With the development of communication technology, handheld radio high-frequency circuit board technology is more and more widely used, such as: wireless pager, mobile phone, wireless PDA, etc., the performance of the radio frequency circuit directly affects the quality of the whole product. One of the biggest characteristics of these handheld products is miniaturization, and miniaturization means that the density of components is very high, which makes the components (including SMD, SMC, bare chip, etc.) interfere with each other 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 topic in the design of RF circuit PCB. The same circuit, different PCB design structure, its performance index will differ greatly. This paper discusses how to maximize the performance of circuit to achieve electromagnetic compatibility requirements when using Protel99 SE software to design rf circuit PCB of palm products.

1. Selection of plate

The substrate of printed circuit board includes organic and inorganic categories. The most important properties of the substrate are dielectric constant ε R, dissipation factor (or dielectric loss) Tan δ, thermal expansion coefficient CET and moisture absorption. ε R affects circuit impedance and signal transmission rate. For high frequency circuits, the permittivity tolerance is the first and more critical factor to consider, and the substrate with low permittivity tolerance should be selected.

2. PCB design process

Because Protel99 SE software is different from Protel 98 and other software, the process of PCB design by Protel99 SE software is briefly discussed.

① Because Protel99 SE adopts the PROJECT database mode management, which is implicit in Windows 99, so we should first set up a database file to manage the circuit schematic diagram and PCB layout designed.

② Design of schematic diagram. In order to realize network connection, all the components used must exist in the component library before the principle design; otherwise, the required components should be made in SCHLIB and stored in the library file. Then, you simply 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 network table can be formed for use in PCB design.

④PCB design. A. CB shape and size determination. The shape and size of PCB are determined according to the position of PCB in the product, the size and shape of the space and the cooperation with other parts. Draw the shape of the PCB using the PLACE TRACK command on MECHANICAL LAYER. B. Make positioning holes, eyes and reference points on PCB according to SMT requirements. C. Production of components. If you need to use some special components that do not exist in the component library, you need to make components before layout. The process of making components in Protel99 SE is relatively simple. Select the “MAKE LIBRARY” command in the “DESIGN” menu to enter the COMPONENT making window, and then select the “NEW COMPONENT” command in the “TOOL” menu to DESIGN components. At this time, just draw the corresponding PAD at a certain position and edit it into the required PAD (including the shape, size, inner diameter and Angle of the PAD, etc., and mark the corresponding pin name of the PAD) at the TOP LAYER with the command of PLACE PAD and so on according to the shape and size of the actual component. Then use the PLACE TRACK command to draw the maximum appearance of the component in the TOP OVERLAYER, select a component name and store it in the component library. D. After components are made, layout and wiring shall be carried out. These two parts will be discussed in detail below. E. Check after the above procedure is complete. On the one hand, this includes the inspection of the circuit principle, on the other hand, it is necessary to check the matching and assembly of each other. The circuit principle can be checked manually or automatically by network (the network formed by schematic diagram can be compared with the network formed by PCB). F. After checking, archive and output the file. In Protel99 SE, you must run the EXPORT command in the FILE option to save the FILE to the specified path and FILE (the IMPORT command is to IMPORT a FILE to Protel99 SE). Note: In the Protel99 SE “FILE” option “SAVE COPY AS…” After the command is executed, the selected file name is not visible in Windows 98, so the file cannot be seen in Resource Manager. This is different from “SAVE AS…” in Protel 98. It doesn’t function exactly the same.

3. Components layout

Because SMT generally uses infrared furnace heat flow welding to weld components, the layout of components affects the quality of solder joints, and then affects the yield of products. For PCB design of rf circuit, electromagnetic compatibility requires that each circuit module does not generate electromagnetic radiation as far as possible, and has a certain ability to resist electromagnetic interference. 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 the design of RF circuit PCB, in addition to the layout of ordinary PCB design, we should also consider how to reduce the interference between 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 rf circuit depends not only on the performance index of RF circuit board itself, but also on the interaction with CPU processing board to a large extent. Therefore, in PCB design, reasonable layout is particularly important.

General layout principle: components should be arranged in the same direction as far as possible, and the bad welding phenomenon can be reduced or even avoided by selecting the direction of PCB entering the tin melt system; According to experience, the space between components should be at least 0.5mm to meet the requirements of tin-melting components. If the space of PCB board allows, the space between components should be as wide as possible. For double panels, one side should be designed for SMD and SMC components, and the other side is discrete components.

Note in layout:

* First determine the position of interface components on the PCB with other PCB boards or systems, and pay attention to the coordination of interface components (such as the orientation of components, etc.).

* Due to the small volume of handheld products, components are arranged in a compact manner, so for larger components, priority must be given to determine the appropriate location, and consider the problem of coordination between each other.

* careful analysis circuit structure, the circuit block processing (such as high frequency amplifier circuit, mixing circuit and demodulation circuit, etc.), as far as possible to separate heavy current signal and the weak current signal, separate digital signal circuit and analog signal circuit, complete the same function of the circuit should be arranged in a certain range, thereby reducing signal loop area; The filtering network of each part of the circuit must be connected nearby, so that not only the radiation can be reduced, but also the probability of interference can be reduced, according to the anti-interference ability of the circuit.

* Group cell circuits according to their sensitivity to electromagnetic compatibility in use. The components of the circuit that are vulnerable to interference should also avoid interference sources (such as interference from the CPU on the data processing board).

4. Wiring

After the components are laid out, wiring can begin. The basic principle of wiring is: under the condition of assembly density, low-density wiring design should be selected as far as possible, and signal wiring should be as thick and thin as possible, which is conducive to impedance matching.

For rf circuit, the unreasonable design of signal line direction, width and line spacing may cause the interference between signal signal transmission lines; In addition, the system power supply itself also exists noise interference, so in the design of RF circuit PCB must be considered comprehensively, reasonable wiring.

When wiring, all wiring should be far away from the border of THE PCB board (about 2mm), so as not to cause or have the hidden danger of wire breaking during PCB board production. The power line should be as wide as possible to reduce the resistance of the loop. 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 lines should be as short as possible and the number of holes should be reduced as far as possible. The shorter the connection between components, the better, to reduce the distribution of parameters and electromagnetic interference between each other; For incompatible signal lines should be far away from each other, and try to avoid parallel lines, and in the positive two sides of the application of mutual vertical signal lines; Wiring in need of corner address should be 135° Angle as appropriate, avoid turning right angles.

The line directly connected with the pad should not be too wide, and the line should be away from the disconnected components as far as possible to avoid short circuit; Holes should not be drawn on components, and should be far away from disconnected components as far as possible to avoid virtual welding, continuous welding, short circuit and other phenomena in production.

In PCB design of rf circuit, the correct wiring of power line and ground wire is particularly important, and reasonable design is the most important means to overcome electromagnetic interference. Quite a lot of interference sources on PCB are generated by power supply and ground wire, among which ground wire causes the most noise interference.

The main reason why the ground wire is easy to cause electromagnetic interference is the impedance of the ground wire. When a current flows through the ground, a voltage will be generated on the ground, resulting in the ground loop current, forming the loop interference of the ground. When multiple circuits share a single piece of ground wire, common impedance coupling occurs, resulting in what is known as ground noise. Therefore, when wiring the ground wire of the RF circuit PCB, do:

* First of all, the circuit is divided into blocks, rf circuit can be basically divided into high frequency amplification, mixing, demodulation, local vibration and other parts, to provide a common potential reference point for each circuit module circuit grounding, so that the signal can be transmitted between different circuit modules. It is then summarized at the point where the RF circuit PCB is connected to the ground, i.e. summarized at the main ground. Since there is only one reference point, there is no common impedance coupling and thus no mutual interference problem.

* Digital area and analog area as far as possible ground wire isolation, and digital ground and analog ground to separate, finally connected to the power supply ground.

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

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

5. Conclusion

The key of RF PCB design lies in how to reduce radiation ability and how to improve anti-interference ability. Reasonable layout and wiring is the guarantee of DESIGNING RF PCB. The method described in this paper is helpful to improve the reliability of RF circuit PCB design, solve the problem of electromagnetic interference, and achieve the purpose of electromagnetic compatibility.