Impedance Matching and Unmatched Impedances

Impedance Matching

Since a transmission line has impedance built in, the natural question to ask is, how does the impedance affect signals that are relayed through a transmission line from one device to another? The answer to this question ultimately depends on the impedance of the devices to which the transmission line is attached. If the impedance of the transmission line is not the same as the impedance of, say, a load connected to it, the signals propagating through the line will only be partially absorbed by the load. The rest of the signal will be reflected back in the direction it came. Reflected signals are generally bad things in electronics. They represent an inefficient power transfer between two electrical devices. How do you get rid of the reflections? You apply a technique called impedance matching. The goal of impedance matching is to make the impedance of two devices that are to be joined equal. The impedance-matching techniques make use of special matching networks that are inserted between the devices.

Unmatched Impedance

A high-impedance transmission line that is connected to a low-impedance load is, analogous to a high-density rope connected to a low-density rope. If you impart a pulse at the left end of the high-density rope (analogous to sending an electrical signal through a line to a load), the pulse will travel along the rope without problems until it reaches the low-density rope (load). At that time, the pulse will induce a longer-wavelength pulse within the low-density rope and will induce a similar wavelength but inverted and diminished pulse that rebounds back toward the left end of the high-density rope. From this analogy, again you can see that only part of the signal energy from the high-density rope is transmitted to the low-density rope.

Techniques for Matching Impedance

This section looks at a few impedance-matching techniques. As a rule of thumb, with most low-frequency applications where the signal’s wavelength is much larger than the cable length, there is no need to match line impedance. Matching impedance is usually reserved for high-frequency applications. Moreover, most electrical equipment, such as oscilloscopes, video equipment, etc., has input and output impedance that match the characteristic impedance of coaxial cables (typically 50 Ω). Other devices, such as television antenna inputs, have characteristic input impedance that match the characteristic impedance of twin-lead cables (300 Ω). In such cases, the impedance matching is already taken care of. A short length of transmission line that is open ended or short-circuit terminated possesses the property of having an impedance that is reactive. By properly choosing a segment of open-circuit or short-circuit line and placing it in shunt with the original transmission line at an appropriate position along the line, standing waves can be eliminated. The short segment of wire is referred to as a stub.