An externally mounted low-pass or band-reject filter reduces electronic noise from finding its way into the power supply or other inputs. The AC input line filter consists of an electronic circuit connected between the house current and the rectifier input stage of the switching power supply. It is designed to block spikes, interference and static from finding its way into sensitive electronic equipment. The most common source of these kinds of issues is from ElectroMagnetic Interference (EMI) because they occur in the frequencies occupied by mobile phones, FM radios, televisions and VCRs. These can be generated by power tools, power lines, automobile ignitions, TV and radio towers and pinball machines, to name a few. Two common types of filters are shown below. The one on the left is for filtering the AC line source and the other is used for various types of low voltage cables.