In digital circuits, there are three areas worth taking seriously

This paper mainly focuses on the concept and design principle of microwave high frequency circuit and PCB design, a frontier category of communication products. The PCB design principle of microwave-grade high frequency circuit is chosen because it has extensive guiding significance and belongs to the current hot application technology of high technology. The transition from microwave circuit PCB design concept to high-speed wireless network (including all kinds of access network) engineering is also in the same thread, because they are based on the same basic principle - dual transmission line theory.

Experienced RF engineers design digital circuits or relatively low frequency circuit PCB, the success rate is very high, because their design concept is based on the "distributed" parameter as the core, and the distributed parameter concept in lower frequency circuits (including digital circuits) the destructive effect is often ignored.

For a long time, many peers completed the design of electronic products (mainly for communication products), often fraught with problems. On the one hand, it is related to the lack of necessary elements in electrical design (including redundancy design, reliability design, etc.), but more importantly, many of these problems occur when people think that the necessary elements have been taken into account. To solve these problems, they often spend their energy on the verification of procedures, electrical principles, parameter redundancy and so on, but seldom spend their energy on the audit of PCB design, and it is often because of PCB design defects, leading to a large number of product performance problems.

Design concept and principle of microwave high frequency circuit PCB

PCB design principles involve many aspects, including the basic principles, anti-interference, electromagnetic compatibility, safety protection, and so on. For these aspects, especially in the high-frequency circuit (especially in the microwave level high-frequency circuit), the lack of relevant ideas, often leads to the failure of the entire research and development project. 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 scope of structural, technological and production efficiency considerations." Many professional RF engineers do not fully realize that this link should be the special focus of the whole design work in RF design, and mistakenly spend their energy on the selection of high-performance components, resulting in a large increase in cost, but little improvement in performance.

It is especially mentioned here that the digital circuit relies on its strong anti-interference, error detection and correction, and can be arbitrarily constructed various intelligent links to ensure the normal function of the circuit. A common digital application circuit with high add-on configuration of various "ensure normal" elements is clearly a product-free initiative. But often in the "not worth it" link, but lead to a series of product problems. The reason is that such functional links, which are not worth constructing reliability assurance from the perspective of product engineering, should be built on the working mechanism of digital circuit itself, but the wrong construction in circuit design (including PCB design) leads to an unstable state of the circuit. The cause of this unstable state belongs to the basic application of the same concept as the similar problem of high frequency circuits.

In digital circuits, there are three areas worth taking seriously:

(1) Digital signal itself belongs to broad spectrum signal. According to the Fourier function results, it contains very 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, the signal transition area within and between each functional link, if arbitrary, will lead to a series of problems. Especially in digital and analog and high - frequency circuit applications.

(2) All kinds of reliability design in digital circuit application is related to the reliability requirements of the circuit in practical application and product engineering requirements, so it is not possible to add various kinds of high-cost "guarantee" parts to the circuit that can fully meet the requirements by conventional design.

(3) The working speed of digital circuits is moving towards high frequency with unprecedented development (for example, the main frequency of the current CPU has reached more than 2GHz, far beyond the lower limit of microwave frequency). Although the reliability assurance function of the related devices is also synchronized, it is based on the internal and typical external signal characteristics of the devices.