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What is a filter and its type classification

Postdate:2013-06-13 3:56:51    Hits:2329Secondary

Filters Overview
The specific frequency frequency or frequency other than the frequency effectively filter circuit is a filter. Its function is get to a particular Usage Frequency or a eliminate one specific frequency, filter is a on the signal have treatment effects of device or circuit.
Main role is: Let no possible useful signal attenuation through the unwanted signal attenuation as large as possible.

Filters generally have two ports, an input signal, an output signal.

Advantage of this feature can be through filter with a square wave group or composite noise, while the get to a particular frequency sine wave.

Filter is formed by an inductor and a capacitor network, can be separated mixed DC current. Power adapter, that by means of this network in the filter net pulsating DC ripple, to obtain relatively pure DC output. The most basic wave filter, is by a capacitor and one inductor is constituted, known L-type filtering. All the various types of filters, are a collection of L-type single filter from. Basic Single-Festival-style filter consists of a series arm AND a parallel arm composed of, series arm is the inductor, parallel arm for the capacitors, shown in Figure 3-67 shown below. In the power supply and sound and frequency circuit middle of the wave filter, the most general-purpose those who for the L-type and π-type two kinds. The L-block filter, its inductive reactance XL and the capacitor anti XC, for any frequency is constant, and the relationship between

XL · XC = K2

Therefore, L-type filters, also known as K constant filter. If a filter component, representing constant K-type having relatively sharp cut-off frequency (i.e. the frequency selectivity), while this cut-off frequency other than the frequency is only a small attenuation ratio, known as m constant filter. The so-called cut-off frequency, i.e. the filter has a sharp resonance frequency. Pass band and band stop filter are all m constant filter, m is the cutoff Usage Frequency and being attenuated other frequencies of the attenuation ratio of the function. Each m constant filter and the impedance of K constant filter the relationship between, by the m constant determines the, this constant between 0 ~ between 1 and. When m is close to zero value, the higher the sharpness of the cutoff frequency, the cutoff frequency for the frequency with which the attenuation rate will decrease. The most together in practical m is 0.6. As for which frequencies need to be cut-off, adjustable resonant arm to determine. m constant filter cutoff frequency attenuation is determined by the total of rallying the effective Q value of size. If the constants of K and m are constant filter composed of cascaded circuit, get sharp filtering effect and good frequency attenuation.

Filter Type
Butterworth response (maximally flat response)
Butterworth response to maximize filter passband flatness. The response is very flat, very close to a DC signal, and then slowly decay to a cutoff frequency of-3dB, the final approach -20ndB/decade attenuation rate, where n is the filter order. Butterworth Butterworth filter particularly suitable for low frequency applications, its for the maintenance of gain flatness, is very important.

Bessel response
Addition to the will be change depend on frequency of the input signal amplitude outer, the filter will also introduced its a delay. Delay allows frequency-based phase shift non-sinusoidal signal distortion. Like

Butterworth response using the pass-band maximizes the amplitude flatness as Bessel response minimizes passband phase linearity.

In some applications which, most important factor is the filter truncation unnecessary speed of the signal. If you could acceptable Passband has some ripple, you can get the 比巴特沃斯 the filter more rapid attenuation. Appendix A contains of the design up to 8-order the concrete Butterworth, Bessel AND CHEBYSHEV response filter the required parameters's form. Two tables for Chebychev response: one for 0.1dB maximum passband ripple; another for 1dB maximum passband ripple.

Order of the filter the number of
Order of the filter is the filter transfer function has several poles. Order also determines the transition zone of the rate of decline, generally each additional step (a pole), will increase a 20dBDec (a 20dB per ten octave).

Filter Design
Filter characteristics can be use it frequency response to describe the, according to their characteristics, the can be divided into low-pass filter, high pass filter, Bandpass filter and band stop filter and so on.

Used to describe the filter performance of the technical indicators are:

The center frequency f0, that is the operating band center
Bandwidth BW
Passband attenuation, the pass band of the maximum attenuation
Stopband attenuation

For the actual filter, having regard to the actual constituent elements of the quality factor value is a finite value (because of the limited material and process level), all the engineering practical filters are lossy filters, Therefore, for these filters should be considered passband minimum insertion attenuation.

Modern filter design, mostly using filter transformation method to be implemented. Mainly through the low-pass prototype filter frequency conversion and impedance conversion, to obtain a new target filter.

Filter selection
Lumped low-pass prototype filter yes modern network Syndicated news method design wave filter the basis for, a variety of low pass, high pass, band pass, band stop filter big are all Pursuant to this characteristics of deduced out of the. For this reason, only to makes the filter design able to simplify the, the precision can be improved.

Ideal low pass filter should enable the all below the cutoff frequency lossless signal pass, while all above the cutoff frequency signals should both be infinite to decay, thereby in the amplitude-frequency characteristic curve render a rectangle at, Guer also called rectangle filtering device (brick-wallfilter). Unfortunately, such desirable properties can not be achieved, all designed merely trying to close the rectangular filter characteristics only. Depending on the approximation of the function, you can get a different response. Although the approximation function function varied, but taking into account actual circuit demand for the use, we usually will choose "Butterworth response" or "Chebyshev response."

"Butterworth response" bandpass filter has a flat response, and "Chebyshev" band-pass filter has a steeper attenuation characteristic. So the specific selection of the characteristics required of the circuit or system specific requirements. However, "Chebyshev filter" for component changes the least sensitive, and both a good selectivity and a good VSWR (in the passband center), so in general applications, it is recommended to use "Chebyshev response" filter.

Filter classification
1, according to the processed signal

According to the processed signal into analog and digital filters two kinds.

2, classified according to the through the signal of the frequency band

Classified according to the through the signal's frequency band divided into low pass, high pass, band-pass and bandstop wave filter four kinds.
Low Pass Filter: It allows the signal a low frequency or DC component to pass and suppresses high-frequency component or the interference and noise.
High-pass filter: It allows the signal of the high frequency component to pass and suppresses the low frequency or DC component.
Band-pass filter: It allows certain frequency band signal passes through, inhibiting lower or higher than the band signal, interference, and noise.
Bandstop filter: it inhibits the certain frequency band of the signal, allowing the band other than the signals.

3, according to the of components used

According to the used components are divided into passive and active filters two kinds.

3.1, passive Noise Filter: only by the passive components (R, L and C) filter composed of, it is the use capacitors and inductance element reactance variation with frequency which changes principle of constituted. Type of filter has the advantage of: the circuit is relatively simple, does not require a DC power supply, and high reliability; disadvantages are: passband signal with energy loss, the load effect is more obvious, the use of inductive components easily lead to electromagnetic induction, the inductance L large filter size and weight are relatively large in the low frequency range is not applicable.

3.2, active filters: composed of passive Components (general use R and C) and active devices (such as integrated op-amp) composed of. The advantage of this type of filter are: passband signal only no energy loss, but also can be enlarged, the load effect is not obvious, the associated multi-level interaction is small, the use of a simple method is easily cascaded filter constituted order device, and the filter the volume of the small, the weight light, does not require magnetic shielding (because they do not using inductive element); drawback is: Passband Range is affected active devices (such as integrated op-amp) the bandwidth of constraints, the need DC power supply, reliable not as good as passive filter high in the high-pressure, high-frequency, high-power applications is not applicable.

4, according to the placement of the filter

According to filter is placed different positions, generally divided into on-board filter and the faceplate filters.

4.1, on-board filter is mounted in the circuit board, such as the PLB, JLB series of filters. This filter has the advantage of economy, the disadvantage is high frequency filtering ineffective. The main reason is:

4.1.1, the filter's between input and output there is no isolation, prone to coupling;

4.1.2, the filter grounding impedance is not very low, weakening the frequency bypass effect;

4.1.3, Filter Banks and the chassis section between the connection will produce two kinds Adverse effects: a is the chassis internal space's electromagnetic interference will have a direct induction to the this period of online, along the cable efferent the chassis, with the help cable radiation, to make wave filter lapsed; the other is the outside interference in the being on-board wave filter prior to filtering, with the help this period of line produce radiation, or directly to the line the PCB circuit occurrence of coupling, causing sensitivity to the problem;

4.2, filter array plates, filtered connectors such as panel filters are generally mounted directly on the shielded enclosure metal panels. Due to the direct installed in the metal panels on the, the filter's

Between input and output completely isolated, grounding good, on the cable interference in the chassis on the port be filtered out, therefore filtering the effect is quite satisfactory. Disadvantage is that early in the design considerations required for installation with the structure.

Digital Filter
With analog filters corresponding to the discrete systems widely used digital filter. Its role is to use the discrete-time system characteristics of the input signal waveform or frequency processing. In other words, the input signal into a certain output signal, so as to achieve the purpose of changing the signal spectrum. Digital filter can generally be achieved in two ways: One way is to use digital hardware assembled into a dedicated device, this device is called a digital signal processor; Another method is the direct use of general-purpose computer, which will need arithmetic compiled program lets general-purpose computer to complete, namely the use of of computer software to achieve.

Analog filter
Analog filter in the test system or a dedicated instrument middle is a kind of commonly used in transform devices. For example: Bandpass filter is used as spectrum analyzer in the selected frequency device; low-pass filter is used as digital signal analysis system in the anti frequency mixed filter; high-pass filter been used acoustic emission detector to exclude low-frequency interference noise; band-stop filter is used as electric eddy current vibrometer in a notch filter, and so forth.

For spectral analysis apparatus of the band-pass filter, according to the center frequency and bandwidth of the question numerical relationship, divided into two:

One kinds is the bandwidth B does not Sui center frequency of the person and change, known as the constant band broadband-pass filter, its center frequency in any frequency bands on the O'clock, the bandwidth Du the same;

Another B is the bandwidth and center frequency ratio is the same, called the constant bandwidth than the band-pass filter, the higher its center frequency, the wider bandwidth.

SAW filters

SAW refers to the surface of the elastic wave propagation, the wave is called elastic surface acoustic wave. SAW propagation velocity of the electromagnetic wave speed is about little more than 100,000 times. SAW filters are used quartz piezoelectric ceramic piezoelectric materials, using its piezoelectric effect acoustic wave propagation physical characteristics made of a filter-specific devices, widely used in televisions and video recorders IF circuits in the, to replace the LC intermediate-frequency filter, make the image, the sound greatly improve the quality.
SAW SAW filters, SAW resonator in the piezoelectric substrate surface is generated and transmitted, and the amplitude with deep substrate of the depth of the material increases rapidly decrease in the elastic wave. Surface acoustic wave (SAW) is the dissemination Yu piezoelectric crystal the surface of the mechanical wave, and its sound velocity of only electromagnetic wave-speed's one hundred thousandth, dissemination attenuation is very small.

SAW sound table devices are In-piezoelectric substrate on the use of microelectronic technology and the Country of interdigital-shaped electric acoustic transducer and the reflectors couplers and other, the use of substrate sheet material piezoelectric effect, through the input interdigital interdigital transducer (IDT) to convert electrical signals into acoustic signals and limit the spread of the substrate surface, the output IDT of the sound signal is restored into an electric signal, electrically - acoustic - electric conversion process, to complete the process signal, to obtain an electronic device for various purposes. Uses advanced microelectronic processing technology to produce surface acoustic wave device, a small size, light weight, high reliability, consistency, versatile and flexible design advantages.

Dielectric filter

Dielectric filter using the dielectric ceramic materials for low loss, high dielectric constant, frequency-temperature coefficient and the thermal expansion coefficient is small, can withstand high power and other features designed and manufactured by a number of long resonators in series or parallel vertical multi-stage ladder line composition. Which is characterized is to insert loss small, resistance to power resistance is good, narrow bandwidth, especially suitable for CT1, CT2, 900MHz, 1.8GHz, 2.4GHz, 5.8GHz, portable telephone, automobile telephone, wireless headsets, wireless microphone, radio, cordless phones and integrated transceiver duplexer filter coupled to the other level.

Filter Installation Precautions
While on-board high frequency filter filtering effect is not ideal, but if properly applied, can meet most of the requirements for electromagnetic compatibility civilian products. When in use should pay attention to the following matters:

A "clean"

If you decide to use the on-board filter in the wiring should pay attention to the cable port of leaving a "clean", filters and connectors are installed in the "clean" on. From the previous discussion, we know that the signal ground line interference is very serious. If the cable directly to the filter capacitor connected to this ground line, can cause serious common-mode radiation. In order to achieve better filtering effect, you must prepare a cleanly. And with the signal ground can only be connected at one point up, the flow point is called "The Bridge", all signal lines are passed from the bridge to reduce the signal loop area.

2, the filter to be juxtaposed

The same set of cables for all conductors unfiltered part - since the filtered parts together. Otherwise, a conductor Louis filter section will another wire that has filtered partial re-contamination 9 so that the cable overall filter failure.

3, the filter should be as close to the cable port

Filter and the panel between the wire distance should be as short as possible. When necessary,, the use of metal plate obscured look, isolation near-field interference.

4, filter and chassis lap

Installation wave filter's dry Zheng ground want to with the metal chassis to reliably lap up and, if the chassis not metal's, in respect in the line board beneath set the a larger metal plate come as the filtering ground. Cleanly with the metal chassis between the of the lap want to guarantee a very low RF impedance. If necessary, can use the electromagnetic gasket lap, to increase lap area, reducing the RF impedance.

Five, filter grounding line is to be short

Taking into account the pin inductance effect, and its importance for discussing previously, the filter circuit board layout and design of local chassis (metal plate) connection structure should pay special attention.

6, the filter line and unfiltered line grouping

In the Port filtering's cables and do not filtering's cable should be as far as possible far away from, to prevent the occurrence of the above of the coupling problem.

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