RF circuit PCB design

fighting

RF circuit PCB design [Copy link]

RF circuit PCB design
introduces the process of RF circuit PCB design using Protel99 SE. In order to ensure circuit performance, electromagnetic compatibility should be considered when designing RF circuit PCB, so the layout principles of components are discussed in detail to achieve the purpose of electromagnetic compatibility.

Keywords: RF circuit PCB electromagnetic compatibility layout

With the development of communication technology, handheld wireless RF circuit technology is increasingly used, such as wireless pagers, mobile phones, wireless PDAs, etc. The performance indicators of the RF circuits 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. If the electromagnetic interference signal is not handled properly, it 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 topic when designing RF circuit PCB. The same circuit, different PCB design structures, will have very different performance indicators. This discussion uses Protel99 SE software to design the RF circuit PCB of handheld products, if the performance indicators of the circuit are maximized to meet the electromagnetic compatibility requirements.

1. Choice of board material

The substrates of printed circuit boards include organic and inorganic types. The most important properties of the substrate are the 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 signal transmission rate. For high-frequency circuits, the dielectric constant tolerance is the first and more critical factor to consider, and a substrate with a small dielectric constant tolerance should be selected.

2 PCB Design Process

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

① Since 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 schematics and PCB layouts.

② Schematic design. In order to achieve network connection, all components used must exist in the component library before the schematic design. Otherwise, the required components should be made in SCHLIB and stored in the library file. Then, you only need to call the required components from the component library and connect them according to the designed circuit diagram.

③After 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. Determine the shape and size of the PCB according to the position of the designed PCB in the product, the size of the space, the shape and the coordination with other components. Use the PLACE TRACK command in the MECHANICAL LAYER layer to draw the shape of the PCB. b. According to the requirements of SMT, make positioning holes, sight eyes, reference points, etc. on the PCB. c. 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 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 production window, and then select the "NEW COMPONENT" command in the "TOOL" menu to design components. At this time, you only need 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 shape, size, inner diameter and angle of the pads, etc., and the corresponding pin names of the pads should also be marked), and then use the PLACE TRACK command to draw the maximum shape of the components in the TOP OVERLAYER layer, and take a component name and save it in the component library. d. After the components are made, layout and wiring are carried out. These two parts are discussed in detail below. e. After the above process is completed, it must be checked. On the one hand, this includes the inspection of the circuit principle, and on the other hand, it is necessary to check the matching and assembly problems between each other. The inspection of the circuit principle can be checked manually or by automatic network inspection (the network formed by the schematic diagram can be compared with the network formed by the PCB). f. After the inspection is correct, the file is archived and output. In Protel99 SE, the "EXPORT" command in the "FILE" option must be used to store the file in the specified path and file (the "IMPORT" command is to import a certain file into Protel99 SE). Note: After executing the "SAVE COPY AS..." command in the "FILE" option in Protel99 SE, the selected file name is invisible in Windows 98, so the file cannot be seen in the resource manager. This is not exactly the same as the "SAVE AS..." function in Protel 98.

3. Component layout

Since SMT generally uses infrared furnace hot flow soldering to achieve component welding, the layout of components affects the quality of solder joints, and thus affects the yield rate of products. For RF circuit PCB design, electromagnetic compatibility requires that each circuit module should not generate electromagnetic radiation as much as possible and have a certain anti-electromagnetic interference capability. Therefore, the layout of components also directly affects the interference and anti-interference capabilities of the circuit itself, which is also directly related to the performance of the designed circuit. Therefore, when designing RF circuit PCB, in addition to considering the layout of 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 anti-interference ability of the circuit itself. 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 on the mutual influence with the CPU processing board. Therefore, when designing PCB, reasonable layout is particularly important.

General layout principle: Components should be arranged in the same direction as much as possible, and the phenomenon of poor soldering can be reduced or even avoided by choosing the direction in which the PCB enters the tin melting system; according to experience, there should be at least 0.5mm spacing between components to meet the tin melting requirements of the components. If the space on the PCB board allows, the spacing between components should be as wide as possible. For double-sided boards, one side should generally be designed for SMD and SMC components, and the other side for discrete components.

Note the following points in the layout:

*First, determine the position of the interface components with other PCB boards or systems on the PCB board. Attention must be paid to the coordination issues between the interface components (such as the orientation of the components, etc.).

*Because handheld devices are very small and the components are arranged very compactly, larger components must be given priority, their corresponding positions must be determined, and their coordination must be considered.

*Carefully analyze the circuit structure, and process the circuit in blocks (such as high-frequency amplification circuit, mixing circuit and demodulation circuit, etc.), separate strong electric signals and weak electric signals as much as possible, and separate digital signal circuits and analog signal circuits. Circuits that perform the same function should be arranged within a certain range as much as possible to reduce the signal loop area; the filtering network of each part of the circuit must be connected nearby, which can not only reduce radiation, but also reduce the chance of interference, based on the circuit's anti-interference ability.

* Group the unit circuits according to their different sensitivity to electromagnetic compatibility during use. When laying out the components in the circuit that are susceptible to interference, try to avoid interference sources (such as interference from the CPU on the data processing board, etc.).

4 Wiring

After the layout of components is basically completed, wiring can begin. The basic principle of wiring is: when the assembly density allows, try to use low-density wiring design, and the signal routing should be as consistent as possible to facilitate impedance matching.

For RF circuits, unreasonable design of the direction, width, and line spacing of signal lines may cause cross interference between signal transmission lines. In addition, the system power supply itself also has noise interference, so when designing the RF circuit PCB, comprehensive consideration must be given to reasonable wiring.

When wiring, all traces should be kept away from the border of the PCB board (about 2mm) to avoid the risk of disconnection or disconnection during the production of the PCB board. The power line should be as wide as possible to reduce loop resistance. At the same time, the direction of the power line and ground line should be consistent with the direction of data transmission to improve anti-interference ability; the signal line should be as short as possible and the number of vias should be minimized; the connection between components should be as short as possible to reduce distributed parameters and mutual electromagnetic interference; incompatible signal lines should be kept away from each other, and parallel routing should be avoided as much as possible, and the signal lines on the two positive sides should be perpendicular to each other; when wiring, the address side that needs to turn a corner should be 135°, and right angles should be avoided.

When wiring, the lines directly connected to the pads should not be too wide, and the routing should be kept away from unconnected components as much as possible to avoid short circuits. Vias should not be drawn on components and should be kept away from unconnected components as much as possible to avoid cold soldering, continuous soldering, short circuits, etc. during production.

In the RF circuit PCB design, the correct wiring of power lines and ground lines is particularly important. Reasonable design is the most important means to overcome electromagnetic interference. Quite a lot of interference sources on PCB are generated by power and ground lines, among which the noise interference caused by ground lines is the largest.

The main reason why ground wires are prone to electromagnetic interference is that they have impedance. When current flows through the ground wire, voltage is generated on the ground wire, which in turn generates a ground wire loop current and forms ground wire loop interference. When multiple circuits share a ground wire, common impedance coupling is formed, which generates the so-called ground wire noise. Therefore, when routing the ground wire of the RF circuit PCB, the following should be done:

*First, divide the circuit into blocks. The RF circuit can basically be divided into high-frequency amplification, mixing, demodulation, local oscillation and other parts. 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, summarize it at the place where the RF circuit PCB is connected to the ground wire, that is, summarize it at the total ground wire. Since there is only one reference point, there is no common impedance coupling, and thus no mutual interference problem.

*The digital area and the analog area should be isolated as much as possible by ground lines, 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 single-point grounding principle, try to reduce the signal loop area, and connect it to the address of the corresponding filter circuit as close as possible.

*If space permits, it is best to isolate each module with a ground wire to prevent signal coupling effects between them.

5 Conclusion

The key to RF circuit PCB design is how to reduce radiation and how to improve anti-interference ability. Reasonable layout and wiring are the guarantee of RF circuit PCB design. The method described in this article is conducive to improving the reliability of RF circuit PCB design, solving the electromagnetic interference problem, and thus achieving the purpose of electromagnetic compatibility.