PCB Blog - PCB RF Circuit Design

RF circuit PCB design

With the development of communication technology, hand-held radio frequency circuit technology is used more and more widely, such as wireless pager, mobile phone, wireless PDA, etc. The performance index of the radio frequency circuit directly affects the quality of the entire product. One of the biggest characteristics of these handheld products is miniaturization, which means that the density of components is very high, which makes the mutual interference between components (including SMD, SMC, bare chips, etc.) very prominent. If the EMI signal is not handled properly, the whole circuit system may not work properly. Therefore, how to prevent and suppress EMI and improve EMC has become a very important topic when designing RF circuit PCB. The performance indicators of the same circuit with different PCB design structures will vary greatly.

First, determine the position of the interface components with other PCB board or systems on the PCB, and pay attention to the coordination between the interface components (such as the direction of the components). Because the size of handheld devices is very small, and the arrangement of components is very compact, for larger components, priority must be given to determine the corresponding location and consider the coordination between them. Carefully analyze the circuit structure, process the circuit in blocks (such as high-frequency amplification circuit, frequency mixing circuit, demodulation circuit, etc.), try to separate the strong current signal from the weak current signal, separate the digital signal circuit from the analog signal circuit, and try to arrange the circuits that complete the same function within a certain range to reduce the signal loop area; The filter network of each part of the circuit must be connected nearby, which can not only reduce the radiation, but also reduce the probability of interference. The unit circuits are grouped according to their sensitivity to electromagnetic compatibility in use. The components in the circuit vulnerable to interference should also be arranged to avoid interference sources (such as interference from the CPU on the data processing board).

After the layout of components is basically completed, the wiring can be started. The basic principle of wiring is to select low-density wiring design as far as possible after the assembly density permits, and the signal wiring should be as thick and thin as possible, which is conducive to impedance matching. For RF circuits, the unreasonable design of the direction, width and 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, it must be comprehensively considered and reasonably wired. During wiring, all wiring shall be far away from the frame of PCB board (about 2mm) to avoid potential wire breakage or breakage during PCB board fabrication. The power line shall be as wide as possible to reduce the loop resistance. At the same time, the direction of power line and ground wire shall be consistent with the direction of data transmission to improve the anti-interference capability; The signal wires shall be as short as possible and the number of vias shall be minimized; The shorter the connection between components, the better, to reduce the distribution parameters and mutual electromagnetic interference; The incompatible signal lines shall be far away from each other, and parallel routing shall be avoided as far as possible, while the signal lines on both sides of the positive direction shall be perpendicular to each other; During wiring, 135 ° angle should be used at the address where corner is required to avoid turning right angles. When wiring, the line directly connected to the bonding pad should not be too wide, and the wiring should be as far away from the unconnected components as possible to avoid short circuit; The vias are not painted on the components and should be as far away from the disconnected components as possible to avoid false soldering, continuous soldering, short circuit and other phenomena in production. In the design of RF circuit PCB, the correct wiring of power line and ground line is particularly important. Reasonable design is the most important means to overcome electromagnetic interference.

Quite a few interference sources on PCB board are generated by power supply and ground wire, among which ground wire causes the most noise interference. The main reason why the ground wire is easy to form electromagnetic interference is that the ground wire has impedance. When a current flows through the ground wire, it will generate voltage on the ground wire, thus generating the loop current of the ground wire and forming the loop interference of the ground wire. When multiple circuits share a section of ground wire, a common impedance coupling will be formed, thus producing the so-called ground wire noise. Therefore, when wiring the ground wire of the RF circuit PCB, we should do the following: First, we should block the circuit. The RF circuit can basically be divided into high-frequency amplification, mixing, demodulation, local oscillator and other parts. We should provide a common potential reference point for each circuit module, that is, the ground wire of each module circuit, so that signals 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 ground wire. Since there is only one reference point, there is no common impedance coupling, so there is no mutual interference problem. The ground wire of the digital area and the analog area shall be isolated as far as possible, and the digital ground shall be 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 principle of single point grounding, minimize the area of the signal loop, and connect to the address of the corresponding filter circuit nearby. If space allows, it is better to isolate each module with ground wire to prevent mutual signal coupling effect.

The key of RF circuit PCB design is how to reduce the radiation ability and how to improve the anti-interference ability. Reasonable layout and wiring is the guarantee of RF circuit PCB design. The method described in this paper is conducive to improving the reliability of RF circuit PCB design, solving the problem of electromagnetic interference, and achieving the purpose of electromagnetic compatibility. With the rise of the Internet of Things technology, it is more and more common for electronic products to carry the wireless communication function. The wireless communication technology is realized by relying on the RF circuit PCB, and requires professional design and simulation analysis tools.