The circuit shown in Fig. 15.13 is a notch filter. At its resonant frequency, the L-C series circuit has zero effective impedance and, as a result, any signal at that frequency is short-circuited. For this reason, the filter is often referred to as a trap.
Consider the following scenario. A system operating at 1 kHz has picked up noise at a fundamental frequency of 10 kHz, as well as some second- and third-harmonic junk. Given this information, we wish to design a filter that will eliminate both the noise and its attendant harmonics.