Abstract: This paper details common problems and solutions in RF circuit design. Keywords: PCB; radio frequency; RF circuit; design 1 Introduction RF PCB design, based on currently published theories, has many uncertainties and is often described as a "black art." Generally, for circuits below microwave frequency (including low-frequency and low-frequency digital circuits), careful planning based on a comprehensive understanding of various design principles guarantees a successful design on the first attempt. For PC-type digital circuits above microwave frequency and high frequency, 2-3 versions of the PCB are needed to ensure circuit quality. For RF circuits above microwave frequency, even more versions of the PCB design are often required, requiring continuous improvement, and this is only possible with considerable experience. This illustrates the difficulties in RF circuit design. 2 Common Problems in RF Circuit Design 2.1 Interference between Digital Circuit Modules and Analog Circuit Modules If analog (RF) circuits and digital circuits work independently, they may function well on their own. However, once they are placed on the same circuit board and use the same power supply, the entire system is likely to become unstable. This is mainly because digital signals frequently oscillate between ground and positive power (>3V) with extremely short periods, often on the order of nanoseconds. Due to the large amplitude and short switching time, these digital signals contain a large number of high-frequency components independent of the switching frequency. In the analog section, the signal transmitted from the wireless tuning circuit to the receiving section of the wireless device is generally less than 1μV. Therefore, the difference between the digital signal and the RF signal can reach 120 dB. Obviously, if the digital signal and the RF signal cannot be well separated, the weak RF signal may be damaged, thus degrading the performance of the wireless device or even rendering it completely inoperable. 2.2 Power Supply Noise Interference RF circuits are quite sensitive to power supply noise, especially voltage spikes and other high-frequency harmonics. Microcontrollers draw a large amount of current suddenly for a short period within each internal clock cycle, due to the use of CMOS technology in modern microcontrollers. Therefore, assuming a microcontroller operates at an internal clock frequency of 1MHz, it will draw current from the power supply at this frequency. Without proper power supply decoupling, voltage spikes on the power lines will inevitably occur. If these voltage spikes reach the power supply pins of the RF section of the circuit, they can cause serious malfunctions. 2.3 Improper Grounding If the grounding of the RF circuit is not handled properly, some strange phenomena may occur. For digital circuit design, most digital circuits function well even without a ground layer. However, in the RF band, even a very short ground wire can act like an inductor. Roughly calculated, the inductance per millimeter is about 1 nH, and the inductive reactance of a 10-ton PCB line at 433 MHz is about 27Ω. Without a ground layer, most ground wires will be too long, and the circuit will not have the designed characteristics. 2.4 Radiated Interference from Antennas to Other Analog Circuits 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). High-frequency signals emitted by the antenna of the RF transmitter may reach the analog input of the ADC. Because any circuit line can emit or receive RF signals like an antenna, if the ADC input is not handled properly, the RF signal may self-oscillate within the ESD diode of the ADC input. This causes ADC deviation. 3 RF Circuit Design Principles and Schemes 3.1 RF Layout Concept When designing RF layout, the following general principles must be prioritized: (1) Isolate the high-power RF amplifier (HPA) and low-noise amplifier (LNA) as much as possible. Simply put, keep the high-power RF transmitting circuit away from the low-power RF receiving circuit. (2) Ensure that the high-power area on the PCB board has at least one whole ground plane, 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 needs to be far away from RF input. (5) Sensitive analog signals should be kept as far away as possible from high-speed digital signals and RF signals. 3.2 Physical and Electrical Partition Design Principles Design partitions can be decomposed into physical partitions and electrical partitions. Physical partitions mainly involve component layout, orientation, and shielding. Electrical partitions can be further decomposed into power distribution, RF traces, sensitive circuits and signals, and grounding partitions. 3.2.1 Physical Partition Principles (1) Component Placement Principles. Component placement is the key to achieving an excellent RF design. The most effective technique is to first fix the components on the RF path and adjust their orientation to minimize the length of the RF path and keep the input away from the output. And separate the high-power circuit and the low-power circuit as far as possible. (2) PCB stacking design principles. The most effective way to stack circuit boards is to place the main ground plane (main ground) on the second layer below the surface layer and to place the RF lines on the surface layer as much as possible. Minimize the via size on the RF path, which can not only reduce the path inductance, but also reduce the number of poor solder joints on the main ground and reduce the chance of RF energy leakage to other areas in the stacked board. (3) RF device and RF wiring layout principles. In physical space, linear circuits such as multi-stage amplifiers are usually sufficient to isolate multiple RF areas from each other, but duplexers, mixers and intermediate frequency amplifiers/mixers always have multiple RF/IF signals interfering with each other. Therefore, this effect must be carefully minimized. RF and IF traces should cross each other as much as possible and be separated by a ground plane as much as possible. The correct RF path is very important to the performance of the entire PCB, which is why component layout usually takes up most of the time in cellular phone PCB design. (4) Design principle to reduce interference coupling of high/low power devices. On a cellular phone PCB, the low noise amplifier circuit can usually be placed on one side of the PCB, while the high power amplifier is placed on the other side, and finally connected to the antenna of the RF end and the baseband processor end on the same side by a duplexer. To ensure that the via does not transfer RF energy from one side of the board to the other, the common technique is to use blind vias on both sides. The adverse effects of vias can be minimized by arranging vias in areas on both sides of the PCB that are not subject to RF interference. 3.2.2 Electrical partitioning principle (1) Power transmission principle. The DC current of most circuits in a cellular phone is quite small, so the wiring width is usually not a problem. However, a separate high current line, as wide as possible, must be set 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 transfer current from one layer to another. (2) Power supply decoupling for high-power devices. If the power supply pins of a high-power amplifier are not adequately decoupled, high-power noise will radiate across the entire board, causing various problems. Grounding of high-power amplifiers is critical and often requires a metal shield. (3) RF input-output isolation principle. In most cases, it is equally important 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, if the outputs of amplifiers and buffers are fed back to their inputs with appropriate phase and amplitude, they may oscillate. In the best case, they will be able to operate stably under any temperature and voltage conditions. In reality, they may become unstable and add noise and intermodulation signals to the RF signal. (4) Filter input-output isolation principle. If the RF signal line has to loop back from the input to the output of the filter, this may severely impair the bandpass characteristics of the filter. To ensure good isolation between the input and output, a ground plane must first be placed around the filter. Secondly, a ground plane should also be placed in the lower area of ​​the filter and connected to the main ground surrounding the filter. Keeping signal lines that need to pass through the filter as far away from the filter pins as possible is also a good approach. In addition, grounding at various points on the board must be done very carefully, otherwise an unwanted coupling path may be introduced unknowingly. (5) Isolation of digital and analog circuits. In all PCB designs, keeping digital circuits as far away from analog circuits as possible is a general principle, and this also applies to RF PCB designs. Common analog ground and ground used to shield and isolate signal lines are usually equally important, and design changes due to negligence may lead to the design being scrapped and restarted. Similarly, RF lines should be kept away from analog lines and some critical digital signals. All RF traces, pads and components should be surrounded by as much ground copper as possible and connected to the main ground as possible. If RF traces must pass through signal lines, then try to place a ground layer connected to the main ground along the RF traces between them. If this is not possible, make sure they are cross-shaped. This will minimize capacitive coupling, and at the same time, place as much ground as possible around each RF trace and connect them to the main ground. In addition. Minimizing the distance between parallel RF traces minimizes inductive coupling. 4. Conclusion The rapidly developing RF integrated circuits offer broad prospects for engineers working in various wireless communication fields. However, RF circuit design requires designers to have practical experience and engineering design capabilities. The experiences summarized in this article can help RF integrated circuit developers shorten development cycles, avoid unnecessary detours, and save manpower and resources.