RF circuit design scheme Wei Na Jiangxi Vocational and Technical College of Industry and Trade Nanchang 330100 Jiangxi Abstract Some of the experiences summarized in this article can help RF circuit developers shorten the cycle, avoid unnecessary detours, and save manpower and material resources Keywords RF circuit design scheme 1. Introduction Radio frequency (RF) PCB design has many uncertainties in the current published theory, and is often described as a "black art". Normally, for circuits in sub-microwave frequency bands (including low-frequency and low-frequency digital circuits), careful planning based on a comprehensive grasp of various design principles is the guarantee of a successful one-time design. For PC digital circuits in frequency bands above microwave and high frequency. Then 2 to 3 versions of PCB are needed to ensure circuit quality. For RF circuits in the microwave and above frequency bands, more versions are often required: PCB design and continuous improvement, and this is under the premise of considerable experience. This shows the difficulties in RF electrical design. 2. Common problems in RF circuit design 1. Interference between digital circuit modules and analog circuit modules. If analog circuits (radio frequency) and digital circuits work independently, they may each work well. However, once the two are put on the same circuit board and work together using the same power supply, the entire system is likely to be unstable. This is mainly because digital signals frequently swing between ground and positive supply (gt; 3V), and the period is extremely short, often on the order of nanoseconds. Due to the larger amplitude and shorter switching time. As a result, these digital signals contain a large number of high-frequency components that are independent of the switching frequency. In the analog part, the signal transmitted from the wireless tuning loop to the receiving part of the wireless device is generally less than lpV. Therefore, the difference between the digital signal and the RF signal will reach 120dB. Obviously, if the digital signal cannot be well separated from the RF signal. Weak RF signals can be corrupted, causing wireless devices to deteriorate or not work at all. 2. Noise interference from the power supply. RF circuits are quite sensitive to power supply noise, especially glitches and other high-frequency harmonics. Microcontrollers will suddenly draw most of the current for a short period of time during each internal clock cycle. This is due to the fact that modern microcontrollers are manufactured using CMOS processes. therefore. Assuming a microcontroller is running at an internal clock frequency of 1MHz, it will draw current from the power supply at this frequency. If proper power decoupling is not taken, voltage glitches on the power line will inevitably occur. If these voltage glitches reach the power pins of the RF part of the circuit, they may cause work failure in severe cases. 3. Unreasonable ground wire. If the ground wire of the RF circuit is not handled properly, some strange phenomena may occur. For digital circuit designs, most digital circuit functions perform well even without a ground plane. In the RF band, even a short ground wire can act like an inductor. Roughly calculated, the inductance per millimeter of length is about 1nH, and the inductive reactance of the 10toniPCB line at 33MHz is about 27Ω. If a ground layer is not used, most ground wires will be longer and the circuit will not have the designed characteristics. 4. Radiated interference from the antenna to other analog circuit parts. In PCB circuit design, there are usually other analog circuits on the board. For example, many circuits have analog-to-digital converters (ADCs) or digital-to-analog converters (DACs). A high-frequency signal from the RF transmitter's antenna may reach the analog input of the ADC. Because any circuit line may emit or receive RF signals like an antenna. If the ADC input is not properly processed, the RF signal may self-excitate within the ESD diode of the ADC input. This causes ADC deviation. 3. RF circuit design principles and solutions 1. RF layout concept.
When designing the RF layout, the following general principles must be given priority: ① Isolate the high-power RF amplifier (HPA) and the low-noise amplifier (LNA) as much as possible. Simply put, keep the high-power RF transmitting circuit as far away as possible. Low-power RF receiving circuit; ② Ensure that the high-power area on the PCB board has at least a whole piece of ground, preferably without vias. Of course, the larger the copper foil area, the better; ③ Circuit and power supply decoupling is also extremely important; ④ RF output usually Need to stay away from RF input; ⑤ Sensitive analog signals should be as far away as possible from high-speed digital signals and RF signals. 2. Design principles for physical partitions and electrical partitions. Design partitions can be broken down into physical partitions and electrical partitions. Physical partitioning mainly involves component layout, orientation and shielding: electrical partitioning can continue to be broken down into partitions for power distribution, RF routing, sensitive circuits and signals, and grounding. 2.1 Principles of physical partitioning. ① Principles of component location layout. Component placement is the key to a good RF design. The most effective technique is to first secure the components located in the RF path and orient them so that the length of the RF path is minimized and the input is moved away from the output. And separate high-power circuits and low-power circuits as far apart as possible. ② PCB stacking design principles. The most effective method of stacking circuit boards is to arrange the main ground plane (main ground) on the second layer below the surface layer, and arrange the RF lines on the surface layer as much as possible. Minimizing the size of vias in the RF path not only reduces path inductance, but also reduces false solder joints on the main ground and reduces the opportunity for RF energy to leak into other areas within the stackup. ③RF devices and their RF wiring layout principles. In physical space, linear circuits like multistage amplifiers are usually sufficient to isolate multiple RF zones from each other, but diplexers, mixers, and IF amplifiers/mixers always have multiple RF/IF Signals interfere with each other. Care must therefore be taken to minimize this effect. RF and IF traces should cross as much as possible and be separated by as much ground as possible. The correct RF path is very important to the performance of the overall PCB, which is why component placement usually takes up the majority of time in cell phone PCB design. ④ Design principles to reduce interference coupling of high/low power devices. On a cell phone PCB, it is often possible to place the low-noise amplifier circuit on one side of the PCB and the high-power amplifier on another side, and ultimately connect them both to the RF side and the baseband processor on the same side via a duplexer on the terminal antenna. Techniques are needed to ensure that vias do not transfer RF energy from one side of the board to the other. A common technique is to use blind vias on both sides. The adverse effects of vias can be minimized by locating them in areas on both sides of the PCB that are free from RF interference. 2.2 Principles of electrical zoning. ①Power transmission principle. The DC current in most circuits in a cell phone is fairly small, so trace width is usually not an issue. However, it is necessary to set a separate high-current line as wide as possible for the power supply of the high-power amplifier to minimize the transmission voltage drop. To avoid too much current loss, multiple vias are needed to pass current from one layer to another. ②Power supply decoupling of high-power devices. If the high-power amplifier is not adequately decoupled at its supply pins, the high-power noise will radiate throughout the board and cause a variety of problems. The grounding of high-power amplifiers is very critical, and it is often necessary to design a metal shield for it. ③RF input. Output isolation principle. In most cases, it is also critical to ensure that the RF output is kept away from the RF input. This also applies to amplifiers, buffers and filters. In the worst case, amplifiers and buffers have the potential to self-oscillate if their outputs are fed back to their inputs with the proper phase and amplitude. In the best case scenario. They will work stably under any temperature and voltage conditions. Actually. They can become unstable and add noise and intermodulation signals to the RF signal. ④ Filter input and output isolation principle. If the RF signal line has to be routed from the input end of the filter back to the output end, this can severely compromise the filter's bandpass characteristics. To keep the input and output well isolated. First a circle of ground must be arranged around the filter.
Secondly, a piece of ground should also be arranged in the lower area of ??the filter and connected to the main ground surrounding the filter. It is also a good idea to keep the signal lines that need to pass through the filter as far away from the filter pins as possible. In addition, great care must be taken when grounding various places throughout the board, otherwise an undesirable coupling path may be introduced unknowingly. ⑤ Digital circuits and analog circuits are isolated. In all PCB designs, it is a general principle to keep digital circuits as far away from analog circuits as possible, and it also applies to RPCB design. Public analog ground and the ground used to shield and separate signal lines are usually equally important. Design changes caused by negligence may cause the nearly completed design to be overthrown and restarted. RF lines should also be kept away from analog lines and some critical digital signals. All RF traces, pads and components should be filled with as much ground copper as possible and connected to the main ground as much as possible. If the RF traces must pass through the signal lines, try to place a ground connected to the main ground along the RF traces between them. If this is not possible, make sure they are criss-crossed. This will minimize capacitive coupling while also placing as much ground as possible around each RF trace and connecting them to the main ground. also. Minimizing the distance between parallel RF traces minimizes inductive coupling. 4. Conclusion The rapid development of radio frequency integrated circuits provides broad prospects for engineers and technicians engaged in various types of wireless communications. But at the same time, the design of radio frequency circuits requires designers to have certain practical experience and engineering design capabilities. Some experiences summarized in this article can help RF integrated circuit developers shorten the development cycle, avoid unnecessary detours, and save manpower and material resources. References: [1] Peng Yipin, Shi Longxing, Lu Shengli, Liu Hao. Monolithic integration of radio frequency circuits [J]. Electronic Devices, 2002, (02) [2] Ma Ning, Chen Li. Radio frequency circuit board resistance Interference Design[J]. China Electronics Business Information (Basic Electronics), 2008, (0