RF and Microwave Engineering: Fundamentals of Wireless Communications

3.4 Summary

In Section 3.1 we provided a description of transmission line effects for harmonic signals. On a transmission line, voltage and current waves can propagate in opposite directions. These waves determine the electric behaviour at the input and load terminals. In order to describe a transmission line as a circuit component we need the three transmission line parameters: length of line _t, characteristic impedance Z₀ and propagation constant γ. The characteristic impedance Z₀ is defined as a ratio between voltage and current on a forward propagating wave. In the case of a loss-less line the characteristic impedance is real and voltage and current are in-phase. For a low-loss line this is still a good approximation. The propagation constant γ = α + jβ is a complex valued with a real part (attenuation constant) and an imaginary part (phase constant). The attenuation constant α describes the exponential decay of a wave that travels along the line. The phase constant β depends on the wavelength and is—for a given frequency—a measure for the propagation velocity c.

In Section 3.2 and 3.3 we looked at transient signals on lines. In order to avoid reflections and bouncing signals transmission lines have to be matched at their terminals. The eye diagram represents a valuable tool to access the quality of digital signals that are propagated along a non-ideal transmission path. With ...