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, Protel99SE 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.
The 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.
PCB design process
Since the use of Protel99SE software is different from software such as Protel98, first, the process of PCB design using Protel99SE software is briefly discussed.
1. Because Protel99SE uses the engineering (PROJECT) database mode management, it is implicit under Windows99, 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.
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 PCB, the size and shape of the space, and the cooperation with other components. Use the PLACETRACK command on the MECHANICALLAYER 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 Protel99SE is relatively simple. After selecting the “MAKELIBRARY” command in the “DESIGN” menu, you will enter the component creation window, and then select the “NEWCOMPONENT” command in the “TOOL” menu to design the components. . At this time, according to the shape and size of the actual components, the corresponding pads can be drawn at a certain position in the TOPLAYER layer with PLACEPAD and the like, and edited into the required pads (including the pad shape, size, inner diameter size and angle). Etc. In addition, the corresponding pin name of the pad should be marked. Then, the maximum shape of the component can be drawn in the TOPOVERLAYER layer by the PLACETRACK command, and a component name can be stored in the component 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 Protel99SE 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 Protel99SE). Note: After the "SAVECOPYAS..." command in the "FILE" option in Protel99SE 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 "SAVEAS..." function in Protel98.