In the previous sections we only looked at ideal inductors. However, every inductors has a certain series resistance. This is not only determined by the resistance of the wire, but also by the kind of material the wire has been wrapped around. This resistance looks very much like the ESR of a capacitor. But with inductors we don't talk about ESR, but about the quality factor Q, which is defined as:

**Q =
X _{L}/r_{L}**

where r_{L} is the series resistance.
X_{L} and r_{L} are both
frequency-dependant, but not to the same extent. Q is therefore also
frequency-dependant.

Assume that the series resistance of the inductor in the previous section is 20 ohms at 159kHz. The quality factor at 159kHz will then be 1k/20 = 50.

The Q plays an important role in determining the notch depth and the bandwidth of LC filters. This will be discussed in a next lesson. This lesson will also teach us how to measure the Q without expensive equipment.