Notch filters, also known as Band-Reject Filters, are used to filter out a single frequency out of a frequency range. These are often used to filter out the 60hz power hum in stereos.
A very simple notch filter uses a tuned LC circuit. When the frequency to be filtered enters the circuit, the LC portion is in resonance, so all current and voltage goes to ground.
The notch frequency of this circuit is 80hz.
