Double transmission line theory for mICrowave circuit design and PCB wiring principle
（1） PCB Concept Based on Double Line Theory
For microwave grade high frequency circuits, each corresponding strip line on the PCB forms a microstrip line (asymmetrical) with the grounding plate. For PCB with more than two layers, they can form both a microstrip line and a strip line (symmetrical microstrip transmission line). Different microstrip lines (double-sided PCBs) or striplines (multilayer PCBs) form coupling microstrip lines with each other, thus forming various complex four port networks, thus forming various characteristics of microwave Circuit PCBs.
It can be seen that the microstrip transmission line theory is the basis for the design of microwave grade high-frequency circuit PCB.
■ For the RF-PCB design above 800MHz, the PCB 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 microstrip theory.
■ For the centralized and distributed parameters of the circuit, although the lower the operating frequency is, the weaker the action characteristics of the distributed parameters are, the distributed parameters always exist. There is no clear boundary 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 circuits and relative IF circuits.
■ The basis and concept of microstrip theory and the design concept of microwave RF circuit and PCB are actually an application aspect of the microwave dual transmission line theory. For RF-PCB wiring, each adjacent signal line (including adjacent on different sides) forms a feature that follows the basic principle of dual line (which will be clearly described later).
■ 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 a complex four port network, thus directly following the coupLED microstrip theory, its basis is still the two-wire theory. Therefore, in design practice, double line theory has more extensive guiding significance.
■ Generally speaking, for microwave circuits, the microstrip theory has a quantitative guiding significance, which belongs to the specific application of the two-wire theory, while the two-wire theory has a broader qualitative guiding significance.
■ It is worth mentioning that all the concepts given by the two-wire theory, on the surface, seem to have nothing to do with the actual design work (especially digital circuits and low-frequency circuits), which is actually an illusion. Double line theory can guide all the conceptual problems in electronic circuit design, especially in PCB circuit design.
Although the two-wire theory is established on the prEMIse of microwave high-frequency circuit, it is only because the influence of distributed parameters in high-frequency circuit becomes significant, which makes the guiding significance particularly prominent. In digital or low-frequency circuits, the distributed parameters can be ignored compared with the concentrated parameter components, and the concept of the two-wire theory becomes fuzzy.
However, how to distinguish between high frequency and low frequency circuits is often overlooked in design practice. What kind of digital logic or pulse circuits are usually? The most obvious low-frequency circuits and middle and low-frequency circuits with nonlinear components can easily reflect some high-frequency characteristics once some sensitive conditions change. The main frequency of high-end CPU has reached 1.7GHz, far exceeding the lower limit of microwave frequency, but it is still a digital circuit. Because of these uncertainties, PCB design is extremely important.
■ In many cases, the passive components in the circuit can be equivalent to the transmission line or microstrip line of a specific specification, and can be described by the double transmission line theory and its related parameters.
In a word, it can be considered that the theory of double transmission lines was born on the basis of integrating the characteristics of all electronic circuits. Therefore, strictly speaking, if every link in the design practice is based on the concept embodied in the double transmission line theory, then the corresponding PCB will face few problems (no matter under what working conditions the circuit is applied).