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Oct 19 2019

Four Typical Electronic Filters (Signal Processing)


In modern telecommunication equipment and various control systems, the filters are extremely widely used. Among all electronic components, the most used and most complicated technology is the filter. The advantages and disadvantages of the filter directly determine the performance of the product. Therefore, the research and production of the filters had always been valued by various countries, and various filters such as RC active filter, digital filter, switched capacitor filter and charge transfer device were made, which reflected the rapid development of the device. By the late 1970s, the monolithic integration of the above several filters had been developed and applied. In the 1980s, we worked on the research of various new types of filters, striving to improve performance and gradually expand the range of applications. From the 1990s to the present, it is mainly devoted to the application of various types of filters to the research and development of various products. Of course, the research on the filter itself is still going on.

EMI filter

Electronic filters are a type of signal processing filter in the form of electrical circuits, you can also say that, any device or system that can pass a particular frequency component of a signal and greatly attenuate or suppress other frequency components is referred to as a filter. With the advantages of simple structure, low cost, high operational reliability, etc., electronic filters have been widely used in our daily lives. There are many kinds of filters and different uses. This article talks about four common filters divided according to different standards.


 Four General Type Filters Four Commonly Used Filters Four Classic Electronic Filters

1) Low Pass Filter

2) High-pass Filter

3) Band-pass Filter

4) Band-stop Filter

a. Digital Filter 

   1) Technique

   2) Comparison

b. Program-controlled Filters

c. Passive Filters (PPF)

d. Active Filters (APF

1) Butterworth Filters

2) Chebyshev Filters

3) Bessel Filters

4) Elliptic Filters

Ⅰ Four General Type Filters

According to the frequency band of the signal passed, it is divided into low-pass, high-pass, band-pass, and band-stop filters.

This Video Talks About Four Types of Typical Electronic Filters

1) Low pass filter

It allows low frequency or DC components in the signal to pass, suppressing high frequency components or interference and noise.

Low pass filter 

From 0 to f2, the amplitude-frequency characteristic is flat, which allows the frequency components below the f2 in the signal to pass without attenuation almost, while the frequency components above f2 are greatly attenuated.


2) High-pass filter

It allows high-frequency components in the signal to pass, suppressing low-frequency or DC components.

High-pass filter 

In contrast to low-pass filtering, the amplitude-frequency characteristics are straight from the frequency f1 to ∞. It allows the frequency components above the f1 in the signal to pass almost without attenuation, while the frequency components below f1 are greatly attenuated.

3) Band-pass filter

It allows signals in certain frequency bands to pass, suppressing signals, interference and noise below or above this band.

Band-pass filter 

Its pass-band is between f1 and f2. It allows the frequency components of the signal above f1 and below f2 to pass without attenuation, while the other components are attenuated.

4) Band-stop filter

It suppresses signals in a certain frequency band and allows signals outside the band to pass, also known as notch filters.

 Band-stop filter

In contrast to band-pass filtering, the stop band frequency is between f1 to f2. It attenuates the frequency components of the signal between f1 and f2, and the other signals not between f1 and f2 can pass almost without attenuation.

The low-pass filter and the high-pass filter are the two most basic forms of the filter. Other filters can be decomposed into two types of filters. For example, the series connection of the low-pass filter and the high-pass filter is band-pass filter, and the low-pass filter and the high-pass filter are connected in parallel as a band-stop filter.

Ⅱ Four Commonly Used Filters

a. Digital filter 

Digital filters is correspond to analog filters, and we use digital filters in commonly used discrete systems. Its main function is to utilize the characteristics of the discrete-time system, where time is a variable, and then process signals from the external input. The input signal here is generally a waveform model in a broad sense, and the signal can be voltage, current, power, etc. Of course, there are similarities like frequency. In actual operation, we can also turn the input signal waveform into an output, that is, in other words, inverting the input and output. Thereby achieve the purpose of modifying the spectrum of the signal.


Digital filters can be processed in a variety of ways. We introduce the two most commonly used in practice. One way is forming a dedicated device for the various components of an integrated circuit, which used as a digital signal processor. It is a digital processor similar to the arm architecture or the single-chip architecture that commonly used. This method has a high commercial value for the needs of batch manufacturing, because the cost of the production is relatively low, and it is welcomed by the market. The other way is to use the x86/x64 commercial or industrial computer that we usually use for simulation. This is completely simulated by application software. This method is also used in laboratory or large-scale digital filtering projects. The cost is high and it is not suitable for production and matching with large quantities. However, it is the best simulation method in the laboratory, which has great advantages in high-level simulation and calculation.


There are two main types of digital filters, one is IIR, which we call an infinite impulse response filter, and the other is FIR, which corresponds to IIR, which is a finite impulse response filter. Both systems have their own characteristics. The FIR filter is a loop signal with no closed loop feedback. Its structure is relatively simple, and the calculation of the phase of the more stringent linear equation can be realized. In general, this filter doesnt used under the case of phase requirement is not strict, but instead it is used. Of course, in many scenarios, we need to perform some real-time processing on the signal. When the signal data is collected more, the performance requirements of the hardware are getting higher and higher, and many single-chip microcomputers on the market can not meet the requirements. For actual functional requirements, the general 8-bit 16-bit or even 32-bit microcontroller and ARM chip can not support the algorithm. Because the emergence of DSP controller designed for digital processing, improve the efficiency of our filter In many cases. DSP can use multiple sets of buses to process multiple sets of real-time data in parallel. The full use of independent algorithms greatly improves the efficiency of our filters. For the short board on the hardware, it can be compensated by the DSP chip to realize the real-time processing and calculation of the digital signal.

Compared with ordinary microprocessors, DSP has great advantages in digital signal processing. It is the inheritance of single-chip microcomputer and ARM. It has made some partial development and improvement for signal processing, which greatly enhances the ability of digital processing. It has The specific data flow format, specific algorithm, and special system structure provide many excellent conditions and foundations for solving complex digital signal processing. The IIR filter can be realized by programming the DSP. And the FIR filter actually has certain defects. This type of system has only zero points. It does not have the good attenuation characteristics as the IIR system, but it also has obvious advantages, that is, it is implemented by non-hardware circuits. There are many main advantages of implementing filters compared to hardware circuits, such as high efficiency, having extremes and feedback.


b. Program-controlled filters

Program-controlled filtering systems are proposed in the shortcomings of traditional filters. For example, traditional filters generate errors during operation, which affect the accuracy of the entire system. And low-precision filters can have many undesirable consequences when used, whats more, the higher the waveform requirements of traditional filters mean more op amps, which is clumsy and costly. Therefore, the digitization of the programmable filter can reduce the uncertainties in the production process and the number of people involved in the production process, which can effectively solve engineering problems such as reliability, intelligence and product consistency in the power module, greatly improving production efficiency and products maintainability.


c. Passive filters (PPF)

A passive filter is a filter circuit composed of a resistor, a reactor, and a capacitor. When having the resonant frequency, the circuit impedance value is the smallest; when having the non-resonant frequency, the circuit impedance ratio is large; and when the circuit component value is adjusted to a certain characteristic harmonic frequency, the harmonic current can be filtered out. In addition, when several harmonic frequencies circuits are combined, the corresponding characteristic harmonic frequencies can be filtered out, and the order harmonics are filtered by the low impedance bypass. The main principle is to design the harmonic frequency to be small for different harmonics, and realize the shunt effect of the harmonic current, that is, provide a bypass channel for the pre-filtered high-order harmonics to realize the purification waveform.

Passive filters can be divided into capacitor filters, power plant filter circuits, L-type RC filter circuits, π-shaped RC filter circuits, multi-section π-shaped RC filter circuits, and π-shaped LC filter circuits. According to the function, it can be divided into single-tuned filter, double-tuned filter and high-pass filter. And passive filters have the following advantages: simple structure, low investment cost, ability to compensate reactive components in the system, improving grid power factor, high stability, simple maintenance, mature technology, etc. Thus it was widely adopted before the emergence of active filters.

The shortcomings of passive filters also have many aspects: they are greatly affected by the parameters of the power grid. The system impedance value and the main frequency of the resonant frequency often change with the working conditions; the frequency band of the harmonic filtering is also narrow, and only the main frequency can be filtered out. Due to the parallel resonance, amplify some order harmonics; coordination between filtering and reactive power compensation and voltage regulation is difficult; as the current flowing through the filter rises, the equipment may be overloaded; large weight and volume require more space; poor running stability and other shortcomings. That why active filters with better overall performance are getting more and more applications.


d. Active filters (APF)

The main function of the active filter is not only to dynamically track and suppress harmonics, but also to compensate for the lower reactive components in the grid. It can compensate for higher harmonic components with fluctuations in amplitude and frequency, and dynamically compensate for varying reactive components of the system. It overcomes the shortcomings of traditional harmonic management schemes and reactive power compensation, and achieves dynamic tracking compensation. The basic principle of APF is to detect the voltage and current signals of current system, generate the compensation current signal by the operation of the command current operation circuit, and amplify the command by the compensation current generation circuit according to the harmonic signal, thereby obtaining the compensation current, and then offset  the higher harmonic components and reactive currents to make sinusoidalization of the system waveform, filtering out the harmonics of the power grid, and improving the power quality.

The active filters can be classified into a voltage-type active filter and a current-type active filter depending on the energy storage components. The voltage type filter is widely used because of its low loss and high efficiency. Current-type filters are less used due to large losses and low efficiency. According to the AC and DC power supply, it can be divided into DC APF and AC APF. According to the circuit topology structure, it can be divided into series APF, parallel APF and series-parallel one, and a hybrid use of APF and PPF.

Compared with passive filters, active filters have many advantages: fast response, sound controllable performance; adaptive function, dynamic trace and compensation for higher harmonics of the system, high stability, independence for the system impedance influence, ability to avoid the occurrence of resonance, flicker suppression, and reactive components for the system.

The cabinet-type system adopts a modular structure, and the contact design is fine. It can be expanded according to the actual needs of the site at any time. And it has strong expandability, modular configuration and small size, which allows designers to have more choices and maximize user space. In addition, it can realize flexible capacity configuration and support subsequent field capacity expansion. On-site installation and maintenance are simple and easy to plug and unplug. The drawer type structure can meet the users separate design of the module, excellent architecture form, and DSP processing capability. When operating with large programmable controller, selecting high-power power electronic components, the system has external communication port. Except that, it can work independently to be attached to other cabinets .

The difference of active filters compared with passive filters is that it requires a power supply and compensation for both harmonics and reactive power. The command current operation circuit and the compensation current generation circuit are two important components of the active filter. The function of the command current operation circuit is to detect the harmonic component and reactive component that the system needs to compensate. The function of the compensation current generator circuit is to issue a compensation current command according to the detected harmonic component and the reactive component, and generate a compensation current. The main component is composed of three parts: a current tracking control circuit, a driving circuit and a main circuit.

 EMI filters

Ⅲ Four Classic Electronic Filters

1) Butterworth filters

The Butterworth filter is characterized in that the frequency response curve in the pass band is as flat as possible, without undulations, and gradually decreases to zero in the blocking band. On the Bode plot of the logarithmic diagonal frequency of the amplitude, starting from a certain corner frequency, the amplitude gradually decreases with the increase of the angular frequency, and tends to negative infinity.

The frequency characteristic of the Butterworth filter is a monotonic function of frequency both in the pass band and in the stop band. Therefore, when the boundary of the pass band meets the requirements of the index, there will be a margin in the pass band. Therefore, a more efficient design method should be to evenly distribute the accuracy within the pass band or stop band, or both. This allows the lower order system to be used, which can be achieved by selecting an approximation function with equal ripple characteristics.


2) Chebyshev filters

The Chebyshev filter is a filter with equal ripple of frequency response amplitude on pass band or stop band. In the stop band, it is monotonously called the Chebyshev I-type filter; the amplitude characteristic is monotonic in the pass band, and with equal-ripple inside the stop band is called the Chebyshev II-type filter. The type of Chebyshev filter used depends on the practical application.


3) Bessel filters

The Bessel filter is a linear filter with maximum flat group delay (linear phase response), and it is  commonly used in audio flyover systems. The analog Bezier filter is depicted as a constant group delay that spans almost the entire pass band, thus maintaining the filtered signal waveform over the pass band.

The Bessel filter has the flattest amplitude and phase response. The phase response of the band pass (usually the users area of interest) is nearly linear. It can be used to reduce the nonlinear phase distortion inherent in all IIR filters. The Bessel linear phase filter is used in audio equipment because it has the same delay to all frequencies below its cutoff frequency. In audio equipment, it must be in the band without damage. Under the premise of the phase relationship of the signal, the out-of-band noise is eliminated. In addition, the Bessel filter has a fast step response and no overshoot or ringing, which makes it a smoothing filter at the output of the audio DAC or an anti-aliasing filter at the input of the audio ADC. It can also be used to analyze the output of class D amplifiers and to eliminate switching noise in other applications to improve the accuracy of distortion measurements and oscilloscope waveform measurements.

4) Elliptic filters

An Elliptic filter, also known as a Cauer filter, is a filter that is corrugated in the pass band and stop band. It goes further than the Chebyshev method at the expense of both the pass band and the stop band undulations in exchange for the steeper nature of the transition zone. Compared to other types of filters, elliptic filters have minimal pass band and stop band fluctuations under the same order.

filter image 

Comparison of the Four Filters

Frequency Range

The Butterworth filter is characterized in that the frequency response curve in the pass band is as flat as possible, without undulations, and gradually decreases to zero in the blocking band.

The Chebyshev filter has a faster attenuation in the transition band than the Butterworth filter, but the amplitude-frequency characteristics of the frequency response are not as flat as the latter. The error between the Chebyshev filter and the ideal filters frequency response curve is minimal, but there is amplitude fluctuations in the pass band.

The Bessel filter has the flattest amplitude and phase response, and its phase response of the band pass is nearly linear.

At same order:

The amplitude-frequency curve of the elliptic filter drops steepest, followed by the Chebyshev filter, again the Butterworth filter, and with the most gradual drop is the Bessel filter.

The Butterworth filter has the flattest pass band and slower stop band.

The ripple of the Chebyshev filter in pass band is equal and in stop band drops faster.

Bessel filter has equal ripples in pass band, and drops slowly in stop band. That is to say,its frequency-selective characteristic of the amplitude-frequency characteristic is the worst. However, it has the best linear phase characteristics.

The elliptical filter has equal ripples in the pass band (flat or equal ripple in stop band), and drops the fastest in the stop band.

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