The principle of impedance matching

Impedance is an umbrella term that includes resistance and both inductive and capacitive reactance. The two reactances are frequency dependent and pull in opposing directions, so to speak. If equal, they cancel out. If unequal, the remainder must be vectorially combined with “straight” resistance to obtain impedance. This quantity, like resistance, is measured in ohms and conforms to Ohm’s law, although it varies with frequency.

Impedance is a property of any load and also of any power source, such as a battery or generator. A power source has a definite internal impedance, which in an electrical circuit acts like a device external to and in series with the power source.

A large source such as utility power, measured at the output of a substation or even at a pole-mounted distribution transformer, has extremely low impedance. It is known as a stiff source because any load likely to be connected won’t reduce it by an amount that’s measurable. Theoretically, when you switch on a night light in the home, you are slowing down every turbine in the North American Grid, but the effect is infinitesimal.

Power sources and loads have definite impedances, and maximum power transfer takes place when the impedances of source and load match. That is to say, when they are equal. There is nothing mysterious about this. It is implicit in the Ohm’s law wheel. If a relatively high-impedance load connects to a low-impedance power source, the current through both will be limited; actually, this can be a good thing. For example, the AC mains typically has an impedance well under 1 Ω. A test meter with 1 kΩ input impedance would draw about 100 mA from a 110-Vac mains. That’s rather a lot. A test meter of 1 MΩ input impedance would draw about 100 μA.

Similarly, if a low-impedance load connects to a high-impedance power source, the voltage at the load will be low and the current through both, as well as the power transfer from source to load, will be low. As mentioned before, however, if impedance in both the load and source are the same, power transfer will be at the maximum possible level.

Another area in which impedance matching assumes great importance is in data transmission. Source and destination generally are configured so their impedances are identical, unless they are intentionally mismatched for the purpose of attenuation. Unlike a power circuit, where the ideal transmission line impedance is zero so the line is invisible in the circuit, in data transfer the media (cabling) is the load from the point of view of the source. And it is the source from the point of view of the load. Accordingly, the transmission line must have an impedance that matches source and load.

You might think this matching would be difficult because different installations require widely differing lengths of cabling. But through the miracle of characteristic impedance, data cable such as coax and unshielded twisted pair (UTP) is manufactured to have standardized impedances that do not vary regardless of length.

If there is an impedance mismatch, the signal will reflect back along the cable from the load. The result can be data collisions, standing waves, loss and corruption. Careful design and installation practices can prevent such difficulties. This is equally relevant in long cable spans and, at high frequency, short circuit board traces.