RF circuit PCB design - component layout

Therefore, in the design of the RF circuit PCB, in addition to the layout of the ordinary PCB design, 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 circuit itself. Anti-jamming capability. According to experience, the effect of the RF circuit depends not only on the performance of the RF board itself, but also on the interaction with the CPU processing board. Therefore, reasonable layout is particularly important when designing the PCB.

The general principle of layout: components should be arranged in the same direction as possible, by selecting the direction of the PCB into the molten tin system to reduce or even avoid the phenomenon of poor soldering; according to the experience, there must be at least 0.5mm spacing between components to meet the melting tin of the components. Requirements, if the space of the PCB board allows, the spacing of components should be as wide as possible. For double panels, one side should be designed with SMD and SMC components on one side and discrete components on the other.

Pay attention to the layout:

* First determine the position of the interface components of 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 components).

*Because the size of the palm-sized products is very small, the components are arranged very compactly. Therefore, for larger components, priority must be given to determine the corresponding position and consider the problem of cooperation with each other.

* Carefully analyze the circuit structure, block the circuit (such as high-frequency amplifier circuit, mixer circuit and demodulation circuit, etc.), separate the strong signal and the weak signal as much as possible, and separate the digital signal circuit from the analog signal circuit. Circuits that perform the same function should be arranged within a certain range to reduce the signal loop area; the filter network of each part of the circuit must be connected nearby, which not only reduces the radiation, but also reduces the probability of interference, according to the circuit. Anti-jamming capability.

* Grouping according to the degree of sensitivity of the unit circuit to electromagnetic compatibility in use. Components that are susceptible to interference in the circuit should also avoid interference sources (such as interference from the CPU on the data processing board) during layout.

Routing

After the layout of the components is basically completed, the wiring can be started. The basic principle of wiring is: after the assembly density is allowed, try to use low-density wiring design, and the signal traces should be as thick as possible, which is good for impedance matching.

For RF circuits, the unreasonable design of the direction, width and line spacing of the signal lines may cause cross-interference between the signal signal transmission lines. In addition, the system power supply itself also has noise interference, so it must be considered comprehensively when designing the RF circuit PCB reasonable wiring.

When wiring, all the traces should be away from the border of the PCB board (about 2mm), so as to avoid the hidden trouble of disconnection or disconnection when the PCB board is made. The power cable should be wide to reduce the loop resistance. At the same time, the direction of the power cable and the ground wire should be consistent with the direction of data transmission to improve the anti-interference ability. The signal line should be as short as possible and minimized. The number of holes; the shorter the connection between the components, the better, in order to reduce the distribution parameters and mutual electromagnetic interference; for the incompatible signal lines should be away from each other, and try to avoid parallel routing, and in the positive two sides The signal line application should be perpendicular to each other; when wiring, the address of the corner should be 135° angle, avoiding the right angle.

When wiring, the line directly connected to the pad should not be too wide. The trace should be separated from the unconnected components as much as possible to avoid short circuit; the via should not be drawn on the component, and should be kept away from the unconnected components as much as possible to avoid production. There are phenomena such as virtual welding, continuous welding, and short circuit.

In the design of RF circuit PCB, the correct wiring 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 sources of interference on the PCB are generated by the power supply and ground, where the ground line causes the most noise interference.

The main reason why the ground wire is easy to form electromagnetic interference is the impedance of the ground wire. When a current flows through the ground, a voltage is generated on the ground, thereby generating a ground loop current, which forms a loop interference of the ground. When a plurality of circuits share a ground line, a common impedance coupling is formed, resulting in so-called ground noise. Therefore, when routing the ground wire of the RF circuit PCB, you should do the following:

* First, the circuit is divided into blocks, the RF circuit can be basically divided into high-frequency amplification, mixing, demodulation, local oscillator and other parts, to provide a common potential reference point for each circuit module, that is, the respective ground 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 line, that is, it is summarized on the total ground line. 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 are isolated as much as possible, and the digital ground is separated from the analog ground 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 the address of the corresponding filter circuit.

* When space permits, it is best to isolate each module with a ground wire to prevent signal coupling effects between each other.

The key to RF circuit PCB design is how to reduce the radiation capability and how to improve the anti-interference ability. Reasonable layout and wiring is the guarantee for designing the RF circuit PCB. The method described in the paper is beneficial to improve the reliability of the RF circuit PCB design, solve the electromagnetic interference problem, and achieve the purpose of electromagnetic compatibility.