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Chapter 13: Filters

By M. Arshad
From Network Analysis & Circuits

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:

  1. Low-pass filters: These filters reject all frequencies above a specified value.

  2. High-pass filters: These filters reject all frequencies below a specified value.

  3. Band-pass filters: A band-pass filter passes or allows transmission of a band of frequencies and rejects all frequencies beyond this band.

  4. 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.

13.2 PARAMETERS OF A FILTER

The following parameters characterize a typical filter:

  1. Characteristic impedance Z 0: This should be chosen so that the filter may fit into a given line or between two pieces of equipment.

  2. Pass band: Filter should have very low (ideally zero) attenuation in the pass band and high attenuation in the stop band or attenuation band.

  3. Stop band: A band in which ideal filters have to stop frequencies.

  4. Cut-off frequency: This frequency demarcates or separates the pass band and the stop band.

13.3 DECIBEL AND NEPER

The attenuation of a filter can be expressed in decibels ( d B) or Nepers.

13.3.1 Bel

A Bel

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Active Filters
Active filters are electronic filters that use active components such as voltage amplifiers or operational amplifiers. The amplifier shapes and stabilizes the behavior of the filter, and serves as a buffer interface to the driven component.
RF Filters and Microwave Filters
RF filters and microwave filters are devices that pass or reject signals by frequency. Basic types include bandpass filters, band reject filters, low pass filters, and high pass filters.
Board Mount RF Filters
Board mount RF filters are devices mounted to a printed circuit board (PCB) which pass or reject signals by frequency.
Passive Filters
Passive filters are implemented using only passive components such as resistors, capacitors and inductors. These filters do not produce any amplification of the input signal.
RF Band Pass Filters
RF band pass filters are active or passive circuits that pass signals from a specific frequency band and reject signals from out-of-band frequencies. Surface acoustic wave (SAW) filters, crystal filters, and cavity filters are types of band pass filters.
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