line is constant and referred to as the characteristic impedance. Trace width, spacing, length, and dielectric properties between the traces and the ground plane control the transmission line’s impedance. The characteristic impedance can be thought of as the resistance to energy transfer associated with wave propagation in a line much longer than the wavelength of the propagating signal. Signal reflections If a signal is propagated through a transmission line to a load with an impedance equal to the line’s characteristic impedance, the signal is fully delivered to the load. If the load impedance differs from the line’s characteristic impedance, then some of the energy incident on the load is reflected back toward the source. The ratio of the amplitude of the reflected voltage, V R , to the amplitude of the incident voltage, V I , is the reflection coefficient (Figure 4). It depends on the load impedance (Z L ) and the transmission line’s characteristic impedance (Z C ). Reflections result from a signal transitioning across a boundary where the media have unmatched impedances (Figure 5). At each interface, the
Figure 3: Transmission lines can be configured as either single-ended (unbalanced) using a signal and a return or ground conductor, or as differential (balanced) with two complementary signal conductors and a ground conductor. (Image source: Amphenol)
Figure 4: The reflection coefficient depends on the load and the transmission line's characteristic impedance. (Image source: Amphenol)
to the current at each point along the conductor. Transmission lines must control their impedance to carry high- speed/high-bandwidth signals without degradation due to reflections. Their instantaneous impedance at each point in the
Transmission line impedance Electrical impedance is a circuit's opposition to a current due to an applied alternating voltage, measured in ohms (Ω). Impedance is the complex ratio of the voltage
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