Although it is the "mother of electronics", in the past, the domestic focus on the technological innovation of the PCB circuit board industry is far less than that of the developed countries. Fortunately, this situation has changed a lot. On August 30th this year, CS Show 2016 will be held in Shenzhen Convention and Exhibition Center as a platform for domestic professional display of PCB product technology and solutions. With professional and dedicated exhibition concept and strong ability to link upstream and downstream industries, it has more in the industry. Strong influence. The organizers of this exhibition will fully display circuit boards such as PCB, FPC, HDI, MPCB, IC carrier board, etc., and will once again fill the blank of no professional circuit board exhibition in China. The layout design of PCB circuit boards has always been a big problem for PCB engineers. The design of RF circuit boards is even more difficult. Today we will share some RF circuit design experience.
The rapid development of radio frequency integrated circuits provides a broad prospect for engineers and technicians engaged in various types of wireless communications. If the grounding 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 plane. In the RF band, even a short ground wire acts like an inductor. Roughly calculated, the inductance per mm length is approximately 1 nH, and the inductive reactance of 10 to PCB lines at 433 MHz is approximately 27 Ω. If the ground plane is not used, most of the ground will be longer and the circuit will not have the design characteristics. This makes it difficult to design RF circuits.
Frequently asked questions about RF circuit design
1. Interference between digital circuit module and analog circuit module
If the analog circuit (RF) and the digital circuit work separately, they may each work well. However, once the two are placed on the same board and work with the same power supply, the entire system is likely to be unstable. This is mainly because digital signals are frequently oscillated between ground and positive power supplies ("3 V"), and the period is particularly short, often in the nanosecond range. Due to the large amplitude and short switching time. These digital signals are made to contain a large number of high frequency components independent of the switching frequency. In the analog section, the signal transmitted from the wireless tuning loop to the receiving portion of the wireless device is typically less than 1 μV. Therefore, the difference between digital and RF signals can reach 120 dB. Obviously, if the digital signal cannot be separated from the RF signal well. Weak RF signals can be corrupted, and as a result, wireless devices will deteriorate or not work at all.
2. Noise interference from the power supply
RF circuits are quite sensitive to power supply noise, especially for glitch voltages and other high frequency harmonics. The microcontroller will suddenly draw most of the current for a short period of time in each internal clock cycle, since modern microcontrollers are fabricated in a CMOS process. Therefore, assuming that a microcontroller is operating at an internal clock frequency of 1 MHz, it will draw current from the supply at this frequency. If you do not use a suitable power supply decoupling, it will cause voltage glitch on the power line. If these voltage spurs reach the power supply pin of the RF portion of the circuit, it can cause a malfunction if it is severe.
3. Radiation interference from the antenna 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 (ADC) or digital-to-analog converters (DACs). The high frequency signal from 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 processing at the ADC input is unreasonable, the RF signal may self-excite in the ESD diode of the ADC input. This causes ADC deviation.
RF circuit design principles and solutions
1. RF layout concept
When designing an RF layout, the following general principles must be prioritized:
Separate high-power RF amplifiers (HPAs) from low-noise amplifiers (LNAs) as much as possible. Simply put, keep high-power RF transmit circuits away from low-power RF receive circuits:
Make sure that there is at least one whole piece of high power area on the PCB. It is better to have no vias on it. Of course, the larger the area of ​​the copper foil, the better;
Circuit and power supply decoupling are also extremely important;
The RF output usually needs to be kept away from the RF input;
Sensitive analog signals should be as far away as possible from high-speed digital and RF signals.
2. Physical partitioning and electrical partition design principles
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 traces, sensitive circuits and signals, and grounding.
3. Physical partitioning principles
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 orientation to minimize the length of the RF path and keep the input away from the output. And separate high power circuits and low power circuits as far as possible.
PCB stack design principles. The most efficient method of board stacking is to arrange the main ground plane (main ground) in the second layer below the surface layer and place the RF lines on the surface layer as much as possible. Minimizing the via size on the RF path not only reduces path inductance, but also reduces the number of solder joints on the primary ground and reduces the chance of RF energy leaking into other areas of the laminate.
RF device and its RF wiring layout principles. In physical space, linear circuits like multistage amplifiers are usually sufficient to isolate multiple RF regions from each other, but duplexers, mixers, and IF amplifiers/mixers always have multiple RF/IFs. The signals interfere with each other. Therefore, this effect must be carefully minimized. The RF and IF traces should be crossed as much as possible, and as much as possible between them. The correct RF path is important to the performance of the entire PCB, which is why component placement typically takes up most of the time in cellular phone PCB design.
Design principles to reduce interference coupling in high/low power devices. On a cellular phone PCB, it is usually possible to place the low noise amplifier circuit on one side of the PCB and the high power amplifier on the other side, and finally connect them to the RF side and baseband processor on the same side through the duplexer. On the antenna of the end. Tips are needed to ensure that the through hole does not transfer RF energy from one side of the board to the other. A common technique is to use blind holes on both sides. The adverse effects of the vias can be minimized by arranging the vias in areas where both sides of the PCB are free of RF interference.
The design of the RF circuit requires the designer to have some practical experience and engineering design capabilities. Some of the lessons summarized in this paper can help RF IC developers shorten the development cycle. Avoid unnecessary detours and save manpower and material resources.
As an important support for electronic components and electronic components, the role of PCB circuit boards in the electronics industry is very important. People in the industry call them "electronic aircraft carriers." With the development of science and technology, the application range of PCB circuit boards will become more and more extensive. To a certain extent, the production technology level of the circuit board has become an important indicator to measure the level of science and technology in a country. In order to better capture the international leading PCB manufacturing technology, and comprehensively display the manufacturing level of the domestic circuit board industry, from August 30th to September 1st, 2016 Shenzhen International Circuit Board Purchasing Exhibition (CS Show 2016) will be held in Shenzhen Convention and Exhibition Center. Officially kicked off, blew the horn of the annual PCB industry.
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