Filters and Attenuators and their Applications


As the name suggests, a filter is a reactive network that freely passes the desired bands of frequencies while almost totally suppressing all other bands. Thus, a filter can extract desirable frequencies from signals that also contain undesirable or irrelevant frequencies. Filters are essential building blocks of any Electronic and Communication system. It is an important component in the frequency domain analysis of a signal.

  • It provides substantially constant transmission over the band, which they accept.
  • Ideally, the Filter amends the relative amplitudes of the various frequency components and the phase characteristics, and its ‘Gain’ depends fully on the signal frequency.
  • The frequency that separates the transmission band or passband and the attenuation band or stopband is called the FC’s cut-off frequency(FC).
  • In a practical case, the higher is the order of the filter, the higher is its ability to distinct passband and stopband.
Fig 1: Use of Filters in video and image processing


Did you know the Karaoke technology used in many music competitions uses Filters as one of their building blocks??

  • Semiconductor Industry: Severe harmonic pollution will inevitably affect the efficiency and longevity of equipment in power distribution systems. Their influence on the output is curtailed with the help of active filters.
  • Petrochemical Industry: Variable-frequency drives find their use in this field and are the reason behind harmonics in the distribution system. The use of active filters suppresses the harmonics and their impact.
  • Automotive Manufacturing: Huge welding machines used in this field create serious problems of power quality. Here, again filters come to the rescue.
  • Hospital System: The hospital has very strict requirements for the continuity and reliability of the power supply. Degradation in the quality of power can result in an inaccurate display of human vitals.
  • Speech Signal Processing: This is one of the earliest fields of application of digital filters.
  • Image Processing: Digital filtering technology is widely operated in the recovery and enhancement of still images and moving images, data compression, noise and interference, image recognition, and tomography.
  • TV, Radar: digital filters and related technologies are important foundations for video compression and audio compression technologies. Digital signal filtering technology is indispensable for signal generation, filtering, processing, estimation of target parameters, and target imaging display.
  • Music: The digital filtering technology has shown great power in editing, synthesizing, and adding special effects such as reverberation and chorus in music. Digital filters can also be used to compose, record, and playback or restore old tapes’ sound quality.
  • Airport: In the low-voltage power distribution system of the airport, there are a large number of harmonic sources, such as airport navigation lights, DC motors, electric furnaces, rolling mills, electric welders, etc. The mainstream technologies used in airports to reduce the effect of harmonics are passive filtering technology and active filtering technology.
  • Signal Processing: Few of the many roles filters play in signal processing are System identification or system modeling, noise canceller, an equalizer for inter-symbol interference, etc.
  • Smartphones: These work in multiple wireless bands simultaneously, including mobile communications, Bluetooth, Wi-Fi, and GPS, to make mobile phones work in more frequency bands and regions. Mobile phones are increasingly demanding RF filtering.
Fig 2: Filtration of noises in signal processing

Filter Classification:

Passive filters can be further classified into following

Fig 3: Circuit of passive low pass filter and its frequency response.
Fig 4: Circuit of High pass filter and its frequency response.
Figure 5: Circuit of Band Pass Filter
Fig 6: Circuit of Band stop filter.
Figure 8: Circuit of All Pass Filter or Phase shift Filter.

Order of the Filter:

Ideally, filters are supposed to strictly allow a certain range of frequency and precisely cut off others. But this is not the case actually, limitations of the real-world elements being the reason. Fig 6. Shows this difference.

Fig 9: frequency response of various filters in ideal case(graph lines in blue) and practical case(graph lines in red)
Fig 10: frequency responses with various orders of a simple low pass filter.


Attenuators are simple passive two-port electronic devices used for attenuation to reduce the strength of signals without causing disturbance to their waveform. They are used, for example, at the input of an electronic instrument to minimize a voltage or current to a value that can be handled by the device. Attenuation is expressed either in decibels (dB) or in nepers.


Attenuators are used in many fields. A few of the applications are mentioned below.

  • In some digital or analog circuits, a surge in voltage may damage the courses, which can be avoided using attenuators to reduce high voltages.
  • It is important to obtain a proper match for transmitter and receiver in fiber-optic communication received using optical attenuators.
  • The microwave power can be controlled using PIN diode attenuators; PIN diode is used as a control element in most electronically variable attenuators.
  • Radiofrequency (RF) attenuators act as resistors and are used to reduce a signal’s level within radio frequency applications and RF circuits.
  • Variable attenuators are used in laboratories when it is necessary to attain a small value of voltage or current for testing purposes.
  • Resistive attenuators can also be used for matching between circuits of different resistive impedances.
  • Fixed attenuators are used to improve the impedance matching in circuits. These are used to protect the circuits from damages caused by high voltages.

Types of Attenuators:

On a broad basis, they are of 2 types symmetrical and asymmetrical. Unsymmetrical being unbalanced with T-, Pi-, L — a kind of configuration. Whereas symmetrical ones are considered to be balanced and available in H-, O-type of arrangement. H- and O- designs are balanced versions of T- and Pi- configurations, respectively.

Fig 11:Various configurations of attenuators
Fig. 12: Various types of attenuators
Fig. 13: Digital Variable attenuator and mechanically operated Variable attenuator.
Fig. 14: RF Step attenuator.
Fig. 15: RF Variable attenuator.
Fig. 16: Fibre Optic attenuator with adaptor.



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