PCB Blog - The concept and design principle of high frequency circuit and PCB board

For high-frequency circuit PCB board design, there is already good CAD software, and its powerful functions are enough to overcome people's lack of design experience and cumbersome parameter retrieval and calculation. Those with little experience should be able to complete the RF components with better quality. But in practice, this is not the case.

1. About CAD-aided design software and network analyzer

For high-frequency circuit design, there are already very good CAD softwares. Its powerful functions are enough to overcome people's lack of design experience and cumbersome parameter retrieval and calculation. Combined with powerful network analyzers, it should be Those with a little experience can complete better quality RF components. However, in practice this is not the case. CAD design software relies on powerful library functions, including component parameters and basic performance indicators provided by most of the world's radio device manufacturers. Many RF engineers mistakenly believe that as long as the tool is used for design, there will be no problem. However, the actual result is always contrary to the wish. The reason is that they give up the flexible application of the basic concepts of high-frequency circuit design and the accumulation of experience in the application of basic design principles under the wrong understanding. As a result, they often make basic application errors in the application of software tools. The RF circuit design CAD software is a transparent visualization software, which uses its various high-frequency basic configuration model libraries to complete the simulation of the actual circuit working state. So far, we can already understand the key link, there are two types of high-frequency basic configuration models, one is a component model in the form of centralized parameters, and the other is a local function model in conventional design. So there are the following problems:

1) The component model and CAD software have been interacting and developing for a long time, and they are becoming more and more perfect. In practice, the authenticity of the model can be basically trusted. However, the application environment considered by the component model (especially the electrical environment of the component application) are all typical values. In most cases, the series of application parameters must be determined empirically, otherwise the actual results are sometimes even farther from the design results without the aid of CAD software.

2) The conventional high-frequency basic configuration model established in CAD software is usually limited to the predictable aspects under the current application conditions, and can only be limited to the basic functional model (otherwise, product development does not need to employ people, and all kinds of products are born by relying on CAD alone. product).

3) It is particularly worth noting that the establishment of a typical functional model is completed by applying components in a typical way and using a typical and perfect process structure (including PCB board structure), and its performance has also reached a "typical" high level. . But in practice, it is a complete imitation, which is far from the model state. The reason is: although the selected components and their parameters are the same, their combined electrical environment cannot be the same. In low frequency circuits or digital circuits, such a difference of a few centimeters is not a big hindrance, but in radio frequency circuits, fatal errors often occur.

4) In the design of CAD software, the fault-tolerant design of the software does not pay attention to whether the wrong parameter setting that is contrary to the actual situation occurs. Therefore, an ideal result is given according to the running path of the software, but in practice it is full of problems. result. It can be known that the key error link is not using the basic principles of RF circuit design to correctly apply the CAD software.

5) CAD software is only a design auxiliary tool. It uses its real-time simulation function, powerful component model library and its function generation function, typical application model library, etc. to simplify people's tedious design and calculation work. So far, it is far from being able to replace artificial intelligence in specific design. The power of CAD software in the aided design of RF PCB boards is an important aspect of the software's popularity. But in practice, many RF engineers are often "understood" by them. The cause is still its fault-tolerant nature of parameter settings. It is often used to obtain an ideal model (including each functional link) by using its simulation function, but only after the actual debugging is found: it is better to use your own experience to design. Therefore, in PCB design, CAD software is still only beneficial to engineers with basic RF design experience and skills, helping them engage in tedious process design (non-basic principle design). There are two types of network analyzers, scalar and vector, which are essential instruments for RF circuit design. The usual practice is to complete the circuit and PCB board design (or use CAD software) according to the basic RF circuit design concepts and principles, complete the sample processing of the PCB board and assemble the prototype as required, and then use the network analyzer to design each link. Network analysis is performed one by one, and it is possible to make the circuit reach the state. But the cost of this work is the actual production of at least 3~5 versions of the PCB, and if there is no basic PCB design principles and basic concepts, the required PCB versions will be more (or the design cannot be completed). In the process of using a network analyzer to analyze the RF circuit, it is necessary to have a complete high-frequency circuit PCB board design concept and principles, and it must be able to clearly know the design defects of the PCB board through the analysis results. Only this one requires the relevant engineers to have considerable experience. In the process of analyzing the network links of the prototype, it is necessary to rely on skilled experimental experience and skills to construct a local functional network. Because in many cases, the circuit defects found by the network analyzer will have many factors at the same time, so it is necessary to use the construction of a local functional network to analyze and thoroughly investigate the cause. This experimental circuit construction must rely on clear high-frequency circuit design experience and skilled circuit PCB board construction principles.

2. The scope of this article

This paper is mainly aimed at the concept and design principles of microwave-grade high-frequency circuits and its PCB board design, a frontier category of communication products. The reason why the PCB design principle of microwave-grade high-frequency circuit is chosen is that this principle has extensive guiding significance and belongs to the current high-tech popular application technology. The transition from the microwave circuit PCB board design concept to the high-speed wireless network (including various access networks) projects is also in the same vein, because they are based on the same basic principle, the dual transmission line theory. Digital circuits or relatively low-frequency circuit PCBs designed by experienced RF engineers have a very high success rate, because their design concept is centered on "distributed" parameters, and the concept of distributed parameters is used in lower-frequency circuits (including The destructive effect in digital circuits) is often ignored by people. For a long time, the design of electronic products (mainly for communication products) completed by many peers is often full of problems. On the one hand, it is related to the lack of necessary links in electrical principle design (including redundancy design, reliability design, etc.), but more importantly, many such problems occur when people think that all necessary links have been considered. In response to these problems, they often spend their energy on checking procedures, electrical principles, parameter redundancy, etc., but seldom spend their energy on reviewing PCB board design, which is often due to PCB board design defects. Causes a lot of product performance issues. PCB board design principles involve many aspects, including basic principles, anti-interference, electromagnetic compatibility, safety protection, and so on. For these aspects, especially in high-frequency circuits (especially microwave-grade high-frequency circuits), the lack of relevant concepts often leads to the failure of the entire R&D project. Many people still stay on the basis of "connecting electrical principles with conductors to play a predetermined role", and even think that "PCB board design belongs to the consideration of structure, technology and improving production efficiency". Many RF engineers also do not fully realize that this link should be the special focus of the entire design work in RF design, and they mistakenly spend their energy on selecting high-performance components, resulting in a significant increase in cost and little performance improvement. What should be especially pointed out here is that the digital circuit relies on its strong anti-interference, error detection and correction, and can arbitrarily construct each intelligent link to ensure the normal function of the circuit. An ordinary digital application circuit with high additional configuration of various "guaranteed normal" links is obviously a measure without a product concept. However, it often leads to a series of product problems in the link that is considered "not worth it". The reason is that this kind of functional link that is not worthy of construction reliability guarantee from the perspective of product engineering should be based on the working mechanism of the digital circuit itself, but the wrong construction in the circuit design (including the PCB board design) causes the circuit to be in a state of failure. unstable state. The cause of this unstable state is a basic application under the same concept as the similar problems of high-frequency circuits.

In digital circuits, there are three aspects to be taken seriously:

1) The digital signal itself belongs to the broad spectrum signal. According to the result of the Fourier function, it contains very rich high-frequency components, so the high-frequency components of digital signals are fully considered in the design of digital ICs. However, in addition to digital ICs, the signal transition areas within and between each functional link, if arbitrary, will lead to a series of problems. Especially in circuit occasions where digital and analog and high-frequency circuits are mixed.

2) Various types of reliability designs in digital circuit applications are related to the reliability requirements and product engineering requirements of circuits in practical applications, and various high-cost "guarantee" parts cannot be added to circuits that have been conventionally designed to meet the requirements.

3) The working speed of digital circuits is moving towards high frequency with unprecedented development (for example, the current CPU, whose main frequency has reached 1.7GHz, far exceeds the lower limit of the microwave frequency band). Although the reliability assurance functions of related devices are also supported simultaneously, they are based on the internal and typical external signal characteristics of the device.

3. An overview of the guiding significance of the dual transmission line theory for the design of microwave circuits and the principles of PCB board wiring

PCB board concept under two-line theory

For microwave-grade high-frequency circuits, each corresponding strip line on the PCB board forms a microstrip line (asymmetrical) with the ground plate. For PCB boards with more than two layers, a microstrip line and a strip can be formed. line (symmetrical microstrip transmission line). Different microstrip lines (double-sided PCB boards) or strip lines (multi-layer PCB boards) form coupled microstrip lines, thereby forming various complex four-port networks, thus forming a microwave-level circuit PCB Various characteristics of the board. It can be seen that the theory of microstrip transmission line is the design basis of microwave-grade high-frequency circuit PCB board. For RF-PCB board design above 800MHz, the PCB board network design near the antenna should fully follow the microstrip theoretical basis (rather than just using the microstrip concept as a tool to improve the performance of lumped-parameter devices). The higher the frequency, the more significant the guiding significance of the microstrip theory becomes. For the centralized parameters and distributed parameters of the circuit, although the lower the operating frequency, the weaker the effect of the distributed parameters, but the distributed parameters always exist. There is no clear dividing line whether to consider the influence of distribution parameters on circuit characteristics. Therefore, the establishment of the concept of microstrip is equally important for the design of digital circuit and relative intermediate frequency circuit PCB. The basics and concepts of microstrip theory and the design concept of microwave-level RF circuits and PCB boards are actually an application of the microwave dual transmission line theory. For RF-PCB board wiring, each adjacent signal line (including adjacent adjacent) All form the characteristics that follow the basic principle of the double line (this will be clearly explained later). Although the common microwave RF circuit is equipped with a ground plane on one side, so that the microwave signal transmission line on it tends to be a complex four-port network, thus directly following the coupled microstrip theory, but its basis is still the two-wire theory. Therefore, in the design practice, the guiding significance of the double-line theory is more extensive. Generally speaking, for microwave circuits, the microstrip theory has quantitative guiding significance, which belongs to the specific application of the two-line theory, and the two-line theory has a broader qualitative guiding significance. It is worth mentioning that all the concepts given by the two-line theory, on the surface, it seems that some concepts have no connection with the actual design work (especially digital circuits and low-frequency circuits), which is actually an illusion. Two-line theory can guide all conceptual issues in electronic circuit design, especially the significance of PCB circuit design concepts is more prominent. Although the two-line theory is established on the premise of microwave high-frequency circuits, this is only because the influence of distributed parameters in high-frequency circuits becomes significant, which makes the guiding significance particularly prominent. In digital or low-frequency circuits, the distributed parameters are negligible compared with the centralized parameter components, and the concept of two-line theory becomes correspondingly vague. However, how to distinguish high-frequency and low-frequency circuits is often overlooked in design practice. Which category does a typical digital logic or pulse circuit fall into? Obviously low-frequency circuits and low-frequency circuits with nonlinear components, once some sensitive conditions change, it is easy to reflect some high-frequency characteristics. The main frequency of the CPU has reached 1.7GHz, far exceeding the lower limit of the microwave frequency, but it is still a digital circuit. Because of these uncertainties, the design of the PCB board is extremely important. In many cases, passive components in circuits can be equivalent to transmission lines or microstrip lines of specific specifications, and can be described by dual transmission line theory and its related parameters. In short, it can be considered that the dual transmission line theory was born on the basis of synthesizing all the characteristics of electronic circuits. Therefore, in a strict sense, if every link in the design practice is first based on the concept embodied in the dual transmission line theory, then the corresponding PCB board circuit will face very few problems (no matter what the circuit is in working conditions).