In everyday parlance a ‘filter’ is a device that removes some component from whatever is passed through it. A drinking-water filter removes salts and bacteria; a coffee filter removes coffee grinds; an air filter removes pollutants and dust. In electronics the word ‘filter’ evokes thoughts of a system that removes components of the input signal based on frequency. A notch filter may be employed to remove a narrow-band tone from a received transmission; a noise filter may remove high-frequency hi