Filters and Attenuators and their Applications

Filters

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 Systems. It is an important component in the frequency domain analysis of a signal. 

Some prominent features are:

• A filter is built from purely reactive elements.
• It provides substantially constant transmission over the band, which they accept.
• Ideally, 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 stop band 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 pass band and stop band.

Applications

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

• Communications Industry: Use of UPSs in any communication system has to lead to an increment in the noise in these communication channels. The use of Active filters reduces the impact of noises and helps attain stability.
• 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 its use in this field and 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 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.

Based on the components used, Filters are of two types. Active filters use active components like Transistors, Op Amps along with resistors, etc., and Passive filters, which uses components lie resistors, inductors, capacitors.

Filter Classification:

Passive filters can be further classified in to following

1. Low Pass Filter
2. High Pass Filter
3. Band Pass Filter
4. Band Stop Filter
5. All Pass Filter

1. Low Pass Filter: These filters are designed to pass a lower band of frequencies while blocking a higher frequency band. These allow passage of all the frequencies ranging from 0 to FC and attenuate the rest.

   

   Fig 3: Circuit of passive low pass filter and its frequency response.

2. High Pass Filter: These filters are designed to pass a higher band of frequencies while blocking lower frequency bands. These allow passage of all the frequencies above FC and attenuate the ones below It.

   

   Fig 4: Circuit of High pass filter and its frequency response.

3. Band Pass Filter: These filters are designed to allow a certain range of frequencies and attenuate the rest out of this range. Usually, there exists a lower cut off frequency or fc1, and a higher amount off frequency  FC2, FC2 - FC1  is the pass band here.

   

   Figure 5: Circuit of Band Pass Filter

4. Band Stop Filter: Also called Notch Filter or simply Notch. It rejects (blocks) only a certain set of frequencies. In general, this frequency set lies in between the low-frequency range and the high-frequency range. That means this filter allows both low and high-frequency components except for a particular field.

   

   Fig 6: Circuit of Band stop filter.

5. All Pass Filter: This type of Filter passes all frequencies alike. They are also called Phase-Shift filter, time-delay filter as the output voltage shifts in phase concerning input voltage without a change in magnitude.

   

   Figure 8: Circuit of All Pass Filter or Phase shift Filter.

Any of the above filter when accurately connected with an Opamp becomes active 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)

The use of higher-order filters can reduce this difference between ideal and practical filters. Higher is the order of the filter; the more complex is its circuit, the closer is its frequency response to the perfect filter.

Attenuators

Attenuators are simple passive two-port electronic devices used for attenuation to reduce the strength of signals without causing disturbance to its 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.

An Attenuator is well the opposite of an amplifier, though the two work by different methods. In an amplifier, the gain is usually much higher than 1, while an attenuator provides loss or gain less than 1.

Applications:

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

• To control the volume of electronic equipment such as speakers, electric guitars, etc., attenuators are used.
• 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.

Attenuators can be classified into 3 basic kinds:

1. Fixed attenuator
2. Variable attenuator
3. Step attenuator

1. Fixed Attenuator:  The function is quite clear by the name. These attenuators are available in fixed values, attenuating the specified magnitude of the power. These are available in a surface mount, waveguide, or connectorized packages. By connecting such boxes or more precisely, Pads in series desired amount of attenuation can be obtained. These can be unidirectional or bidirectional based on their application requirement.

   These attenuators typically consist of a resistive network that allows heat to dissipate at a certain rate.

   Resistance is developed for a chip-based attenuator by the deposition of various materials onto a thermally conductive substrate. Depending on the process—thick film or thin film—the physical dimensions and the material used will all yield the desired resistance value.

   

   Fig. 12: Various types of attenuators

2. Variable Attenuator: can provide a different attenuation level as there is an option to regulate and get the required attenuation level. This can be achieved by varying the input voltage applied to the control line. A variable attenuator operates through its entire dynamic range, in contrast with a generally restricted switch to one of two states – ‘on’ or ‘off.’
   Mechanically variable attenuators are generally adjusted with a tuning screw or control knob. The necessity for mechanical adjustment means that such variable attenuators are usually unsuited to system requirements. However, electronically controlled variable attenuators are available in both current-controlled and voltage-controlled versions. These are used in many systems, test and lab applications. 

   

   Fig. 13: Digital Variable attenuator and mechanically operated Variable attenuator.

3. Step Attenuator: are primarily fixed attenuators, as they are still passive components that comprise various resistive networks to generate a particular attenuation. The attenuation value can be selected based upon a manual push-button or the spin of a rotary switch. Step attenuators, unlike variable attenuators, can only generate an attenuation value based on pre-allocated steps. For example, a push-button step attenuator can go from 0 to 45.5 dB and, depending on the buttons' arrangement, can be increased in increments of 0.5 dB. These could be digitally operated or even programmable along with mechanical versions.

   

   Fig. 14: RF Step attenuator.

Based on the application, the Attenuators have quite a range of RF attenuators and Optical attenuators.

1. RF Attenuators:  These have many uses like reducing high power signals for test equipment, level control, impedance matching. Available in fixed, switched, or even continuously variable. This attenuation may be required to protect a circuit part from receiving a signal level that is too high.
   Also, an attenuator may be used to provide an accurate impedance match as most fixed attenuators offer a well-defined impedance, or attenuators may be used in various areas of signal level control.

   

   Fig. 15: RF Variable attenuator.

2. Optical Attenuators:  Fibre optic attenuator, also called optical attenuator, is a passive device used to reduce an optical signal's power level. Because too much light can make a fibre optic receiver saturated, the light power must be reduced using a fibre optic attenuator to achieve the best fibre optic system performance.

   They usually work on Optics' various principles, including the gap-loss principle, absorptive principle, or reflective principle.

   Fixed Optical Attenuator, Optical Variable Attenuator, Single Mode and Multimode Fibre Optic Attenuator are a few of the types commonly in use. 

   

   Fig. 16: Fibre Optic attenuator with adaptor.

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