Electronic Design - PCB board design skills of RF circuit design

Layout of PCB design process components

PCB wiring precautions With the development of communication technology, handheld radio frequency circuit technology is more and more widely used, such as: wireless pagers, mobile phones, wireless PDAs, etc. The performance indicators of radio frequency circuits directly affect the overall quality of products.

One of the biggest features of these handheld products is miniaturization, which means that the density of components is very high, which makes the 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 in the design of radio frequency circuit PCBs. The same circuit, different PCB design structure, its performance indicators will be very different.

This discussion uses Protel99SE software to design the RF circuit PCB of handheld products. If the circuit performance index is maximized, the electromagnetic compatibility requirements will be met.

Board selection PCB substrates include two types, organic and inorganic. The most important characteristics of the substrate are the dielectric constant εr, the dissipation factor (or dielectric loss) Tan delta, the thermal expansion coefficient CET and the moisture absorption rate. εr affects circuit impedance and signal transmission rate.

For high-frequency circuits, the dielectric constant tolerance is the primary consideration for the more critical factor, and the dielectric constant tolerance of the small substrate should be selected.

PCB design process

Because the use of Protel99SE software is different from Protel98 and other software, firstly discuss the process of PCB design using Protel99SE software.

1. Because Protel99SE is used for project (PROJECT) database mode management, it is implicit under Windows99, so you should first set up a database file to manage the design of circuit schematics and PCB layout. 2. The design of the schematic diagram. In order to realize the network connection, between the principle design, the used components must exist in the library, otherwise, the required components in the storage file should be made in SCHLIB.

Then, just call the required components from the library and connect them according to the circuit diagram you designed.

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

4. PCB design. Determine the shape and size of a.PCB. Based on the design of the PCB, the shape and size of the PCB are determined on the basis of the location, space size, shape and other components of the product.

Use the PLACETRACK command in the MECHANICALLAYER layer to draw the shape of the PCB.

B. According to the requirements of SMT, make positioning holes, visual 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 library, you need to make the components before layout. The process of making components in Protel99SE is relatively simple. After entering the Component Production window, select the "MAKELIBRARY" command in the "DESIGN" menu, and then select the "TOOL" menu in the "NEWCOMPONENT" command to design the component. At this time, it is only necessary to draw the corresponding gasket in a certain position according to the shape and size of the actual component in the TOPLAYER layer, and edit the corresponding gasket (including gasket shape, size, inner diameter) size and angle. In addition to the corresponding pin name of the pad should be marked),

Then use the PLACETRACK command in the TOPOVERLAYER layer to draw the largest shape of the component, and put the component name in the library.

D. After the production of components, layout and wiring, these two parts will be discussed in detail below. E. The above process must be checked after completion. This 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 of the inspection can be checked manually or automatically by the network (the schematic diagram of the network and the PCB formation of the network can be compared). F. After checking the error, archive and output the file. In Protel99SE, the "EXPORT" command in the "file" option must be used to store the file in the specified path and file (the "IMPORT" command is used to transfer the file to Protel99SE).

This is not exactly the same as the "SAVEAS..." function in Protel98. Component layout Since SMT generally uses infrared furnace heat flow soldering to achieve component welding, the layout of components will affect the quality of solder joints, which in turn affects the yield of products. In PCB design, a reasonable layout is particularly important. The general layout principle: components should be arranged in the same direction as much as possible. By choosing the PCB direction to enter the soldering system, the soldering phenomenon can be reduced or even avoided; according to experience, there must be at least 0.5mm spacing between components to meet the requirements of the components, if the PCB The space of the board allows, and the spacing of the components should be as wide as possible.

For double panels, one side should be designed for SMD and SMC components, and the other side should be designed for discrete components.

After the wiring is basically completed, the wiring can be started.

The basic principles of PCB wiring are as follows: After the assembly density is permitted, try to choose a low-density wiring design, and make the signal wiring as thick as possible, which is conducive to impedance matching.

For radio frequency circuits, unreasonable design of signal line direction, width, and line spacing may cause cross interference between signal transmission lines. In addition, the system power supply itself has noise interference, so the PCB must be considered and reasonable when designing radio frequency circuits. wiring. When wiring, all lines should be far away from the PCB board frame (about 2mm) to avoid PCB production due to wire breakage or hidden dangers of wire breakage. The power line should be wide to reduce loop resistance, and at the same time, make the power line, the direction of the ground line and the data transmission cooperate with each other to improve the anti-interference ability; the signal line should be as short as possible, and the number of holes should be minimized; the connection between the components As short as possible

; Incompatible signal lines should be far away from each other and try to avoid parallel wiring, while the signal lines on both sides of the front should be applied perpendicular to each other; the wiring at the address that needs a corner should be located at the corner of 135° to avoid turning right angles.

The line directly connected between the wiring and the pad should not be too wide. The line should be as far away from the disconnected component as possible to avoid short circuit, and the hole should not be connected to the component, and should be as far away as possible from the disconnected component, so because there is no virtual soldering, continuous soldering, Short-circuit and other phenomena. In the RF circuit PCB design, the correct wiring of the power line and the ground line is particularly important, and a reasonable design is the most important means to overcome electromagnetic interference.

Quite a lot of interference sources on the PCB are generated by the power supply and the ground wire, and the ground wire causes the largest noise interference. The main reason that the ground wire easily forms electromagnetic interference is the existence of the ground wire impedance. When the current flows through the ground wire, a voltage is generated on the ground wire, and a ground loop current is generated, forming a loop interference of the ground wire. When multiple circuits share a ground wire, a common impedance coupling is formed, resulting in so-called ground noise.

Therefore, when wiring the ground wire of the RF circuit PCB, you should do: *First, block the circuit. The RF circuit can basically be divided into high-frequency amplification, mixing, demodulation, vibration and others. Part, to provide a common potential reference point for each circuit module, that is, each module circuit of each ground wire, so that the signal can be transmitted between different circuit modules. Then, the RF circuit PCB access to the ground wire is summarized, that is, the total ground wire is summarized.