Issue link: https://iconnect007.uberflip.com/i/1467185
54 DESIGN007 MAGAZINE I MAY 2022 digital data rates with pico-sec- ond rise times, the printed cir- cuit board ceased to be a passive interconnecting substrate and became a complex component in its own right. A PCB trace behaved like a transmission line when its physical length was of the same order as the electrical wavelength of the transmitted signal. He examined the properties of a transmission line: in addi- tion to its DC resistance it expe- rienced a propagation delay depending on the dielectric characteristics of the material sur- rounding it, which also contributed to losses as well as those attributed to copper roughness and line length. e transmission line had a characteristic impedance determined by trace width, dielectric spacing, and relative permit- tivity. All of these parameters needed to be considered in order to ensure signal integrity, which became increasingly difficult to main- tain as frequency increased. PCB material selection and stack-up, trace width, dielectric separation, design and layout, and component placement were all critical. Reischer explained how a transmission line could be considered in terms of a series of inductors and capacitors and described how a signal was propagated along it, charging each successive capacitor element. e goal was to achieve constant impedance across the entire length of the line, with a uniform voltage and current wave-front in order to ensure good sig- nal integrity. e propagation speed of an elec- tromagnetic wave was a function of the speed of light and the permittivity of the dielectric, which for air or vacuum was 1.0. In typical PCB materials with relative permittivity around 4.0, the propagation speed was reduced from the speed of light to about half that value. e "critical line length," beyond which a conductor would be considered a transmission line, was related to the rise time of the signal. Early semiconductor devices had rise times of around 5 nano- seconds, giving a bandwidth of 70 MHz and a critical length of 36 cm. Current gallium arse- nide devices had rise times of 0.3 nanoseconds, giving band- width of 1.166 GHz and a critical length of 2 cm. He clarified the differences in propagation characteristics between low-speed and high- speed signals, and he explained the ways in which electromag- netic waves were reflected at impedance dis- continuities. Impedance matching offered a solution, and he gave an example of a typical transmission system where the impedances of the source, the transmission line, and the ter- mination were matched at 50 ohms. is was generic in a radio frequency environment; 75 ohms was typical in video applications, 90 ohms in USB transmission, and 100 ohms for Ethernet. Reischer commented that his system exam- ple illustrated the essential interaction between the three companies contributing to the webi- nar: Phoenix Contact supplying interconnects, NCAB Group supplying printed circuit boards, Anna Brockman