Analysis of Frequently Asked Questions about Radio Frequency Integrated Circuits[Copy link]
Radio frequency (RF) PCB design has many uncertainties in the currently published theories and is often described as a "black art". Generally speaking, for circuits below the microwave frequency band (including low-frequency and low-frequency digital circuits), careful planning based on a comprehensive understanding of various design principles is the guarantee of a successful one-time design. For PC-type digital circuits above the microwave frequency band and high frequency, 2~3 versions of PCB are required to ensure the quality of the circuit. For RF circuits above the microwave frequency band, more versions of PCB design are often required and continuously improved, and this is based on considerable experience. This shows the difficulty of RF electrical design. Interference between digital circuit modules and analog circuit modules If the analog circuit (RF) and the digital circuit work separately, they may work well separately. However, once the two are placed 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 the positive power supply (>3 V), and the cycle is extremely short, often in the nanosecond range. Due to the large amplitude and short switching time, these digital signals contain a large amount 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 1μV. Therefore, the difference between the digital signal and the RF signal can reach 120dB. Obviously, if the digital signal and the RF signal cannot be separated well, the weak RF signal may be damaged, so that the performance of the wireless device will deteriorate or even fail to work at all. Noise interference of power supply RF circuits are quite sensitive to power supply noise, especially to glitch voltages and other high-frequency harmonics. Microcontrollers will suddenly absorb most of the current for a short period of time in each internal clock cycle. This is because modern microcontrollers are manufactured using CMOS technology. Therefore, assuming that a microcontroller runs at an internal clock frequency of 1MHz, it will draw current from the power supply at this frequency. If proper power decoupling is not adopted, voltage glitches will inevitably occur on the power line. If these voltage glitches reach the power pins of the RF part of the circuit, it may cause work failure in serious cases. Unreasonable ground line If the ground line of the RF circuit is not handled properly, some strange phenomena may occur. For digital circuit design, most digital circuit functions perform well even without a ground layer. In the RF frequency band, even a very short ground line will act like an inductor. Roughly calculated, the inductance per millimeter length is about 1nH, and the inductive reactance of a 10mm PCB line at 433MHz is about 27Ω. If a ground layer is not used, most ground lines will be long and the circuit will not have the designed characteristics. Radiated interference of 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/digital conversion (ADC) or digital/analog converter (DAC). The high-frequency signal emitted by the antenna of the RF transmitter may reach the analog input of the ADC. Because any circuit line may emit or receive RF signals like an antenna. If the processing of the ADC input is not reasonable, the RF signal may self-excite in the ESD diode of the ADC input, thereby causing ADC deviation. 2.RF circuit design principles and solutions RF layout concepts When designing RF layout, the following general principles must be met as a 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 away from the low-power RF receiving circuit; (2) Ensure that there is at least one whole ground in the high-power area of the PCB board, preferably without vias on it. Of course, the larger the copper foil area, the better; (3) Circuit and power supply decoupling is also extremely important; (4) The RF output usually needs to be away from the RF input; (5) Sensitive analog signals should be kept as far away from high-speed digital signals and RF signals as possible. Design principles for physical partitioning and electrical partitioning Design partitioning can be divided into physical partitioning and electrical partitioning. Physical partitioning mainly involves component layout, direction, and shielding; electrical partitioning can be further divided into power distribution, RF routing, sensitive circuits and signals, and grounding. Physical partitioning principles (1) Component location layout principles. Component layout 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 direction so as to minimize the length of the RF path, keep the input away from the output, and separate high-power circuits and low-power circuits as far as possible. (2) PCB stacking design principles. The most effective circuit board stacking method is to arrange the main ground plane (main ground) on the second layer below the surface, and to place the RF line on the surface as much as possible. Minimize the size of the vias on the RF path, which can not only reduce the path inductance, but also reduce the number of cold solder joints on the main ground, and reduce the chance of RF energy leaking to other areas in the stacked board. (3) Principles of RF device and RF wiring layout. 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, so this effect must be carefully minimized. RF and IF traces should be crossed as much as possible, and a ground should be placed between them as much as possible. Correct RF paths are 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 principles to reduce interference coupling between high/low power devices. On a cellular phone PCB, the low noise amplifier circuit can usually be placed on one side of the PCB and the high power amplifier on the other side, and finally connected to the antennas on the RF and baseband processor sides on the same side through a duplexer. Skills must be used to ensure that the through hole does not transfer RF energy from one side of the board to the other side. A common technique is to use blind vias on both sides. The adverse effects of through holes can be minimized by arranging through holes in areas on both sides of the PCB that are not subject to RF interference. Electrical partitioning principles (1) Power transmission principles. 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 up for the power supply of the high power amplifier to minimize the transmission voltage drop. To avoid too much current loss, multiple vias are used to transfer current from one layer to another. (2) Power supply decoupling of high-power devices. If the power supply pins of the high-power amplifier are not adequately decoupled, the high-power noise will be radiated to the entire board and cause a variety of problems. The grounding of the high-power amplifier is quite critical and often requires a metal shielding cover. (3) RF input/output isolation principle. In most cases, it is also critical to ensure that the RF output is far 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 the appropriate phase and amplitude, they may produce self-oscillation. In the best case, they will be able to operate stably under any temperature and voltage conditions. In fact, 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 go back from the input end of the filter to the output end, this may seriously damage the bandpass characteristics of the filter. In order to isolate the input and output well, first of all, a circle of ground must be arranged around the filter. Secondly, a ground must also be arranged in the lower area of the filter and connected to the main ground around the filter. It is also a good idea to keep the signal line that needs to pass through the filter as far away from the filter pin as possible. In addition, the grounding of various places on the entire board must be very careful, otherwise an undesirable coupling channel may be introduced unknowingly. (5) Isolation of digital circuits and analog circuits. In all PCB designs, it is a general principle to keep digital circuits away from analog circuits as much as possible. It also applies to RF PCB design. The common analog ground and the ground used to shield and separate signal lines are usually equally important. Design changes caused by negligence may cause the completed design to have to be torn down and re-done. Similarly, RF lines should 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 trace must pass through the signal line, try to arrange a layer of ground connected to the main ground along the RF trace between them. If this is not possible, make sure they are cross-crossed, which can minimize capacitive coupling. At the same time, try to lay more ground around each RF trace and connect them to the main ground. In addition, minimizing the distance between parallel RF traces can minimize inductive coupling. 4) Filter input/output isolation principle. If the RF signal line has to go back from the input end of the filter to the output end, this may seriously damage the bandpass characteristics of the filter. In order to isolate the input and output well, first of all, a circle of ground must be arranged around the filter. Secondly, a ground must 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 line that needs to pass through the filter as far away from the filter pin as possible. In addition, the grounding of various places on the entire board must be very careful, otherwise an undesirable coupling channel may be introduced unknowingly. (5) Isolation of digital circuits and analog circuits. In all PCB designs, it is a general principle to keep digital circuits away from analog circuits as much as possible. It also applies to RF PCB design. The common analog ground and the ground used to shield and separate signal lines are usually equally important. Design changes caused by negligence may cause the completed design to be torn down and re-done. Similarly, RF lines should 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 trace must pass through the signal line, try to arrange a layer of ground connected to the main ground along the RF trace between them. If this is not possible, make sure they are cross-crossed, which can minimize capacitive coupling. At the same time, try to lay more ground around each RF trace and connect them to the main ground. In addition, minimizing the distance between parallel RF traces can minimize inductive coupling. 4) Filter input/output isolation principle. If the RF signal line has to go back from the input end of the filter to the output end, this may seriously damage the bandpass characteristics of the filter. In order to isolate the input and output well, first of all, a circle of ground must be arranged around the filter. Secondly, a ground must 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 line that needs to pass through the filter as far away from the filter pin as possible. In addition, the grounding of various places on the entire board must be very careful, otherwise an undesirable coupling channel may be introduced unknowingly. (5) Isolation of digital circuits and analog circuits. In all PCB designs, it is a general principle to keep digital circuits away from analog circuits as much as possible. It also applies to RF PCB design. The common analog ground and the ground used to shield and separate signal lines are usually equally important. Design changes caused by negligence may cause the completed design to be torn down and re-done. Similarly, RF lines should 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 trace must pass through the signal line, try to arrange a layer of ground connected to the main ground along the RF trace between them. If this is not possible, make sure they are cross-crossed, which can minimize capacitive coupling. At the same time, try to lay more ground around each RF trace and connect them to the main ground. In addition, minimizing the distance between parallel RF traces can minimize inductive coupling.
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