A filter is a frequency-selective device that allows certain frequency components in a signal to pass while greatly attenuating other frequency components. Using this frequency selection function of the filter, interference noise can be filtered out or spectrum analysis can be performed. In other words, any device or system that can pass a specific frequency component in a signal and greatly attenuate or suppress other frequency components is called 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 describe the process of changing the value of various physical quantities over time. Through the action of various sensors, this process is converted into a time function of voltage or current, which is called the time waveform of various physical quantities, or a signal. Because the independent variable time is a continuous value, it is called a continuous time signal, and it is also habitually called an analog signal (Analog Signal).