TABLE OF CONTENTS 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:
Low-pass filters: These filters reject all frequencies above a specified value.
High-pass filters: These filters reject all frequencies below a specified value.
Band-pass filters: A band-pass filter passes or allows transmission of a band of frequencies and rejects all frequencies beyond this band.
Band-stopfilters: A band-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.
The following parameters characterize a typical filter:
Characteristic impedance Z 0: This should be chosen so that the filter may fit into a given line or between two pieces of equipment.
Pass band: Filter should have very low (ideally zero) attenuation in the pass band and high attenuation in the stop band or attenuation band.
Stop band: A band in which ideal filters have to stop frequencies.
Cut-off frequency: This frequency demarcates or separates the pass band and the stop band.
The attenuation of a filter can be expressed in decibels ( d B) or Nepers.
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