RF circuit PCB design

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 Selection of board

The substrates of printed circuit boards include two major 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 the circuit impedance and signal transmission rate. For high-frequency circuits, 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, the process of using Protel99 SE software for PCB design is briefly discussed first.

① Since Protel99 SE uses the project (PROJECT) database mode management, which is implicit under Windows 99, a database file should be established first 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 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 network table can be formed for use in PCB design.

④PCB design

a. Determine the shape and size of the PCB.

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, eyes, reference points, etc. on the PCB.

c. Make 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 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 soldering, the layout of components affects the quality of solder joints, and thus affects the yield rate of the product. 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 ability. 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 addition to considering the layout of ordinary PCB design when designing RF circuit PCB, 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 RF circuit depends not only on the performance indicators of RF circuit board itself, but also on the interaction with CPU processing board. Therefore, reasonable layout is particularly important when designing PCB.

General layout principle:

components should be arranged in the same direction as much as possible, and the phenomenon of poor welding can be reduced or even avoided by selecting the direction of PCB entering the tin melting system; according to experience, there should be at least 0.5mm spacing between components to meet the tin melting requirements of components. If the space of PCB board allows, the spacing of components should be as wide as possible. For double-sided board, one side should be designed as SMD and SMC components, and the other side should be discrete components.

Attention should be paid to the layout:

• First determine the position of interface components with other PCB boards or systems on the PCB board, and pay attention to the coordination between interface components (such as the direction of components, etc.).
• Because the volume of handheld products is very small and the arrangement between components is very compact, large components must be given priority, the corresponding position must be determined, and the coordination between them must be considered.
• Carefully analyze the circuit structure, process the circuit in blocks (such as high-frequency amplifier circuit, mixer circuit and demodulation circuit, etc.), separate strong electric signals from weak electric signals as much as possible, separate digital signal circuits from analog signal circuits, and arrange the circuits that complete the same function 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 anti-interference ability of the circuit.
• Group the unit circuits according to their different sensitivity to electromagnetic compatibility during use. For the components of the circuit that are susceptible to interference, the interference source should be avoided as much as possible during layout (such as interference from the CPU on the data processing board, etc.).

4 Wiring

After the layout of the components is basically completed, the wiring can be started. The basic principle of wiring is: when the assembly density permits, try to use low-density wiring design, and the signal routing should be as consistent as possible, which is conducive to 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 wiring should be kept away from the border of the PCB board (about 2mm) to avoid the risk of wire breakage or wire breakage during PCB board production. 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 wiring should be avoided as much as possible, and the signal lines on the two sides of the forward direction should be perpendicular to each other; when wiring, the address side where a corner is required 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. in production.

In the PCB design of radio frequency circuits, the correct routing of power lines and ground lines is particularly important, and reasonable design is the most important means to overcome electromagnetic interference. A considerable number of interference sources on PCBs are generated through power and ground lines, among which the noise interference caused by ground lines is the largest.

The main reason why ground lines are prone to electromagnetic interference is that there is impedance in the ground line. When current flows through the ground line, voltage will be generated on the ground line, thereby generating ground line loop current and forming ground line loop interference. When multiple circuits share a ground line, common impedance coupling will be formed, resulting in the so-called ground line noise. Therefore, when wiring the ground wire of the RF circuit PCB, the following should be done:

• First, the circuit is divided into blocks. The RF circuit can basically be divided into high-frequency amplification, mixing, demodulation, local oscillation and other parts. A common potential reference point, that is, the ground wire of each module circuit, should be provided for each circuit module, so that the signal can be transmitted between different circuit modules. Then, it is summarized at the place where the RF circuit PCB is connected to the ground wire, that is, the total ground wire. Since there is only one reference point, there is no common impedance coupling, and thus there is no mutual interference problem.
• The digital area and the analog area should be isolated by ground wire as much as possible, 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, minimize the signal loop area, and connect it to the address of the corresponding filter circuit nearby.
• If space permits, it is best to isolate each module with a ground wire to prevent signal coupling effects between each other.

5 Conclusion

The key to the design of RF circuit PCB lies in how to reduce radiation capability and how to improve anti-interference capability. Reasonable layout and wiring are the guarantee of designing RF circuit PCB. 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.