Electronic Design - The substrates of printed circuit boards include two categories

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 RF 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.

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.

1. Because Protel99 SE uses the project (PROJECT) database mode management, it is implicit under Windows 99, so you should first create a database file to manage the settings

The schematic circuit diagram and PCB layout of the meter.

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.

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

4. PCB 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, sight 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 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 making window, and then select the "NEW COMPONENT" command in the "TOOL" menu to perform meta Device design. At this time, it is only necessary to draw the corresponding pad at a certain position with PLACE PAD and other commands on the TOP LAYER layer according to the shape and size of the actual component and edit it to the required pad (including pad shape, size, inner diameter size, etc.) And the angle, etc. should also be marked with the corresponding pin name of the pad), and then use the PLACE TRACK command to draw the maximum shape of the component 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, an inspection must be carried out. On the one hand, it includes the inspection of the circuit principle. On the other hand, it is necessary to check the matching and assembly problems between each other. The circuit principle can be checked manually or automatically by the network (the network formed by the schematic diagram can be compared with the network formed by the PCB). f. After the inspection is correct, archive and output the file. In Protel99 SE, you must use the "EXPORT" command in the "FILE" option to store the file in the specified path and file (the "IMPORT" command transfers 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 invisible in Windows 98, so the file cannot be seen in the Explorer. This is not exactly the same as the "SAVE AS..." function in Protel 98.

3 Layout of components

When designing the RF circuit PCB, in addition to the PCB layout of the ordinary design, it is mainly 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 resistance of the circuit itself. 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.

Attention should be paid to the layout:

*First determine the position of the interface components with 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 the components, etc.).

*Because the volume of the handheld products is very small, the arrangement of the components is very compact, 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, divide the circuit into blocks (such as high-frequency amplifier circuit, mixing circuit and demodulation circuit, etc.), separate strong and weak electrical signals as much as possible, and separate digital signal circuits from analog signal circuits, Circuits that complete the same function should be arranged within a certain range as much as possible to reduce 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 interference. According to the circuit The anti-interference ability.

* 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 radio frequency circuits, the unreasonable design of signal line direction, width, and line spacing may cause cross* interference between signal signal transmission lines; in addition, the system power supply itself also has noise interference, so it must be integrated when designing the radio frequency circuit PCB Consider, 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.

The main reason that the ground wire easily forms electromagnetic interference is the impedance of the ground wire. When a current flows through the ground wire, a voltage will be generated on the ground wire, thereby generating a ground loop current and forming a loop interference of the ground wire. When multiple circuits share a section of ground, a common impedance coupling will be formed, resulting in so-called ground noise.

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.