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 by 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 by the line will only be slightly 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 strength move 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, similar 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 (similar to sending an electrical signal by 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 generate a longer-wavelength pulse within the low-density rope and will generate a similar wavelength but inverted and reduced 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. additionally, 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-rule cables (300 Ω). In such situations, the impedance matching is already taken care of. A short length of transmission line that is open ended or short-circuit terminated possesses the character of having an impedance that is reactive. By properly choosing a part 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 groups can be deleted. The short part of wire is referred to as a stub.

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