PCB Tech - RF circuit PCB design

RF circuit PCB design process Layout of components

RF circuit PCB wiring considerations

With the development of communication technology, handheld wireless RF circuit PCB technology is used more and more widely, such as: wireless pagers, mobile phones, wireless PDAs, etc. The performance indicators of the RF circuit PCB 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 Protel99SE software to design the radio frequency circuit PCB of handheld products, how to achieve the circuit's performance indicators to the greatest extent to meet the electromagnetic compatibility requirements.

RF circuit PCB designSelection 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.

RF PCB design process

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

1. Because Protel99SE uses the project (PROJECT) database mode management, it is implicit under Windows99, so you should first create a database file to manage the designed circuit schematic and PCB layout.

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 PLACETRACK command to draw the appearance of the PCB on the MECHANICALLAYER 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 Protel99SE is relatively simple. Select the "MAKELIBRARY" command in the "DESIGN" menu to enter the component production window, and then select the "NEWCOMPONENT" command in the "TOOL" menu to design the components . At this time, it is only necessary to draw the corresponding pad at a certain position with PLACEPAD and other commands on the TOPLAYER layer according to the shape and size of the actual component and edit it to the required pad (including the pad shape, size, inner diameter size and angle In addition, the corresponding pin name of the pad should be marked), and then use the PLACETRACK command to draw the maximum shape of the component in the TOPOVERLAYER 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 Protel99SE, 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 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 cannot be seen in the Explorer. This is not exactly the same as the "SAVEAS..." function in Protel98.

RF PCB Layout of components

Since SMT generally uses infrared furnace heat flow welding to realize the welding of components, the layout of the components affects the quality of the solder joints, which in turn affects the yield of products. For the design of RF circuit PCB, electromagnetic compatibility requires that each circuit module does not produce electromagnetic radiation as much as possible, and has a certain degree of anti-electromagnetic interference ability. Therefore, the layout of components also directly affects the interference and anti-interference ability of the circuit itself., This is also directly related to the performance of the designed circuit.

Therefore, in addition to the layout of the ordinary PCB design, the PCB design of the radio frequency circuit must also consider how to reduce the mutual interference between the various parts of the radio frequency circuit, how to reduce the interference of the circuit itself to other circuits, and the circuit itself. The anti-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.

The general principle of the layout: components should be arranged in the same direction as much as possible, and poor soldering can be reduced or avoided by choosing the direction of the PCB entering the soldering system; according to experience, there must be at least 0.5mm spacing between components to meet the soldering of components It is required that if the space of the PCB board allows, the spacing of the components should be as wide as possible. For double-sided boards, one side should be SMD and SMC components, and the other side should be discrete components.

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, the priority must be given to the corresponding position, and the mutual cooperation should be considered.

*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.

RF circuit PCB design wiring

After the layout of the components is basically completed, the 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 the radio frequency circuit, the unreasonable design of the signal line's direction, width, and line spacing may cause cross interference between the signal signal transmission lines; in addition, the system power supply itself also has noise interference, so comprehensive consideration must be taken when designing the radio frequency circuit PCB, 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, make the direction of the power cord and ground wire consistent with the direction of data transmission to improve the anti-interference ability; the signal line should be as short as possible, and minimize excessive 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.

When wiring, the line directly connected to the pad should not be too wide, and the trace should be as far away as possible from unconnected components to avoid short circuits; vias should not be drawn on the components, and should be as far away as possible from unconnected components to avoid production Phenomenon such as virtual welding, continuous welding, short circuit and so on.

In the PCB design of radio frequency circuits, the correct wiring of power lines and ground lines is particularly important. 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, of which the noise interference caused by the ground wire is the largest.

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. Therefore, when wiring the ground wire of the RF circuit PCB, you should do:

*First of all, the circuit is divided into blocks. The radio frequency circuit can basically be divided into high-frequency amplification, mixing, demodulation, local oscillator and other parts. A common potential reference point is provided for each circuit module, that is, the respective ground wire of each module circuit., So that the signal can be transmitted between different circuit modules. Then, it is summarized in the place where the RF circuit PCB is connected to the ground wire, that is, it is summarized in the main ground wire. Since there is only one reference point, there is no common impedance coupling, so there is no mutual interference problem.

*The digital zone and the analog zone should be separated from the ground as much as possible, and the digital ground should be separated from the analog ground, and finally connected to the power ground.

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

*In the case of space permitting, it is best to isolate each module with a ground wire to prevent the signal coupling effect between each other.

The key to the RF circuit PCB design 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 achieve the purpose of electromagnetic compatibility.