A filter is a frequency selective device that passes a specific frequency component of a signal and greatly attenuates other frequency components. With this frequency selection of the filter, interference noise or spectral analysis can be filtered out. In other words, any device or system that can pass a particular frequency component of a signal and greatly attenuate or suppress other frequency components is referred to as a filter. A filter is a device that filters waves. "Wave" is a very broad physical concept. In the field of electronic technology, "wave" is narrowly limited to the process of describing the fluctuations of various physical quantities over time. This process is converted into a time function of voltage or current through the action of various types of sensors, which is called a time waveform of various physical quantities, or a signal. Because the independent variable time is continuous, it is called continuous time signal, and it is customarily called analog signal.