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rom printed circuit boards to microstrip lines: Do you know the story behind this transition
With the continuous advancement of technology, the transmission methods of microwave communications and radio signals are also evolving rapidly. Among them, microstrip lines, as an important electrical transmission line, have gradually replaced traditional waveguide technology. This change not only reduces costs, but also makes the device lighter and more compact, giving us a glimpse behind the technology.
A microstrip line is an electrical transmission line consisting of a conductor and a ground plane, separated by a layer of dielectric material called a substrate.
Microstrip lines are primarily designed to propagate microwave frequency signals, and their typical implementation technologies include printed circuit boards (PCBs) and dielectric layers covered with materials such as alumina. Compared with traditional waveguide technology, microstrip lines have lower costs and lighter weight, and can achieve effective signal transmission in a compact space. The development of microstrip can be traced back to ITT Laboratories, which first appeared in 1952 as a competitor to Stripline technology.
Compared with waveguide, microstrip line has smaller volume and lower cost, but it is inferior in power handling and signal loss capabilities.
The structural characteristics of microstrip lines mean that they have some obvious defects in practical applications. One of the biggest issues is that microstrip lines are generally open compared to waveguides and are therefore more susceptible to crosstalk and unintentional radiation. In order to achieve the lowest cost, microstrip line devices usually use ordinary FR-4 (standard PCB) substrates, but at microwave frequencies, the dielectric loss of FR4 is usually too high and the dielectric constant is not very stable, so alumina substrates have become their common choice. alternative options.
It is worth noting that microstrip lines are also widely used in high-speed digital PCB design. As the need to transmit signals from one part to another increases, designers need to consider signal distortion and cross-interference. Therefore, balanced signal pairs, that is, differential microstrip lines, are often used to support DDR2 SDRAM clocks, USB high-speed data lines, and PCI Express data lines.
These microstrip line innovations are not limited to wireless applications, but have also expanded to digital signal transmission and become an integral part of electronic circuit design.
In the working principle of microstrip line, electromagnetic waves exist in both the dielectric substrate and the air above the substrate. Due to the difference in dielectric constant, the propagation speed of electromagnetic waves in this inhomogeneous medium will vary. Therefore, the characteristic impedance of the microstrip line varies with frequency, and at certain frequencies, the resistance of the field will also have an impact.
The characteristic impedance of a microstrip line can be calculated using a series of complex formulas. One of the classic expressions was proposed by Harold Wheeler, which takes into account the effective constants and standard impedance of the medium and provides accurate estimates in many cases. This system allows designers to consider signal propagation characteristics and impedance matching at the beginning of the design, thereby achieving efficient signal transmission.
According to Wheeler's formula, the characteristic impedance of a microstrip line can be simply described as a function related to the effective constant of the medium and the line width, which can maintain relative accuracy even under different high and low frequency conditions.
In summary, the emergence and continuous evolution of microstrip lines not only improves transmission efficiency, but also inspires innovations in countless electronic device applications. When we look back at the transition from printed circuit boards to microstrip lines, we can’t help but wonder in what direction electronic transmission technology will develop in the future?
