Filters are frequency-selective devices. They pass signals of desired frequencies and block others. They are necessary in order to select the desired signal from the range of signals transmitted, and also to attenuate unwanted noise and interference. Filters are classified by the general shape of the ampitude-frequency response into low-pass filters, high pass filters. bandpass filters and bandstop filters.

   In the diagram below we are using a bandpass filter which passes a range of frequencies within its passband limits and attenuates all other frequencies outside its passband which is defined by a lower cut-off frequency limit and an upper cut-off frequency limit