Anti-interference design of radio frequency PCB circuit board

With the development of electronic communication technology, wireless radio frequency circuit technology is used more and more widely. The functional objectives of the radio frequency circuit directly affect the quality of the entire product. The anti-interference design of the radio frequency PCB circuit board is of great significance in reducing the electromagnetic information radiation of the system. . The density of RF circuit boards is getting higher and higher, and the interference between good and bad RF PCB printed circuit board designs has a great impact. The functional objectives of the same circuit and different RF PCB printed circuit board design structures will be very different. If the electromagnetic interference signal is not handled properly, it may cause the entire circuit system to malfunction. Therefore, how to prevent and suppress electromagnetic interference and improve electromagnetic compatibility has become a very important topic when designing radio frequency circuit boards.

Electromagnetic compatibility EMC refers to the ability of an electronic system to operate normally in accordance with design requirements in a regulated electromagnetic environment. The electromagnetic interference suffered by electronic systems not only comes from the radiation of electric fields and magnetic fields, but also is affected by the common impedance of the lines, coupling between wires and circuit structure. When developing and designing circuits, the printed circuit board should be designed to be as less susceptible to external interference as possible, and to interfere with other electronic systems as little as possible.

The first task when planning a PCB board is to analyze the circuit and confirm the key circuits. This is to identify which circuits are interference sources, which circuits are sensitive circuits, and to clarify how the interference sources may interfere with the sensitive circuits. Radio frequency circuits operate at high frequencies, and interference sources mainly interfere with active circuits through electromagnetic radiation. Therefore, the purpose of anti-interference design of radio frequency circuit boards is to reduce the crosstalk between the electromagnetic radiation of the PCB board and the circuits on the PCB board.
1. RF circuit board planning
1.1 layout of equipment
Since SMT generally uses infrared furnace warm flow welding to realize the welding of components, the layout of components affects the quality of solder joints and thus the product yield. For radio frequency circuit board design, electromagnetic compatibility requires that each circuit module produces as little electromagnetic radiation as possible and has a certain anti-electromagnetic interference capability. Therefore, the structure of components also affects the interference and anti-interference capabilities of the circuit itself, which directly Relevant to the function of the planned circuit. Therefore, in addition to the general PCB layout when planning the RF circuit board, we must also 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 circuit itself. Anti-interference ability.
According to experience, the quality of the RF circuit effect not only depends on the functional objectives of the RF circuit board itself, but also largely depends on the interaction with the CPU processing board. Therefore, when planning the RF circuit board, reasonable layout is particularly important. The general principle of the layout is that components should be placed in the same direction as much as possible. By selecting the direction in which the RF PCB circuit board enters the tin melting system, the phenomenon of poor welding can be reduced or even prevented; according to experience, there must be at least 0.5mm distance between components to be satisfactory. Regarding the tin melting requirements of components, if the space of the PCB board allows, the distance between components should be as wide as possible. For double-sided panels, one side should generally be designed with SMD and SMC components and the other side with discrete components.
Attention should be paid to the following when laying out RF circuit boards:
① First confirm the position of the interface components with other PCB boards or systems on the RF PCB printed circuit board. It is necessary to pay attention to the coordination issues between the interface components (the direction of the Canadian dollar components, etc.);
② Group the unit circuits according to their different sensitivity to electromagnetic compatibility in use. The components in the circuit that are susceptible to interference should also be designed to avoid interference sources (such as interference from the CPU on the data processing board, etc.);
③Since handheld products are very small in size and components are placed very compactly, it is necessary to give priority to larger components, confirm the corresponding location, and consider mutual cooperation issues;
④ Carefully analyze the circuit structure, divide the circuit into blocks (add high-frequency extension circuits, mixing circuits and demodulation circuits, etc.), separate strong electric signals and weak electric signals as much as possible, and separate digital signal circuits and analog signal circuits. Circuits that perform the same function should be arranged within a certain layout as much as possible to reduce the signal loop area; the filter network of each part of the circuit must be connected nearby, which not only reduces radiation, but also reduces the chance of interference and improves efficiency. The anti-interference ability of the circuit.

1.2 Wiring
After the layout of the components is basically completed, wiring can begin. The basic principle of wiring is: when the assembly density allows, try to use a low-density wiring plan, and the signal traces should be as thick and thin as possible to facilitate impedance matching.
Regarding radio frequency circuits, unreasonable planning 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 comprehensive considerations must be taken when planning the radio frequency circuit board. Proper wiring. When wiring, all traces should be about 2mm away from the frame of the PCB board to prevent breakage or risk of breakage during PCB board manufacturing.
The power lines should be as wide as possible to reduce loop resistance, and at the same time, the power lines and ground wires should be in the same direction as the data transmission to improve anti-interference capabilities; the signal lines should be as short as possible, and the number of vias should be reduced as much as possible ;The shorter the connections between components, the better, in order to reduce the spread of parameters and electromagnetic interference between each other; Incompatible signal lines should be kept as far away from each other as possible, and parallel routing should be avoided as much as possible, and the signals on the front and back sides The lines should be perpendicular to each other; when wiring, the angle should be 135° where corners are required, and avoid turning at right angles.
The lines directly connected to the pads during wiring should not be too wide, and the wiring should be as far away from unconnected components as possible to prevent short circuits; vias should not be drawn on components, and should be kept as far away from unconnected components as possible to prevent high-frequency In the production of microwave RF circuit boards, phenomena such as virtual soldering, continuous soldering, and short circuits occur. In RF circuit board planning, the correct wiring of power lines and ground wires is particularly important. Reasonable planning is the most important means to defeat electromagnetic interference.
A considerable number of interference sources on RF PCB circuit boards are generated through power supplies and ground wires, among which the ground wire causes the largest noise interference. The main reason why the ground wire easily causes electromagnetic interference is that the ground wire has impedance. When a current flows through the ground wire, a voltage will be generated on the ground wire, and then a ground loop current will be generated, forming loop interference in the ground wire. When multiple circuits share a section of ground wire, common impedance coupling will occur, resulting in so-called ground wire noise.

Therefore, when routing the ground wire of the RF circuit board, you should do the following:
① When the circuit is divided into blocks, the radio frequency circuit can basically be divided into high-frequency extension, mixing, demodulation, local oscillator and other parts. A common potential reference point must be provided for each circuit module, that is, the respective ground wire of each module circuit , so that signals can be transmitted between different circuit modules. Then, it is collected at the place where the radio frequency circuit board is connected to the ground wire, that is, it is collected at the main ground wire. Since there is only one reference point, there is no common impedance coupling, and there is no mutual interference problem;
②The digital area and the analog area should be separated by ground wires as much as possible, and the digital ground and analog ground should be separated and finally connected to the power ground;
③ Pay attention to the single-point grounding principle for the ground wires inside each part of the circuit, minimize the signal loop area, and connect it to the ground wire of the corresponding filter circuit nearby;
④ When space permits, it is best to isolate each module with a ground wire to prevent signal coupling effects between each other.

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