as Protel 98, the process of PCB design using Protel99 SE software is briefly discussed.
1 Because Protel99 SE uses the engineering (PROJECT) database mode management, it is implicit under Windows 99, so you should first set up a database file to manage the designed schematic and PCB layout.
2 schematic design. In order to achieve network connection, the components used must be in the library between the principle design. Otherwise, the required components should be made in the SCHLIB and stored in the library file.
Then, simply call the required components from the library and connect according to the designed circuit diagram.
3 After the schematic design is completed, a network table 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 PCB, the size and shape of the space, and the cooperation with other components. Use the PLACE TRACK command on the MECHANICAL LAYER layer to draw the shape of the PCB. b. According to the requirements of SMT, make positioning holes, eye, reference points, etc. on the PCB. c. Production of components. If you need to use some special components that do not exist in the library, you need to make the components before layout. The process of making components in Protel99 SE is relatively simple. After selecting the “MAKE LIBRARY†command in the “DESIGN†menu, you will enter the component creation window, and then select the “NEW COMPONENT†command in the “TOOL†menu to execute the element. Device design. At this time, it is only necessary to draw the corresponding pads at a certain position in the TOP LAYER layer according to the shape and size of the actual components, and edit them into the required pads (including the pad shape, size, inner diameter size). And angle, etc., should also mark the corresponding pin name), then use the PLACE TRACK command to draw the maximum shape of the component in the TOP OVERLAYER layer, take a component name into the library. d. After the components are manufactured, layout and wiring are performed. These two parts are discussed in detail below. e. The above process must be checked after completion. This aspect includes the inspection of the circuit principle, on the other hand, it must also check the matching and assembly problems between each other. The inspection of the circuit principle can be checked manually, or it can be automatically checked by the network (the network formed by the schematic diagram is compared with the network formed by the PCB). f. After checking the error, archive and output the file. In Protel99 SE you must use the "EXPORT" command in the "FILE" option to place the file in the specified path and file (the "IMPORT" command is to transfer a file to Protel99 SE). Note: After the “SAVE COPY AS...†command in the “FILE†option in Protel99 SE is executed, the selected file name is not visible in Windows 98, so the file is not visible in the Explorer. This is not exactly the same as the "SAVE AS..." function in Protel 98.
3 component layoutSince SMT generally uses infrared furnace heat flow welding to achieve component soldering, the layout of the components affects the quality of the solder joints, which in turn affects the yield of the product. For the design of RF circuit PCB, electromagnetic compatibility requires that each circuit module does not generate electromagnetic radiation as much as possible, and has certain anti-electromagnetic interference capability. Therefore, the layout of components 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 the design of the RF circuit PCB, in addition to the layout of the ordinary PCB design, it is also necessary to consider how to reduce the mutual interference between the various parts of the RF circuit, how to reduce the interference of the circuit itself to other circuits, and the circuit itself. Anti-jamming capability. According to experience, the effect of the RF circuit depends not only on the performance of the RF board itself, but also on the interaction with the CPU processing board. Therefore, reasonable layout is particularly important when designing the PCB.
The general principle of layout: components should be arranged in the same direction as possible, by selecting the direction of the PCB into the molten tin system to reduce or even avoid the phenomenon of poor soldering; according to the experience, there must be at least 0.5mm spacing between components to meet the melting tin of the components. Requirements, if the space of the PCB board allows, the spacing of components should be as wide as possible. For double panels, one side should be designed with SMD and SMC components on one side and discrete components on the other.
Pay attention to the layout:
* First determine the position of the interface components of other PCB boards or systems on the PCB board, you must pay attention to the coordination problems between the interface components (such as the direction of components).
*Because the size of the palm-sized products is very small, the components are arranged very compactly. Therefore, for larger components, priority must be given to determine the corresponding position and consider the problem of cooperation with each other.
* Carefully analyze the circuit structure, block the circuit (such as high-frequency amplifier circuit, mixer circuit and demodulation circuit, etc.), separate the strong signal and the weak signal as much as possible, and separate the digital signal circuit from the analog signal circuit. Circuits that perform the same function should be arranged within a certain range to reduce the signal loop area; the filter network of each part of the circuit must be connected nearby, which not only reduces the radiation, but also reduces the probability of interference, according to the circuit. Anti-jamming capability.
* Grouping according to the degree of sensitivity of the unit circuit to electromagnetic compatibility in use. Components that are susceptible to interference in the circuit should also avoid interference sources (such as interference from the CPU on the data processing board) during layout.
4 wiringAfter the layout of the components is basically completed, the wiring can be started. The basic principle of wiring is: after the assembly density is allowed, try to use low-density wiring design, and the signal traces should be as thick as possible, which is good for impedance matching.
For RF circuits, 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 considered comprehensively when designing the RF circuit PCB. , reasonable wiring.
When wiring, all the traces should be away from the border of the PCB board (about 2mm), so as to avoid the hidden trouble of disconnection or disconnection when the PCB board is made. The power cable should be wide to reduce the loop resistance. At the same time, the direction of the power cable and the ground wire 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 minimized. The number of holes; the shorter the connection between the components, the better, in order to reduce the distribution parameters and mutual electromagnetic interference; for the incompatible signal lines should be away from each other, and try to avoid parallel routing, and in the positive two sides The signal line application should be perpendicular to each other; when wiring, the address of the corner should be 135° angle, avoiding the right angle.
When wiring, the line directly connected to the pad should not be too wide. The trace should be separated from the unconnected components as much as possible to avoid short circuit; the via should not be drawn on the component, and should be kept away from the unconnected 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 sources of interference on the PCB are generated by the power supply and ground, where the ground line causes the most noise interference.
The main reason why the ground wire is easy to form electromagnetic interference is the impedance of the ground wire. When a current flows through the ground, a voltage is generated on the ground, thereby generating a ground loop current, which forms a loop interference of the ground. When a plurality of circuits share a ground line, a common impedance coupling is formed, resulting in so-called ground noise. Therefore, when routing the ground wire of the RF circuit PCB, you should do the following:
* First, the circuit is divided into blocks, the RF circuit can be basically divided into high-frequency amplification, mixing, demodulation, local oscillator and other parts, to provide a common potential reference point for each circuit module, that is, the respective ground of each module circuit So that the signal can be transmitted between different circuit modules. Then, it is summarized in the place where the RF circuit PCB is connected to the ground line, that is, it is summarized on the total ground line. Since there is only one reference point, there is no common impedance coupling, and thus there is no mutual interference problem.
* The digital area and the analog area are isolated as much as possible, and the digital ground is separated from the analog ground and finally connected to the power ground.
* The ground wire inside each part of the circuit should also pay attention to the single-point grounding principle, minimize the signal loop area, and connect the address of the corresponding filter circuit.
* When space permits, it is best to isolate each module with a ground wire to prevent signal coupling effects between each other.
5 ConclusionThe key to RF circuit PCB design is how to reduce the radiation capability and how to improve the anti-interference ability. Reasonable layout and wiring is the guarantee for designing the RF circuit PCB. The method described in the paper is beneficial to improve the reliability of the RF circuit PCB design, solve the electromagnetic interference problem, and achieve the purpose of electromagnetic compatibility.
Electric Water Kettle,Stainless Steel Electric Water Kettle,Electric Hot Water Kettle,Electric Water Boiler Kettle
Shandong Sangle Group Co.,Ltd. , https://www.sangle-group.com