TABLE OF CONTENTS Transmission lines are commonly used to connect test and measurement instruments to circuits under test. Transmission lines are used to control the effects of inductance and capacitance which are unavoidable in high-frequency systems. Coaxial transmission lines are the most common and provide the best shielding of the signals being measured.
Measurement error can be introduced due to impedance mismatch at either end of a transmission line. These errors must be understood and minimized in order to ensure an accurate measurement.
When connecting DC circuits, the major concern is the resistance of the wires. According to Ohm's law, a drop in voltage will occur when a current flows through a wire with nonzero resistance. Inductance and capacitance are not a concern for strictly DC voltages and currents.
For circuits with AC voltages and currents, the inductance and capacitance of wires start to come into play. A typical wire exhibits self-inductance and has some capacitance to other nearby conductors. The higher the frequency, the more significant the effect of inductive and capacitive reactance. Uncontrolled, these reactive effects can distort signals by loading the driving circuit and causing reflections on the wire. Transmission lines are used to solve these problems by controlling the inevitable inductance and capacitance.
Signals do not travel down a wire infinitely fast. It takes a finite amount of time for a signal to propagate from one place to another. For circuits and systems that have short connections (relative to the wavelength of interest),...
