This article mainly focuses on the concepts and design principles of microwave-grade high-frequency circuits/microwave circuits and high-frequency PCB design in a cutting-edge field of communication products. The reason for choosing microwave-grade high-frequency circuits/microwave circuits and high-frequency PCB design principles is that this aspect of the principle has a wide range of guiding significance and is a high-tech hot application technology at the time. The transition from the microwave circuit high-frequency PCB design concept to the high-speed wireless network (including various access networks) project is also in the same vein, because they are based on the same basic principle: the double transmission line theory.

    Experienced high-frequency microwave RF engineers design digital circuits or relatively low-frequency circuit high-frequency PCB boards , and the first-time success rate is very high, because their design concept is centered on “distributed” parameters, and the destructive effect of the concept of distributed parameters in low-frequency circuits (including digital circuits) is often overlooked.

    For a long time, the design of electronic products (mainly for communication products) completed by many peers often has many problems. On the one hand, it is of course related to the lack of necessary links in electrical principle design (including redundancy design, reliability design, etc.), but more importantly, many of these problems arise when people think that all necessary links have been considered. In response to these problems, they often spend their energy on checking the program, electrical principle, parameter redundancy, etc., but rarely spend their energy on reviewing the high-frequency PCB board design. It is often because of the defects in the high-frequency PCB board design that a large number of product performance problems occur.

    The design principles of high-frequency PCBs involve many aspects, including basic principles, anti-interference, electromagnetic compatibility, safety protection, and so on. Regarding these aspects, especially in high-frequency circuits/microwave circuits (especially in microwave-level 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 “high-frequency PCB design belongs to the consideration of structure, process and improving production efficiency”. Many professional high-frequency microwave RF engineers have not fully realized that this link should be the key to the entire high-frequency PCB design work in RF design, and mistakenly spend their energy on selecting high-performance components, resulting in a significant increase in costs and minimal improvement in performance.

    It should be pointed out that digital circuits rely on their strong anti-interference, error detection and correction, and the ability to arbitrarily configure various intelligent links to ensure the normal function of the circuit. For a general digital application circuit, it is obviously a behavior without product concept to configure various “normal” links with high additionality. However, it often leads to a series of product problems in the links that are considered “unworthy”. The reason is that this kind of functional link that is not worthy of structural reliability assurance from the perspective of product engineering should be based on the working mechanism of the digital circuit itself, but the error structure in the circuit design (including high-frequency PCB board design) causes the circuit to be in an unstable state. The cause of this unstable state is the same as the similar problems of high-frequency circuits/microwave circuits.

    In digital circuits, there are three aspects that deserve serious attention:

    1. Various reliability designs in the application of digital circuits are related to the reliability requirements of the circuits in actual applications and the product engineering requirements. It is not possible to add various high-cost “guarantee” parts to the circuits that can fully meet the requirements using conventional designs.

    2. The operating speed of digital circuits is moving towards high frequency at an unprecedented rate (for example, the main frequency of the current CPU has reached 1.7GHz, far exceeding the lower limit of the microwave frequency band). Although the reliability assurance function of related devices is also matched synchronously, it is based on the internal and typical external signal characteristics of the device.

    3. Digital signals themselves belong to broad spectrum signals. According to the Fourier function results, they contain abundant high-frequency components, so the high-frequency weight of digital signals is fully considered in the design of digital ICs. However, in addition to digital ICs, if the signal transition areas within and between various functional links are arbitrary, a series of problems will occur. Especially in circuits where digital and analog and high-frequency circuits (high-frequency PCB boards) are mixed.

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