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Classification of Electronic Filters

Author: Apogeeweb
Date: 2 Aug 2019
active and passive filters


The electronic filter is an arrangement of electronic components used in a circuit to transmit signals within a given frequency range, rejecting others. Electronic filters are used in radios, television receivers, telephone systems, etc.

Electronic Filters: Types of Electronic Filters



Ⅰ Types of Electronic Filters

Ⅱ Common Electronic Filters

2.1 Passive Filters

2.2 Active Filters

2.3  Digital Filters

2.4 Low Pass Filters

2.5 Switched Capacitor Filter (SCF)

Ⅲ Several New Digital Filters

3.1 Self-adaption DF 

3.2 Complex DF

3.3 Multidimensional DF

Ⅳ Other New-type Filters

4.1 Electronically Programmed CCD Transversal Filter

4.2 Crystal Filter

4.3 SAW Filters

4.4 Dielectric Filters

4.5 Active Power Filters

Ⅰ Types of Electronic Filters

(1) According to the processing signal type, it can be divided into analog filter and discrete filter. The analog filter can be divided into three categories: active, passive, and heterogeneous; discrete filters can be divided into three categories: digital, sampling, and hybrid. Of course, each category can continue to be divided. In short, their classifications can form a tree structure. In fact, some filters are difficult to belong to because of their cross-functional characteristics. For example, switched-capacitor filters can be either sampling analog filters, hybrid filters, or active filters. 

(2) According to the physical quantities selection, it can be divided into four types, such as frequency selection, amplitude selection, time selection (for example, channel signals in PCM system), and information selection (as a matched filter).

(3) According to the frequency passband range, it can be divided into five categories: low-pass, high-pass, band-pass, band-stop, and all-pass filter. In addition, the comb filter belongs to band-pass and band-stop filter because it has periodic band pass and band stop.

high-pass electronic filter

Ⅱ Common Electronic Filters

There are many types of filters, and the following are several types that have developed rapidly in recent years.

2.1 Passive Filter

Passive filter, also known as LC filter, is a filtering circuit composed of a combination of inductor, capacitor and resistor. It can filter out one or more harmonics. The most common passive filter structure is the inductor in series with the capacitor, forming a low impedance bypass for the main subharmonics. Single-tuned filters, double-tuned filters, and high-pass filters are all passive filters.

The principle of passive filtering is to use a reactor and a capacitor to form a low-impedance channel for a certain harmonic to let this harmonic flow in causing short-circuit. In addition, a single filter reactor or filter capacitor cannot be filtered. And the filtering range is not limited, so that several filtering channels are designed and calculated several times when the harmonics exceed the standard.

The passive filter has the advantages of low investment, high efficiency, simple structure and convenient maintenance, and it is widely used in the distribution network at this stage. Since the filter characteristics are greatly affected by the system parameters, only a few harmonics can be eliminated. Harmonic waves, and some subharmonics will produce amplification, even resonance phenomena and other factors will be caused. With the development of power electronics technology, people will gradually turn the research direction of filtering into active filters.

2.2 Active Filters

Active filter is a new type of power electronic device for dynamically suppressing harmonics and compensating for reactive power. It can compensate for harmonics and varying reactive powers that vary in size and frequency.

As the name implies, we can know that the device needs to provide power (to compensate the harmonics of the main circuit), and its application can overcome the shortcomings of traditional harmonic suppression and reactive compensation such as LC filters (it can only be fixedly compensated), realizing dynamic trace compensation for the harmonics and reactive power.

The active filter consists of the following active components: operational amplifier, negative resistor, negative capacitor, negative inductor, frequency varistor (FDNR), generalized impedance converter (GIC), negative impedance converter (NIC), positive impedance converter (PIC), negative impedance inverter (NII), positive impedance inverter (PII), four controlled sources, and so on.

2.3  Digital Filters

Corresponding to analog filters, digital filters are widely used in discrete systems. Its role is to process the input signal waveform or frequency using the characteristics of a discrete-time system. In other words, the input signal is converted into a certain output signal, thereby achieving the purpose of changing the signal spectrum.

Digital filters can generally be made in two ways: one is to assemble a specialized device using digital hardware, and the device is called a digital signal processor; the other is to directly use a computer to make the required computational programming program, that is, using computer software.

2.4 Low Pass Filters

The low-pass filter refers to the circuit in which allows low-frequency signals to pass through without passing medium and high-frequency signals in the car amplifier. Its function is to filter out the mid-range and high-pitched components in the audio signal, and enhance the bass component to drive the woofer of the speaker. Since the car power amplification part is a full-band power amplifier, the class AB amplification design is usually adopted, filtering out the signal of the low frequency band and only pushing the middle and high frequency is the best choice for saving power and ensuring sound quality. In addition, high-pass filters are often paired with low-pass filters, whichever is used to send a certain frequency of sound to the unit that should be sent.

The low pass filter is an electronic filtering device that allows signals that pass below the cutoff frequency to pass, but signals that are higher than the cutoff frequency cannot pass.

For different filters, the attenuation of the signal at each frequency is different. When used in audio applications, it is sometimes referred to as a high-cut filter, or a treble-cut filter.

The low-pass filter concept has many different forms, including electronic circuits (such as the hiss filter used in audio equipment, digital algorithms for smoothing data, acoustic barriers, image blurring, etc.) Eliminating short-term fluctuations and retaining long-term trends provides a smooth form of signal.

Recommended Reading: What Is A Low Pass Filter Circuit?

2.5 Switched Capacitor Filter (SCF)

A major issue in technological transformation in the 1980s was the realization of comprehensive large-scale integration (LSI) of various electronic systems. The most used filters are a hindrance to realized this goal, because RC active filters cannot implement LSI, neither passive filters and mechanical filters, thus people have to find new ways.

In the 1950s, the concept of SCF was proposed. Because the integration process was not enough at that time, it did not attract people's attention. In 1972, a scientist named Fried in the United States published an analog resistor R with switch and capacitor, saying that the performance of SCF depends only on the ratio of capacitance, independent of the absolute value of the capacitor, which had attracted people's attention. In 1979, single-chip SCFs in some developed countries have become commodities. Now, SC technology has matured. SCF is realized by using MOS technology and recognized as a major breakthrough in network theory and integration in the 1980s. Currently, the MOS capacitor value is generally within 0pF to 100pF, which has voltage coefficient of (10 ~ 100) x 10-6/V and a temperature coefficient of (10 ~ 100) x 10-6/°C. Both coefficients are almost close to the ideal realm.

SCF has the following advantages: it can be integrated on a large scale; it has high precision because its performance depends on the ratio of capacitance, and the error of MOS capacitance is less than one thousandth; with many functions, almost all electronic components and functions can be achieved by SC technology; it is simpler than digital filters, because A / D, D / A conversion is not required; its power is small, less than 10mW. The application of SCF is mainly based on the application of audio frequency range, and the working frequency is within 100kHz.

Applications in signal processing include: program-controlled SCF, analog signal processing, vibration analysis, adaptive filters, music synthesis, resonance spectrum, speech synthesizer, tone selection, speech coding, audio analysis, equalizer, demodulator , phase-locked circuit, discrete Fourier transform, etc. In short, SCF has wide application prospects in many fields such as instrumentation measurement, medical instrumentation, data or information processing.

dc filter

Note: SCF still has many topics to study

(1) Due to the limited sampling frequency of the op amp and control MOS switches, the SCF can only be used in the audio range. In recent years, although there is an SC circuit without an op amp, but its operating frequency is only within 1MHz.

(2) The channel resistance of non-MOS switches and non-ideal op amp characteristics can cause errors in the SCF.

(3) The parasitic capacitance of the switched capacitor itself distorts the frequency response of the SCF.

(4) The thermal noise of the MOS switch and the MOS op amp limits the dynamic range of the SCF. The SCF that is ultimately implemented in the MOS process, because it is a time-varying network, it is impossible to accurately analog with discrete components. Thus, the well-designed CAD technology is a better way to solve this problem.

In addition, there are still many issues to be studied in sensitivity analysis, noise analysis and so on.

RFI Filter Technology

Ⅲ Several New Digital Filters

3.1 Self-adaption DF 

The optimal control, adaptive control and self-learning control all involve multi-parameter and multi-variable complex control systems, which are all topics of modern control theory research. Self-adaption DF has a strong self-learning and self-tracking function. It has been widely used in the fields of radar and sonar beamforming, suppression of slowly varying noise interference, processing of noise signals, adaptive equalization of communication channels, echo cancellation of long-distance telephones, etc., which promotes the development of modern control theory. Self-adaption DF has the following simple algorithms: W-LMS, M-LMS, TDO, difference LMS and C-LMS.

3.2 Complex DF

In the case where the input signal is a narrowband signal processing system, a complex DF technique is often employed. To reduce the sampling rate and save all the information contained in the signal, the orthogonal dual-path detection method can be used to extract the complex envelope of the narrow-band signal, and then the complex envelope is converted into a complex sequence by A/D conversion. The signal processing system is a complex DF. It has many functions: suppressing clutter interference with Doppler shift in MTI radar; multiplexing TDM/FDM signal multiplexing between digital communication network and analog communication network.

3.3 Multidimensional DF

Multidimensional DF (usually 2D DF) is used in data processing of image processing, earthquake, and petroleum exploration. The design of multidimensional DF often extends from the one-dimensional DF design. Multidimensional DF can also play a very good role in the processing of two-dimensional images with fuzzy and random noise interference. In addition, there is a wave DF, which facilitates large-scale integration and digital simulation of passive and active filter networks. Therefore, it is receiving attention and research.

The topics to be studied for DF are: coefficient sensitivity, rounding noise and limit cycle, stability of multidimensional inverse-return filter, various hardware and software DF implementations, and so on.

In short, DF plays an extremely important role in digital signal processing technology, and it is very meaningful for its research, production and application.

mini filter

Ⅳ Other New-type Filters

In order to adapt to various needs, a number of new filters have emerged. Here are a few new filters that have been widely used.

4.1 Electronically Programmed CCD Transversal Filters (FPCCDTF)

Charge coupled device (CCD) and fixed weighting transversal filters (TF), in signal processing, are comparable in performance and cost to digital filters and various signal processing components. This filter is mainly used for self-adaption filtering; matched filtering of PN sequence and Chirp waveform; generalized frequency domain filter and correlation product operation; speech signal and phase equalization; beamforming of phased array system and ghosting of television signal elimination and other applications. Of course, more applications need to be further developed.

4.2 Crystal Filters

It was developed to accommodate single sideband technology. In the 1970s, the generation of integrated crystal filters made it a leap forward. In the past ten years, crystal filters have been devoted to the following research: having a good time domain response to get a better design; seeking new materials; expanding the working frequency; transforming the process to make it integrated. It is widely used as a carrier filter in a multiplexed system, as a frequency selective filter in a single band communication machine, and as an intermediate frequency filter in a spectrum analyzer and a sonar device.

4.3 SAW Filters

Acoustic surface waves refer to the propagation of sound waves on the surface of an elastomer. Its transforming speed is about 100,000 times smaller than the speed of the electromagnetic wave. The SAW filter is a special filter device made of piezoelectric materials such as quartz crystal and piezoelectric ceramic to use the piezoelectric characteristics and surface acoustic wave propagation. It is widely used in TV and video recorder intermediate frequency circuits to replace the mid-frequency LC filter, and the quality of images and sounds is greatly improved.

It is an ideal UHF device. Its amplitude-frequency characteristics and phase characteristics can be controlled separately to meet the requirements, and it has the characteristics of small size, long-term stability and simple process. Typically used in television broadcast transmitters as vestigial sideband filters and in the color television receivers tuned system surface comb filters.

And it is a combination of electronics and acoustics and can be integrated, so it is the most promising of all passive filters.

4.4 Dielectric Filters

The dielectric filter is designed and manufactured by the dielectric ceramic material with low loss, high dielectric constant, small temperature coefficient of temperature and small thermal expansion coefficient, and can withstand high power. The ladder is composed of several long resonators in multiple stages in series or in parallel.

It features small insertion loss, high power resistance and narrow bandwidth. So that it is suitable for level-coupled filtering of portable phones, car phones, wireless headsets, wireless microphones, radios, cordless phones, and integrated transceiver duplexers.

4.5 Active Power Filters (APF)

Active power filter is a kind of electronic device that dynamically suppresses harmonics and compensates reactive power. It can compensate harmonics and reactive power with varying frequency and size, which can make up for the shortcomings of passive filters. The better compensation characteristics of the source filter is an ideal compensation harmonic device.


Frequently Asked Questions about Electronic Filters Types

1. What are 4 types of filters?
Filters can be active or passive, and the four main types of filters are low-pass, high-pass, band-pass, and notch/band-reject (though there are also all-pass filters).


2. What are the types of active filters?
Types of Active Filters
Active Low Pass Filter
Active High Pass Filter
Active Band Pass Filter
Active Band Stop Filter


3. What are the commonly used filters?
Some common filter families and their particular characteristics are:
Butterworth filter – no gain ripple in pass band and stop band, slow cutoff
Chebyshev filter (Type I) – no gain ripple in stop band, moderate cutoff
Chebyshev filter (Type II) – no gain ripple in pass band, moderate cutoff


4. What is difference between active and passive filters?
The major difference between active and passive filter is that an active filter uses active components like transistor and op-amp for the filtering of electronic signals. As against, a passive filter uses passive components like resistor, inductor and capacitor to generate a signal of a particular band.


5. Why active filter are better than passive filter?
Active filters have some definite advantages over passive versions, including the ability to provide signal gain, higher input and lower output impedances, no need for buffer amplifiers, and less dependency on inductors, which add expense. Most electronic filters are linear.


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Filter (Signal Processing) Basics in Electronics
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Four Typical Electronic Filters (Signal Processing)

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