Filtering is a basic and important technology in signal processing. Using filtering techniques, the desired signal can be extracted from various signals and unwanted interference signals can be filtered out. Filters are an important component of the signal's frequency domain analysis.
There are many types of filters, and various filters have different performance characteristics. Therefore, when choosing a filter, it is usually necessary to consider the customer's actual use environment and customer performance requirements in order to make a correct, effective, and reliable choice.
Filters are divided into analog filters and digital filters. Analog filters are used to process analog signals or continuous signals. Digital filters are used to process discrete digital signals.
Analog filters can be widely used in distribution networks of industrial, commercial and institutional groups, such as power systems, electrolytic plating companies, water treatment equipment, petrochemical companies, large shopping malls and office buildings, precision electronics companies, and airports/ports. Systems, medical institutions, etc.
Communications industry
In order to meet the operational needs of large-scale data center equipment rooms, the use of UPS in communications distribution systems has increased dramatically. According to investigations, the main harmonic source equipment for communication low-voltage power distribution systems is UPS, switching power supply, and inverter air conditioners.
The resulting harmonic content is high and the displacement power factor of these harmonic source devices is extremely high. By using an active filter, the stability of the communication system and the power distribution system can be improved, the service life of the communication equipment and the power equipment can be extended, and the distribution system can be more in line with the design specifications of the harmonic environment.
Semiconductor IndustryThe third harmonic in most semiconductor industries is very serious, mainly due to the large number of single-phase rectifier devices used in the enterprise. The 3rd harmonic belongs to the zero-sequence harmonics, which has the characteristics of convergence at the neutral line, resulting in excessive pressure on the neutral line, and even the phenomenon of ignition. There is a great hidden danger of production safety.
Harmonics can also cause circuit breakers to trip, delaying production time. The third harmonic forms a circulation in the transformer, accelerating the aging of the transformer. Severe harmonic pollution inevitably affects the efficiency of equipment use and the life of the distribution system.
Petrochemical industry
Due to the need of production, there are a lot of pump loads in the petrochemical industry, and many pump loads are equipped with frequency converters. The extensive application of frequency converters has greatly increased the harmonic content in the distribution system of the petrochemical industry.
At present, most of the frequency converter's rectification links are applied 6 pulses to convert AC to DC, so the resulting harmonics are mainly 5, 7, and 11 times. The main hazards are the damage to the power equipment and the deviation in the measurement. The use of active filters can solve this problem.
Chemical fiber industry
In order to greatly increase the melting rate, increase the melting quality of glass, extend the furnace life, and save energy, it is common in the chemical fiber industry to use electrical melting and heating equipment to send electricity directly to fuel-heated glass tank kilns by means of electrodes. These devices generate a large amount of harmonics, and the three-phase harmonics have large differences in the frequency spectrum and amplitude.
Steel/IF heating industry
The medium-frequency furnaces, rolling mills, electric arc furnaces and other equipment commonly used in the iron and steel industry have a major impact on the power quality of the power grid, causing frequent overload protection of the capacitor compensation cabinet, serious heat generation on the transformer and power supply lines, frequent fuse blowing, etc., and even causing Voltage drop, flicker.
Automotive Manufacturing
Welding machine is an indispensable device in the automobile manufacturing industry. Due to the randomness, rapidity, and impact of welding machines, a large number of welding machines are used to cause serious power quality problems, resulting in robots with unstable welding quality and high degree of automation. Because the voltage is unstable and cannot work, the reactive power compensation system cannot be used normally.
Harmonic control of DC motorsLarge DC motor sites need to first convert AC power to DC power through rectifier equipment. Because of the large load capacity of such projects, severe harmonic pollution exists on the AC side, causing voltage distortion and serious accidents.
The use of automated production lines and precision equipment
In the case of automated production lines and precision equipment, harmonics will affect their normal use, causing malfunctions in intelligent control systems, PLC systems, and so on.
Hospital systemThe hospital has very strict requirements on the continuity and reliability of power supply. Automatic restoration of power supply for type 0 sites is T ≤ 15S. Automatic recovery of power supply for type 1 sites is 0.5S ≤ T ≤ 15S. Automatic recovery of power supply for type 2 sites is T ≤ 0.5. S, voltage harmonic distortion THDu ≤ 3%, X-ray machine, CT machine, nuclear magnetic resonance are highly harmonic content load.
Theater / Stadium
SCR dimming systems and large-scale LED devices are all harmonic sources. During operation, a large number of third-order harmonics are generated. This not only causes the power equipment of the power distribution system to be inefficient, but also causes the light to flicker. The weak electric circuit of the cable TV or the like generates noise and even malfunctions.
In modern telecommunication equipment and various control systems, digital filters are also widely used. Here are some of the most successful areas of application.
Voice processing
Speech processing is one of the earliest applications of digital filters and is one of the earliest fields that have driven the development of digital signal processing theory. This area mainly includes five aspects:
First, speech signal analysis. That is, the waveform characteristics, statistical characteristics, model parameters, etc. of the speech signal are analyzed and calculated.
Second, speech synthesis. That is, using dedicated digital hardware or running software on a general-purpose computer to generate speech;
Third, speech recognition. That is, using a dedicated hardware or computer to recognize what the person is saying, or to identify the speaker;
Fourth, speech enhancement. That is, the concealed speech signal is extracted from noise or interference.
Fifth, speech coding. It is mainly used for voice data compression. At present, a series of international standards for speech coding have been established and used for communication and audio processing.
Image Processing
Digital filter technology has been successfully applied to the restoration and enhancement of still and moving images, data compression, noise and interference, image recognition and tomography, and has also been successfully applied to radar, sonar, ultrasonic and infrared signals. Visible image imaging.
In the field of modern communication technology, almost no branch is not affected by digital filtering technology. Source code, channel coding, modulation, multiplexing, data compression, and adaptive channel equalization are widely used digital filters, especially in digital communications, network communications, image communications, multimedia communications, etc. Digital filters are almost impossible to move. Among them, the software radio technology considered to be the future development direction of communication technology is based on digital filtering technology.
TV, radar
The replacement of analog television by digital television is an inevitable trend. The popularity of high-definition television is just around the corner. The accompanying video disc technology has formed an industry with a huge market; videophones and videoconferencing products are constantly being updated.
The achievements and standardization of video compression and audio compression technologies have contributed to the vigorous development of the television industry. Digital filters and related technologies are important foundations for video compression and audio compression technologies.
The frequency band occupied by radar signals is very wide, and the data transmission rate is also very high. Therefore, compressing the amount of data and reducing the data transmission rate are the primary issues faced by digital processing of radar signals. The advent of digital devices has prompted advances in radar signal processing technology.
In modern radar systems, the digital signal processing part is indispensable, because the digital filtering technology is inseparable from the signal generation, filtering, processing to the target parameter estimation and target imaging display. Radar signal digital filters are one of the most active research areas today. Sonar signal processing is divided into two major categories, namely, active sonar signal processing and passive sonar signal processing. Many theories and techniques involved in active sonar systems are the same as those of radar systems.
musicThe digital filter has opened up a new situation for the music field. The digital filtering technology has shown great power in editing, synthesizing, and adding special effects such as reverberation and chorus to the music. Digital filters can also be used to compose, record, and play, or to restore the sound quality of old tapes.
Application of Active Power Filter in Airport
The fundamental reason for the generation of harmonics in power systems is the transmission and distribution of electricity and equipment that have non-linear volt-ampere characteristics. When the current flows through a non-linear load, a non-linear relationship with the applied voltage results in the formation of a non-sinusoidal current, thereby generating harmonics. Harmonic pollution increasingly threatens the safety, stability, and economic operation of the power system, which has a great impact on the linear load and other users of the same network.
As a convenient mode of transportation, aircraft brings a variety of choices to people's daily traffic life, and the airport is also expanding year by year. However, in the low-voltage distribution system of the airport, there are a large number of harmonic sources, such as airport navigation lights, DC motors, electric furnaces, rolling mills, welding machines, etc. These harmonic sources have large current distortions and a wide spectrum of harmonics. Reactive demand changes quickly and so on.
The harmonics generated by such loads endanger the normal operation of the distribution system and even cause serious electrical accidents. Taking the airport navigation lighting system as an example, the navigation aid lighting load equipment is increasing. The airport lighting station uses a large amount of thyristor dimming equipment, resulting in a large number of harmonic currents, causing pollution to the power quality, and additional current and additional Thermal effects also cause some damage to the safety of various types of electrical equipment and cable lines. Therefore, it is very important to analyze and rectify the harmonic problems in the airport navigation lighting station.
At present, there are two major mainstream methods for harmonic control of power systems: passive filtering and active filtering. High power semiconductor dimming equipment used in airport lighting stations generates a large number of higher harmonics (mainly all odd harmonics other than 3rd harmonic), and the passive filter must be separate for each harmonic. To design a single-resonance filter, the design parameters must be related to the system impedance (computing the system impedance is cumbersome, and the system is expanded year by year, and the system impedance will also change); passive filtering cannot completely eliminate harmonics, but there is a danger of amplifying resonance; The aging of the capacitor will also offset the originally designed resonant point and will not achieve the goal of filtering out the target harmonics; the passive filter system is suitable for single and stable loads.
Compared with passive filters, active filter systems have high controllability and fast response (≤1ms), can compensate for various harmonics, can suppress flicker, compensate for reactive power, and have one-machine multi-energy characteristics; The cost-effectiveness is more reasonable; the filter characteristics are not affected by the system impedance, and the danger of resonance with the system impedance can be eliminated; with the adaptive function, the changing harmonics can be automatically tracked and compensated.
The basic principle is to detect the harmonic current from the load circuit of the harmonic source (to be compensated object), and a compensation current waveform equal to the magnitude of the harmonic current is generated by the compensation device to cancel the harmonic source load. The resulting harmonic currents cause the grid-side current to contain only the fundamental component.
Governance effect:
Application of FIR Filter in Audio System
Normally, we use IIR EQ to correct the frequency response curve of a sound system or a path. This is the purpose of our use of this equalizer. In fact, in most cases, it can help us achieve this goal. There are some differences between PEQ and GEQ in actual use, but whatever form of EQ is, as long as its function is powerful enough, it can basically achieve our intended purpose.
Unfortunately, while IIR EQ corrects the frequency response curve of the system or channel according to our personal will, it also brings a byproduct - the phase response of the audio system or channel is destroyed. Moreover, the general rule is: The larger the IRR EQ changes the frequency response, the more severe it is to destroy the corresponding system or channel phase response.
The use of a high-pass filter in the audio system (also known as an IIR EQ) to indicate the effect on the phase
However, in the highly developed science and technology today, FIR, a technology that has been widely used in communications and other fields, can be applied to sound systems. This is indeed a good thing.
Because it solves the problem that cannot be solved by IIR EQ, it is as another type of EQ, it can only correct the frequency response of the sound system without affecting its phase response; it can also only be the phase of the sound system. Do a correction without affecting the frequency response (isn't this similar to an "all-pass filter"? It is, but it is much more flexible and functional than the AP); it can also correct the system's frequency response and phase response at the same time. .
In this sense, FIR EQ is almost omnipotent, except that it has no correction ability to the impulse response of the system. It is true, but it also has side effects!
Comparing Results Before and After Frequency and Phase Processing of Signals Above 500Hz Using FIR Processor
Because the FIR filter is a digital filter that cannot be implemented with analog circuitry, it takes more or less time cost to process the signal. In other words, sound systems that use FIR filters have additional delays, and IIR can't be criticized because it can be implemented using analog circuits. Anything has two sides and good sides have bad ones. The other side. Although, the time cost is also a factor that we must consider, but at least for the mid-high frequency signal, we do not need too much distress for the time cost of a few milliseconds.
How much time and cost are required depends mainly on the frequency range that needs FIR processing. The lower the frequency of the sound, the longer the cycle. It is very simple that we can think of, as the FIR of the digital signal processor, it needs at least one cycle time of the sound signal corresponding to the lower limit frequency to process it. For example, for an ideal situation, for a 500 Hz sound signal, the FIR filter needs at least 2 ms to process. Of course, this time delay is generally acceptable. But if you want to deal with signals as low as 50Hz, it may take 20ms or even longer, which will become a very annoying problem for live performances.
In general, the audio industry must be constantly making choices. Because there is always no best solution, only a more appropriate solution in the light of the present. For better phase response of the frequency response, we will consider using FIR filters, but at the same time we do not want too much delay to be generated in the system. So, in the real situation, many manufacturers choose to use FIR to process the high-frequency part of the system, while IIR EQ and the classical frequency-dividing circuit handle the low-frequency and ultra-low-frequency parts.
Application of Adaptive Filtering in Signal Processing
The various applications of adaptive filters mainly include:
1. System modeling, where the adaptive filter is used as a model to estimate the characteristics of the unknown system.
2. An adaptive noise canceller, wherein the adaptive filter is used to estimate and cancel noise components in the desired signal;
3. A digital communication receiver, wherein the adaptive filter is used for channel identification and provides an equalizer for intersymbol interference;
4. An adaptive antenna system in which an adaptive filter is used for beam direction control and can provide a null in the beam pattern to eliminate unwanted interference.
System identification or system modeling
For a real physical system, people are mainly concerned with their input and output characteristics, that is, the transmission characteristics of the signal, without requiring a complete understanding of its internal structure. The system can be one or more inputs, or it can have one or more outputs. The identification of communication systems is a very important issue in communication systems. The so-called system identification essentially estimates or determines the characteristics of the system and the unit impulse response or transfer function of the system based on the input and output signals of the system.
System identification and modeling is a very broad concept and has important implications in areas such as control, communications and signal processing. In fact, system identification and modeling are not limited to traditional engineering fields, but can also be used to study social systems, economic systems, and biological systems.
This section only discusses system identification and modeling issues in communication and signal processing. The filter is used as a model of the communication channel, and the identification of the communication channel is performed by using an adaptive system identification method, so that the communication channel can be further processed in an equalized manner.
If the communication channel is seen as a "black box", only the input and output of the "black box" is known; an adaptive filter is used as a model for this "black box" and the filter has the same inputs and outputs as the "black box" . The adaptive filter “matches†the output of the filter with the output of the “black box†by modulating its own parameters.
"Match" here usually refers to matching in the least-squares sense. In this way, the filter simulates the transmission behavior of the communication channel to the signal. Although the structure and parameters of adaptive filters are not the same as the actual communication channels, they maintain a high degree of agreement on the input and output responses.
Therefore, in this sense, the adaptive filter is the model of this unknown "black box" system. And it can also be found that if the adaptive filter has enough freedom (adjustable parameters), the adaptive filter can simulate this "black box" to any degree.
Assuming that the unknown channel is a Finite Impulse Response (FIR) structure, construct an adaptive filter of the FIR structure, using a pseudo-random series as the input signal x(n) of the system, and send it to the unknown channel system and the adaptive filter.
By adjusting the coefficients of the adaptive filter so that the mean square error of the error signal e(n) is minimized, the output y(n) of the adaptive filter is approximately equal to the output d(n) of the communication system. It can be shown that the presence of additive noise v(n) does not affect the final convergence of the adaptive filter to the optimal Wiener solution.
It can be assumed that two FIR systems with the same input and similar output should have similar characteristics. Therefore, the characteristic of the adaptive filter or its unit impulse response can be used to approximately replace the characteristic or unit impulse response of the unknown system.
Application of FBAR Filter in Smartphones
A very important part of a modern smartphone is an RF filter. Just like its basic principle, filters are mainly used to pass and reject unnecessary frequencies so that many receivers in the handset can only process the expected signal. .
In the past, mobile phones usually operated only in a few bands in specific regions of the world. However, for modern mobile phones, they basically work in multiple wireless bands at the same time, including mobile communication, Bluetooth, WiFi, and GPS. Manufacturers also hope to design products that can work in different regions of the world and with different telecom operators. To make mobile phones work in more bands and regions, the demand for radio frequency filters from mobile phones is increasing.
In previous generations of wireless technologies, filtering requirements were not difficult to achieve, and only surface acoustic wave filters may be used, but as operators' networks evolved to CDMA and 3G, in order to make use of current 4G/LTE services, The smartphone itself has become more complicated, so handset manufacturers have begun to expand the use of FBAR technology to solve the unique issues that will be discussed in the forthcoming discussion of 4G/LTE.
4G/LTE mobile phones that can operate in multiple frequency bands
The latest smart phone products must be designed to work in multiple frequency bands around the world. The overall size of a multi-band smart phone will not be larger than the previous generation, so if you want to add more filters in the same space reserved for RF front-end circuits It is very obvious, then, that the filter itself must be very small. With the Microcap micro-encapsulation technology, FBAR filters can satisfy most space-constrained applications with chip-scale packaging.
Since the FBAR is a matrix material, it can provide very good power handling capability without the use of parallel structures such as those commonly found in SAW filters. In addition, the size of the FBAR device also shrinks with increasing frequency, making the FBAR very It is suitable for the new 4G/LTE frequency band applications from 2300MHz to 2700MHz and the future 3.5GHz.
4G/LTE smartphones running at higher data rates
Compared with 3G services, the download speed of 4G/LTE under the same data volume can reach about 10 times, which means that the amount of data that can be downloaded in the same time can reach 10 times. There are several ways to achieve higher Data rate, 4G/LTE will use different modulation methods depending on the detected signal strength. Simply put, the higher the SNR, the higher the data rate, as converted from QPSK to QAM16/64 modulation.
In multi-band 4G/LTE mobile phones with single-pole and multi-throw switches combined with multiple duplexers, the detected signal may be too low and affect the data rate. The low insertion loss of the FBAR helps to maximize the input signal strength. Higher data throughput, resulting in a better user experience and higher data capacity.
Mobile phones using frequency-division-multiplex modulation use duplexers that allow signal transmission and reception at the same time. Since the transmit and receive filters are connected to the same antenna port, the filter isolation between them is very important. Higher isolation will occur. The noise in the receive band is minimized, which can increase SNR and data rate.
Another method to increase the data rate is to increase the download data rate through carrier aggregation and carrier aggregation with more than one frequency band working at the same time. Some new LTE frequency bands occupy relatively small spectrum, so this is a network operator can effectively improve The method of communication capacity.
Since the transmission and reception of each frequency band will work simultaneously, it is not possible to use a switch to use a multiplexer to combine the respective transmit and receive filters onto the same antenna port. When combined in a multiplexer configuration, Avago's FBAR Filters can provide low signal loss paths and help maximize data rates.
Smartphones use multiple wireless signals at the same time
It is difficult to find smart phones without Wi-Fi connectivity at present. Depending on the operating frequency of the mobile phones, the signals sent by the mobile phones may interfere with the normal operation of Wi-Fi if not properly filtered.
When using a smartphone as a Wi-Fi hotspot, Wi-Fi will work simultaneously with 4G/LTE wireless signals. If there is no excellent filtering capability, Wi-Fi transceivers may be blocked or transmitted over LTE signals in the Band 7 band. influences.
The vast majority of mobile phones today also support GPS and even GLONASS services. Since GPS/GLONASS signals are usually very low power, approximately -125dBm to -150dBm, all nearby GPS signals may affect the GPS/GLONASS receivers. Sensitivity, the AGPS-F001 pre-filter plus LNA module provides superior out-of-band masking and linearity performance for mobile networks, PCS and WiFi signals due to its steep filtering and wideband attenuation capabilities.
FBAR technical advantages
Battery life is an important feature that is often used to test mobile phones and compare them with each other. On the receiving side, we discussed how the lower insertion loss of FBARs can be compensated by the higher loss associated with RF front-end combining multiple bands. Another benefit of the higher data rate of 4G/LTE handsets is that by allowing the handset to detect weaker signals, the coverage of mobile communications can be expanded to avoid poor reception or even dropped calls.
On the transmit side, the lower transmit filter insertion loss represents the lower output power required by the power amplifier at the same antenna transmit power, compared to the insertion loss of Avago's Band 4 duplexer compared to other filter technologies. The improvement is about 0.2dB ~ 0.5dB, which is equivalent to saving up to 50mA of current consumption, so it can provide longer battery life and talk time.
When most applications are still based on 3G services, only a few bands can benefit from FBAR technology. With the popularity of 4G/LTE multi-band smart phones, FBAR technology features such as low insertion loss, steep filter curve, and high isolation And miniaturized sizes have become the reason for all major smartphone manufacturers to quickly introduce this technology.
Current filter, duplexer, and multiplexer products using FBAR technology have been introduced into the design of smart phones in 15 different working frequency bands in the United States, Europe, and Asia. With the advent of new filtering challenges, FBAR technology will continue to become Provide answers to the preferred choices and become mainstream technologies.
The application fields of filters are so wide that it is impossible to completely list them. In addition to the above fields, there are many other fields of application. For example, it is widely used in the military for navigation, guidance, electronic countermeasures, and battlefield reconnaissance; in power systems it is used for energy distribution planning and automatic detection; in environmental protection it is used for the automatic monitoring of air pollution and noise disturbances, In the economic field, it is applied to stock market forecasting and economic benefit analysis.
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