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PCB design principles involve many aspects, including basic principles, anti-interference, electromagnetic compatibility, safety protection, etc. In particular, the development of high-frequency circuits (especially in high frequency PCB) leads to the lack of relevant concepts in high frequency PCB. Many people still stay on the basis of "connecting electrical principles with conductors to play a predetermined role", and even think that "PCB design belongs to the consideration of structure, process and improving production efficiency". Many RF engineers do not fully realize that this link should be the special focus of the whole design work in RF design, and they mistakenly spend their energy on selecting high-performance components, which results in a sharp increase in cost but little improvement in performance.

In particular, the digital circuit relies on its strong anti-interference, detection and error correction, and can arbitrarily construct various intelligent links to ensure the normal function of the circuit. An ordinary digital application circuit with high additional configuration of various "ensure normal" links is obviously a measure without product concept. But often in the "not worth" link, but lead to a series of product problems. The reason is that this kind of functional link which is not worth constructing reliability guarantee from the perspective of product engineering should be based on the working mechanism of digital circuit itself, which is only the wrong structure in circuit design (including PCB design), which leads to the circuit in an unstable state. This kind of unstable state is a basic application under the same concept as the similar problem of high frequency PCB.

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

(1) Digital signal itself belongs to broad spectrum signal. According to the results of Fourier function, it contains rich high-frequency components, so the high-frequency components of digital signals are fully considered in the design of Digital IC. However, in addition to digital IC, if the signal transition area within and between each functional link is arbitrary, it will lead to a series of problems. Especially in the mixed application of digital, analog and high frequency circuits.

(2) All kinds of reliability design in the application of digital circuits are related to the reliability requirements and product engineering requirements of circuits in practical application, so it is impossible to add various high-cost "guarantee" parts to the circuits that meet the requirements by conventional design.

(3) The working rate of digital circuits is moving towards high frequency with unprecedented development (for example, the main frequency of CPU has reached 1.7GHz far beyond the lower limit of microwave band). Although the reliability guarantee function of related devices is also synchronized, it is based on the internal and typical external signal characteristics of the device.

For microwave level high frequency circuit, each corresponding stripline on PCB forms a microstrip line (asymmetric type) with the grounding plate. For PCB with more than two layers, it can form a microstrip line and a stripline (symmetrical microstrip transmission line). Different microstrip lines (double sided PCB) or stripline (multi-layer PCB) form coupling microstrip lines with each other, thus forming various complex four port network, thus forming various characteristics of microwave level circuit PCB.

It can be seen that the microstrip transmission line theory is the design basis of microwave high frequency circuit PCB.

For the rf-pcb design above 800MHz, the PCB network design near the antenna should fully follow the microstrip theory (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 microstrip theory.

For the lumped parameters and distributed parameters of the circuit, the lower the working frequency, the weaker the function characteristics of the distributed parameters, but the distributed parameters always exist. There is no clear demarcation line whether to consider the influence of distributed parameters on circuit characteristics. Therefore, the establishment of microstrip concept is also important for the PCB design of digital circuit and relative intermediate frequency circuit.

The basis and concept of microstrip theory and the design concept of microwave level RF circuit and PCB are actually an application aspect of microwave dual transmission line theory. For rf-pcb wiring, each adjacent signal line (including adjacent in different planes) has the characteristics of following the basic principle of two lines (for which, the following will be clearly described).

Although the common microwave RF circuit is equipped with a ground plate on one side, which makes the microwave signal transmission line on it tend to be a complex four port network, thus directly following the coupled microstrip theory, its foundation is still the two-wire theory. Therefore, in the design practice, the double line theory has more extensive guiding significance.

Generally speaking, for microwave circuits, microstrip theory has a quantitative guiding significance, which belongs to the specific application of two-line theory, while the two-wire theory has more extensive qualitative guiding significance.

It is worth mentioning that all the concepts given by the two-wire theory, on the surface, seem to have no connection with the actual design work (especially digital circuits and low-frequency circuits), but they are actually an illusion. The two wire theory can guide all the conceptual problems in electronic circuit design, especially in PCB circuit design.

Although the dual line theory is established on the premise of microwave high-frequency circuit, it is only because of the influence of distributed parameters in high-frequency circuit that the guiding significance is particularly prominent. In digital or medium and low frequency circuits, compared with lumped parameter components, distributed parameters can be ignored, and the concept of two-wire theory becomes fuzzy.

However, how to distinguish between high frequency circuit and low frequency circuit is often neglected in design practice. What kind of general digital logic or pulse circuits belong to? Obviously, the low-frequency circuit and medium low-frequency circuit with nonlinear components can easily reflect some high-frequency characteristics once some sensitive conditions change. The main frequency of CPU has reached 1.7GHz, which is far beyond the lower limit of microwave frequency, but it is still a digital circuit. Because of these uncertainties, PCB design is very important.

In many cases, the passive components in the circuit can be equivalent to the transmission line or microstrip line of specific specifications, and can be described by the double transmission line theory and its related parameters.

In a word, it can be considered that the double transmission line theory was born on the basis of synthesizing the characteristics of all electronic circuits. Therefore, strictly speaking, if the concept embodied in the dual transmission line theory is taken as the principle in every link of design practice, then the corresponding PCB circuit will face few problems (no matter what working conditions the circuit is applied to).