13.1 INTRODUCTION

An electric wave filter, or simply a filter, is an electric network which passes
or allows unattenuated transmission of an electric signal within a certain
frequency range, and stops transmission of electric signals outside this range.

Filters may be classified according to their uses:

(a) Low-pass filters: These filters reject all frequencies above a specified
value.
(b) High-pass filters: These filters reject all frequencies below a specified
value.
(c) Band-pass filters: A band-pass filter passes or allows transmission of a
band of frequencies and rejects all frequencies beyond this band.
(d) Band-stop filters: Aband-stop or elimination filter rejects transmission
of a limited band of frequencies but allows transmission of all other
frequencies.

An ideal filter offers zero attenuation in the pass band, and infinite
attenuation in the stop band.