Learn the fundamentals of signal integrity
The dielectric constant (Dk) and dissipation factor (Df) are significant characteristics of dielectrics that impact transmission lines. The Dk determines the signal propagation speed on the line. For example, a material with a lower Dk has a higher propagation velocity. The Df represents the energy loss within the material as the signal travels down the transmission line. A lower Df indicates less signal attenuation, especially at high frequencies. Common dielectrics are air and various plastics. A typical printed circuit board (pc board) substrate is a dielectric called flame retardant 4 (FR-4), a composite of woven fiberglass cloth impregnated with flame- retardant epoxy resin. Standard transmission line configurations are coaxial cable, twisted pair, pc board stripline, and pc board microstrip. The two conductors are identified as the signal and return paths. The voltage on a transmission line is measured between the conductors along the line, and the current is measured through either of the conductors. In SI, a transmission line is a distributed electrical component that carries transverse electromagnetic (TEM) or quasi-TEM waves between two conductors. These waves
Figure 2: Transmission lines propagate energy along the line using alternating orthogonal electric and magnetic fields. (Image source: Amphenol)
contain alternating electric (E) and magnetic (H) fields that are perpendicular to the wave’s direction of travel (Figure 2). A changing electric field creates a changing magnetic field as an alternating series of transformations, propagating the TEM wave along the transmission line in a direction perpendicular to both fields. Transmission line connections between circuit elements are configured as single-ended or differential connections (Figure 3).
A single-ended configuration uses a signal line and a ground line. The signals are not identical, and the configuration is considered an unbalanced propagation mode. A differential configuration uses two complementary signal lines and a ground line, generally run separately. Differential signals are an example of a balanced propagation mode because the signal of interest is the mathematical difference between the two signal elements.
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