The waveguide filter is an electronic filter made with waveguide technology. Waveguides are hollow metal tubes in which electromagnetic waves can be transmitted. Filter is a device used to allow signals at multiple frequencies to pass (passband), while others are rejected (stopband). Filters are basic components of electronic engineering design and have many applications. This includes signal selection and noise limitation. Waveguide filters are most useful in microwave frequency waves, where they are a comfortable size and have low losses. Examples of the use of microwave filters are found in satellite communications, telephone networks, and television broadcasts.
Waveguide filters were developed during World War II to meet the needs of radar and electronic countermeasures, but then soon found civil applications such as use in microwave links. Much of postwar development relates to mass and weight reduction of these filters, first by using new analytical techniques that lead to the elimination of unnecessary components, then by innovations such as double mode cavities and new materials such as ceramic resonators.
The special feature of the waveguide filter design concerns the transmission mode. The system based on the cable pair performs and similar technology has only one transmission mode. In the waveguide system, a number of modes are possible. This can be a disadvantage, because fake modes often cause problems, and gain, because the design of multiple modes can be much smaller than the design of an equivalent single waveguide mode. The main advantage of waveguide filters over other technologies is their ability to handle their high power and low losses. The main drawback is their cost and cost when compared to technologies such as microstrip filters.
There are different types of different waveguide filters. Many of them consist of a combined resonator chain of several types that can be modeled as ladder network LC circuits. One of the most common types consists of a number of coupled resonance cavities. Even in this type, there are many subtypes, mostly distinguished by clutch means. This type of coupling includes a hole, [w] iris, [x] and post. Other types of waveguide filters include dielectric resonator filters, insert filters, finline filters, wavy waveguide filters, and stub filters. A number of waveguide components have a filter theory applied to their design, but the goal is something other than filtering the signal. Such devices include impedance matching components, directional couplers, and diplexers. These devices often take the form of filters, at least in part.
Video Waveguide filter
Coverage
The general meaning of waveguide , when this term is used is not eligible, is a hollow metal type, but other waveguide technologies are possible. The scope of this article is limited to the type of metal-tube. The post-wall waveguide structure is something of a variant, but related enough to be included in this article - the waves are mostly surrounded by conduction materials. It is possible to build waveguides from dielectric rods, the most famous example being the optical fiber. This subject is outside the scope of the article with the exception that dielectric stem resonators are sometimes used within hollow metal waveguides. Transmission lines [o] technologies such as conducting cables and microstrip can be regarded as waveguides, but not commonly referred to as such and also beyond the scope of this article.
Maps Waveguide filter
Basic concepts
Filters
In electronics, filters are used to allow signals from a particular frequency band to pass while blocking another. They are the basic building blocks of electronic systems and have many applications. Among the use of waveguide filters is the construction of duplexer, diplexer, [d] and multiplexer; selectivity and noise limitation in receiver; and harmonic distortion suppression in the transmitter.
Waveguides
Waveguides are metal channels used to limit and direct radio signals. They are usually made of brass, but aluminum and copper are also used. Generally they are rectangular, but other cross-section such as circle or ellipse may be. The waveguide filter is a filter consisting of a waveguide component. It has many of the same application ranges as other filter technologies in electronics and radio engineering but is very different mechanically and in principle of operation.
The technology used to build filters is selected to a large extent by expected operating frequency, although there is a large amount of overlap. Low frequency applications such as electronic audio use filters consisting of discrete capacitors and inductors. Somewhere in the very high frequency band, the designer switched to using components made from cutting transmission lines. [p] These types of designs are called distributed element filters. Filters made of discrete components are sometimes called equivalent element filters to distinguish them. At higher frequencies, the microwave band, the design switches to a waveguide filter, or sometimes a combination of waveguides and transmission lines.
Waveguide filters have much in common with transmission line filters rather than equivalent element filters; they do not contain discrete capacitors or inductors. However, the design of the waveguide may often be equivalent (or so) with the equivalent element design. Indeed, the design of a waveguide filter often begins with the design of a lumped element and then converts that design element into a waveguide component.
Mode
One of the most important differences in the operation of a waveguide filter is compared to the transmission line design concerning the transmission mode of electromagnetic waves carrying signals. In the transmission line, waves are associated with electrical current on a pair of conductors. The conductor limits the current to parallel to the line, and consequently both the magnetic and electrical components of the electromagnetic field are perpendicular to the direction of the wave travel. This transversal mode is set TEM [l] (transverse electromagnetic). On the other hand, there are so many modes that actually have an empty waveguide, but the TEM mode is not one of them. The waveguide mode is set either TE [m] (transverse) or TM [n] (transverse magnet), followed by a pair of suffixes which identifies the correct mode.
The diversity of this mode can cause problems in the waveguide filter when fake mode is generated. Designs are usually based on a single mode and often incorporate features to suppress unwanted modes. On the other hand, profits can be gained from choosing the right mode for the app, and sometimes even using more than one mode at a time. Where only one mode is used, the waveguide can be modeled like a conduction transmission path and the results of the transmission line theory can be applied.
Cutoff
Another characteristic feature for waveguide filters is that there is a definite frequency, cutoff frequency, below no transmission can occur. This means that in theory a low-pass filter can not be made in waveguides. However, designers often take lumped elements of low-pass filter design and turn it into a waveguide implementation. These filters are consequently low-pass by design and can be considered as low-pass filters for all practical purposes if the cutoff frequency is below the frequency of interest to the application. Frequency cutout waveguide is a function of transmission mode, so at certain frequencies, waveguide can be used in some modes but not others. Likewise, the wavelength guides [h] (< g ) and characteristic impedance [b] ( Z 0 ) from the guide at the given frequency also depends on the mode.
Dominant mode
The mode with the lowest cutoff frequency of all modes is called the dominant mode. Between the cutoff and the next highest mode, this is the only mode that is likely to be transmitted, which is why it is described as dominant. All the false modes generated are quickly attenuated throughout the guide and immediately disappear. Practical filter designs are often made to operate in dominant mode.
In a rectangular waveguide, the TE mode 10 [q] (shown in figure 2) is the dominant mode. There is a frequency band between the dominant mode cutoff and the next highest mode termination in which the waveguide can be operated without the possibility of producing a fake mode. The next highest cutoff mode is TE 20 , [r] exactly twice TE 10 mode, and TE 01 [s] which is also twice TE 10 if the waveguide used has a commonly used aspect ratio 2: 1. The lowest cutoff mode TM is TM 11 [t] (shown in picture 2) that times the dominant mode in 2: 1 waveguide. Thus, there is an octave where the dominant mode is free of false mode, although surgery too close to the cutoff is usually avoided due to phase distortion.
In a circular waveguide, the dominant mode is TE 11 [u] and shown in figure 2. The next highest mode is TM 01 . [v] The range in which the dominant mode is guaranteed to be a false free mode is less than that in a rectangular waveguide; the highest-to-lowest-frequency ratio is about 1.3 in a circular waveguide, compared to 2.0 in the rectangle guide.
Landing mode
Evanescent mode is a mode below the cutoff frequency. They can not propagate to the waveguide for any distance, dying exponentially. However, they are important in the function of certain filter components such as irises and posts, which are described later, because energy is stored in evanescent waves.
Advantages and disadvantages
Like the transmission line filter, the waveguide filter always has many passbands, a replica of the unified element prototype. In most designs, only the lowest frequency passband is useful (or two lowest in case of a stop-band filter) and the rest is considered an unwanted fake artifact. This is an intrinsic property of technology and can not be designed out, although the design can have control over the frequency positions of false bands. As a result, in the filter design provided, there is a frequency above which the filter will fail to perform its function. For this reason, the correct low-pass and high-pass filters can not exist in the waveguide. At some high frequency there will be a false passband or stopband which interfere with the desired function of the filter. But, similar to the situation with the cutoff waveguide frequency, the filter can be designed so that the first fake band edge is far above the interesting frequency.
The frequency range at which a waveguide filter is useful is largely determined by the size of the required waveguide. At lower frequencies, the waveguide needs to be impractical to keep the cutoff frequency below the operational frequency. On the other hand, filters whose operating frequencies are so high that their wavelengths below millimeters can not be produced by normal machine-shop processes. At this high frequency, optical fiber technology began to be an option.
Waveguides are low-loss media. The disadvantages in waveguides are largely derived from ohmic dissipation caused by induced currents in the waveguide wall. The rectangular waveguide has a lower loss than the circular waveguide and is usually the preferred format, but the circular mode TE 01 is very low and has applications in remote communications. Losses can be reduced by polishing the internal surface of the waveguide wall. In some applications that require strict filtering, the walls are coated with a thin layer of gold or silver to increase surface conductivity. Examples of such requirements are satellite applications that require low losses, high selectivity, and linear group delay from the filter.
One of the main advantages of the waveguide filter over TEM mode technology is its resonator quality. The quality of the resonator is characterized by a parameter called a Q factor, or just Q . The Q of the waveguide resonator in the thousands, folds higher than the TEM mode resonator. The resistance of the conductor, especially in the wound inductor, limits the Q of the TEM resonator. This increase Q leads to better-performing filters in waveguides, with larger stop band rejection. The limitations for Q in waveguides largely derive from the ohmic losses in the wall described earlier, but the internal wall plating silver can more than double Q .
Waveguides have good power handling capabilities, leading to filter applications on the radar. Despite the performance advantages of waveguide filters, microstrip is often the preferred technology because of its low cost. This is especially true for consumer goods and lower microwave frequencies. Microstrip circuits can be manufactured with cheap printed circuit technology, and when integrated on the same printed boards with other circuit blocks, they incur a little extra cost.
History
The idea of ​​a waveguide for electromagnetic waves was first proposed by Lord Rayleigh in 1897. Rayleigh proposed that the coaxial transmission line could have the center of the conductor removed, and the waves would still propagate to the interior of the remaining cylindrical conductors even though it no longer exists into a complete conductor electrical circuit. He describes this in terms of a wave that bounces repeatedly from the external wall of the outer conductor in a zigzag way as it moves down the waveguide. Rayleigh was also the first to realize that there is a critical wavelength, the cutoff wavelength, proportional to the diameter of the cylinder, on which the wave propagation is not possible. However, interest in waveguides is reduced because lower frequencies are more suitable for long distance radio communications. Rayleigh's results were temporarily forgotten and had to be rediscovered by others in the 1930s when interest in microwaves was revived. Waveguides were first developed, in a circular shape, by George Clark Southworth and J. F. Hargreaves in 1932.
The first analog filter design that went beyond a simple single resonator was invented by George Ashley Campbell in 1910 and marked the beginning of the filter theory. The Campbell filter is a unified design of the capacitor and inductor elements suggested by his work with the loading coil. Otto Zobel and others quickly developed this further. The development of distributed element filters began in the years before World War II. A major paper on this issue was published by Mason and Sykes in 1937; a patent filed by Mason in 1927 may contain the first published filter design using distributed elements.
The work of Mason and Sykes focused on the format of coaxial cables and balanced pairs of cables, but other researchers then applied these principles to waveguides as well. Much of the development on the waveguide filters carried out during World War II was driven by the need for radar screening and electronic countermeasures. Many of these are at the MIT Radiation Laboratory (Rad Lab), but other laboratories in the US and UK are also involved like the UK Telecommunications Research Establishment. Among the famous scientists and engineers at Rad Lab are Julian Schwinger, Nathan Marcuvitz, Edward Mills Purcell, and Hans Bethe. Bethe only at Rad Lab in a short time but produced his aperture theory there. The aperture theory is important for waveguide cavity filters, which were first developed at Rad Lab. Their work was published after the war of 1948 and included an early description of the dual-mode cavity by Fano and Lawson.
Theoretical work after the war included a parallel line theory from Paul Richards. The corresponding channel is the network in which all elements have the same length (or in some cases multiples of the length of the unit), although they may differ in other dimensions to provide different characteristic impedances. [a] The Richards transform enables each design of the equivalent element to be taken "as is" and transformed directly into the design of a distributed element using a very simple transformation equation. In 1955 K. Kuroda published a transformation known as Kuroda's identity. This makes Richard's work more useful in unbalanced formats and wizards by eliminating the elements associated with troubled series, but it was some time before Kuroda's work in Japan became widely known in the English-speaking world. Another theoretical development is Wilhelm Cauer's network filter synthesis filter in which he uses the Chebyshev approach to determine element values. Cauer's work was largely developed during World War II (Cauer was killed near the end of the war), but could not be widely published until hostilities end. While Cauer's work concerns lumped elements, it is of some importance to the waveguide filter; Chebyshev filter, a special case of Cauer synthesis, is widely used as a prototype filter for waveguide design.
The design of the 1950s began with a prototype of lumped elements (a technique still in use today), arriving after various transformations on the desired filter in the form of a waveguide. At that time, this approach yields fractional bandwidth of no more than about 1 / 5 . In 1957, Leo Young at Stanford Research Institute published a method for designing filters that begin with the prototype of distributed elements, the prototype impedance stepping. This filter is based on a quarter wave pull transformer of various widths and is capable of producing designs with bandwidth up to one octave (fractional bandwidth / 3 ). Young's paper specifically discusses cavity resonators that are combined directly, but the procedures can equally be applied to other types of directly related resonators.
The first account issued from cross-linked filters is due to John R. Pierce at Bell Labs in a 1948 patent. Cross-coupled filters are filters where unrestricted resonators are directly combined. Therefore, an additional degree of freedom allows designers to create filters with enhanced performance, or, alternatively, with fewer resonators. One version of the Pierce filter, shown in Figure 3, uses a circular waveguide cavity resonator to connect between rectangular guide cavity resonators. This principle was initially not widely used by the designers of waveguide filters, but it was used extensively by mechanical filter designers in the 1960s, especially R. A. Johnson at Collins Radio Company.
The initial non-military applications of waveguide filters reside in the microwave connections used by telecommunication companies to provide their network backbone. This link is also used by other industries with large fixed networks, especially television broadcasters. Such applications are part of a large capital investment program. They are now also used in satellite communication systems.
The need for independent frequency delays in satellite applications led to more research into the waveguide incarnation of crosslinked filters. Previously, satellite communications systems used a separate component to delay even distribution. The additional degree of freedom gained from the crossover filter holds the possibility of designing a flat delay to the filter without compromising other performance parameters. Components that simultaneously function both as filters and equalizer will save weight and valuable space. The need for satellite communications also encouraged research into a more exotic resonator mode in the 1970s. The main advantages in this regard are the work of E. L. Griffin and F. A. Young, who investigated the better mode for the band 12-14 GHz when it began to be used for satellites in the mid-1970s.
Another space-saving innovation is a dielectric resonator, which can be used in other filter formats as well as waveguide. The first use of this in the filter was by S. B. Cohn in 1965, using titanium dioxide as a dielectric material. The dielectric resonator used in the 1960s, however, has a very bad temperature coefficient, usually 500 times worse than a mechanical resonator made of invar, which causes instability of filter parameters. Dielectric materials of time with better temperature coefficients have dielectric constants that are too low to be useful for saving space. This changed with the introduction of ceramic resonators with very low temperature coefficients in the 1970s. The first was from MassÃÆ'Â © and Pucel using barium tetratitanate at Raytheon in 1972. Further improvements were reported in 1979 by Bell Labs and Murata Manufacturing. Barium nonatitanate resonator Bell Labs has 40 dielectric constants and Q from 5000-10000 at 2-7 GHz . Modern temperature stabilizers have a dielectric constant of about 90 at microwave frequencies, but research continues to find materials with both low losses and high permittivity; Lower permittivity materials, such as zirconium stannate titanate (ZST) with 38th dielectric constant, are still sometimes used for low property loss.
An alternative approach to designing smaller waveguide filters is provided by the use of non-propagating evanescent modes. Jaynes and Edson proposed an evanescent waveguide filter in the late 1950s. The method for designing this filter was made by Craven and Young in 1966. Since then, waveguide mode evanescent wave has seen a successful use in which the size or weight of the waveguide is an important consideration.
The relatively new technology used in hollow-metal-waveguide filters is a finline, a type of planar dielectric waveguide. Finline was first described by Paul Meier in 1972.
Multiplexer history
The multiplexer was first described by Fano and Lawson in 1948. Pierce was the first to describe a multiplexer with passocolous passbands. Multiplexing using directional filters was created by Seymour Cohn and Frank Coale in the 1950s. The multiplexer with unexpected resonator compensation at each intersection is largely the work of E. G. Cristal and G. L. Matthaei in the 1960s. This technique is still occasionally used, but the availability of modern computing power has led to the use of more general synthesis techniques that can directly produce the appropriate filters without the need for these additional resonators. In 1965, RJ Wenzel found that filters ended singly, [k] instead of normally ending multiple, complementary - exactly what is needed for diplexer. [c] Wenzel was inspired by a lecture by circuit theorist Ernst Guillemin.
Multi-channel, multi-octave multiplexer was investigated by Harold Schumacher at Microphase Corporation, and the results were published in 1976. The principle that multiplexer filters can be matched when combined together with modifying some of the first elements, thus removing the compensation resonator, was discovered accidentally by EJ Curly around 1968 when he blurs a diplexer. A formal theory for this was provided by J. D. Rhodes in 1976 and generalized to multiplexers by Rhodes and Ralph Levy in 1979.
From the 1980s, planar technology, especially microstrip, tended to replace other technologies used to build filters and multiplexers, especially in products intended for the consumer market. The new innovation of post-wall waveguide enables waveguide design to be implemented on flat substrates with low-cost manufacturing techniques similar to those used for microstrip.
Components
Waveguide filter design often consists of two different components that are repeated several times. Typically, one component is a resonator or discontinuity with a equivalent circuit equivalent to an inductor, capacitor, or LC resonance circuit. Often, this type of filter will take its name from the style of this component. These components are separated by a second component, the length of the guide acting as an impedance transformer. The impedance transformer has the effect of making an alternative instance of the first component seem to be a different impedance. The net result is the equivalent equivalent element circuit of the ladder network. Eliminating element filters are usually ladder topologies, and such circuits are a typical starting point for the design of waveguide filters. Figure 4 shows a ladder like that. Normally, the waveguide component is a resonator, and the equivalent circuit is the LC resonator instead of the indicated capacitor and inductor, but the circuit like figure 4 is still used as a prototype filter with the use of band-pass or band-stop transformation.
Filter performance parameters, such as stopband rejection and transition rate between passband and stopband, are enhanced by adding more components and thus increasing the length of the filter. Where components are repeated identically, filters are the design of image parameter filters, and performance is improved only by adding more identical elements. This approach is commonly used in filter designs that use a large number of closely spaced elements such as waffle-iron filters. For designs with wider elements, better results can be obtained by using a network synthesis filter design, such as the common Chebyshev filter and the Butterworth filter. In this approach the circuit elements do not all have the same value, and consequently the components are not all of the same dimensions. Furthermore, if the design is enhanced by adding more components then all element values ​​must be calculated again from the beginning. In general, there will be no common value between the two design instances. Chebyshev waveguide filters are used where strict filtering requirements, such as satellite applications.
Transformer impedance
An impedance transformer is a device that makes the impedance on its output port appear as a different impedance on its input port. In waveguide, this device is only a short wave guide. Especially useful is a quarter wave transformer that has a long impedance? g /4. This device can convert capacitance to inductance and vice versa. It also has properties that are useful for converting elements that connect to shunts into serial-linked elements and vice versa. The elements connected to the circuit are not difficult to apply in a waveguide.
Reflections and discontinuities
Many waveguide filter components work by introducing sudden changes, discontinuities, to the transmitting properties of the waveguide. Such discontinuities are equivalent to the truncated impedance elements placed at that point. It appears in the following way: discontinuity causes partial reflection of the transmitted wave back down the guide in the opposite direction, the ratio of the two known as the reflection coefficient. This is entirely analogous to the reflection on the transmission line where there is an established relationship between the reflection coefficient and the impedance causing the reflection. This impedance must be reactive, that is to say, it must be either capacitance or inductance. It can not be a resistance because no energy is absorbed - it's all either transmitted forward or reflected. Examples of components with this function include iris, stubs, and posts, all described later in this article under the type of filter where they occur.
Impedance stages
The impedance step is an example of a device that introduces discontinuity. This is achieved by step change in the physical dimension of the waveguide. This results in a step change in the characteristic impedance of the waveguide. This step can be in E-plane [f] (high change [j] ) or H-path [g] (change width [i] ) of the waveguide.
Resonant cavity filter
Cavity resonator
The basic component of a waveguide filter is a cavity resonator. It consists of shortwave waves that are blocked at both ends. The waves trapped inside the resonator are reflected back and forth between the two ends. The given cavity geometry will resonate at the characteristic frequency. Resonance effects can be used to selectively pass certain frequencies. Its use in the filter structure requires that some waves be left out of one cavity to the other through the coupling structure. However, if the opening in the resonator remains small then the valid design approach is to design the cavity as if it is completely closed and the error will be minimal. A number of different coupling mechanisms are used in different filter classes.
The nomenclature for mode in cavities introduces a third index, for example TE 011 . The first two indexes describe the waves moving up and down along the cavity, that is, they are transverse-like numbers for the mode in the waveguide. The third index describes the longitudinal mode caused by the interference pattern of forward travel waves and reflections. The third index equals the half-wavelength number along the guide. The most commonly used modes are the dominant mode: TE 101 in the rectangular waveguide, and TE 111 in the circular waveguide. Circular mode TE 011 is used if very low loss (hence high Q ) is required but can not be used in dual mode filters due to circular symmetry. The better modes for square waveguides in dual-mode filters are TE 103 and TE 105 . However, even better is the circular waveguide mode TE 113 that can reach Q 16,000 at 12 GHz .
Tuning screw
The tuning screw is a screw inserted into the external resonant cavity that can be adjusted externally with a waveguide. They provide resonance frequency tuning by inserting more, or less threads to the waveguide. An example can be seen in the postal filter in Fig. 1: each cavity has a secured tuning screw with peanut butter and a locking-locking compound. For screws inserted only small distances, the equivalent circuit is a shunt capacitor, increasing in value when the screw is inserted. However, when the screws have been inserted the distance?/4 it resonates with the equivalent circuit series LC. Putting it further causes the impedance to change from capacitive to inductive, that is, the arithmetic sign is changed.
Iris
An iris is a thin metal plate along a waveguide with one or more holes in it. This is used to unify the two waveguide lengths and is a means to introduce discontinuities. Some iris geometry may be shown in figure 5. An iris that reduces rectangular waveguide width has the same circuit of shunt inductance, whereas a high limit is equivalent to shunt capacitance. An iris that limits both directions is equivalent to a parallel LC resonance circuit. Circuit LC series can be formed by adjusting the distance of the iris portions of the waveguide wall. Narrowband filters often use slices with small holes. This is always inductive regardless of the shape of the hole or its position on the iris. A simple circular hole for the machine, but the elongated hole, or hole in a cross shape, is advantageous in allowing the selection of certain clutch modes.
Iris is a form of discontinuity and works with a higher evanescent mode. The vertical edges are parallel to the electric field (field E) and TE excite mode. The energy stored in TE mode is mostly in the magnetic field (H field), and consequently the equation of the structure is the inductor. The horizontal edge is parallel to the H field and the excite TM mode. In this case the energy stored mostly in the E field and the equivalent equivalent is a capacitor.
It's easy enough to create a mechanically adjustable slice. A thin metal plate can be pushed in and out of the narrow slot on the side of the waveguide. Iris construction is sometimes chosen because of this ability to create variable components.
Iris-coupled Filter
An iris-coupled filter consists of a cascade of impedance transformers in the form of a waveguide resonance cavity coupled together by an iris. In high power applications, capacitive slices are avoided. The reduction of the height of the waveguide (the direction of the E plane) causes the electric field strength in the gap to increase and warp (or the dielectric damage if the waveguide is filled with the insulator) will occur at a lower power than it should be..
Post filter
The post performs a bar, usually circular, stays internally throughout the height of the waveguide and is another means of introducing discontinuities. The thin post has a circuit equivalent to a shunt inductor. A line of posts can be seen as an inductive iris shape.
The post filter consists of several post lines along the width of the waveguide that separates the waveguide into the resonant cavity as shown in figure 7. Different quantities of posts can be used in each line to achieve various inductance values. An example can be seen in Figure 1. Filters operate in the same way as iris-coupled filters but differ in construction methods.
Post-wall waveguide
A post-wall waveguide, or integrated waveguide substrate, is a newer format that seeks to combine the advantages of low radiation losses, high Q , and high power handling of traditional hollow metal pipe waveguides with small sizes and ease of manufacture of planar technology (such as the widely used microstrip format). It consists of a pierced isolated substrate with two post lines doing that stands on for the side wall of the waveguide. The top and bottom of the substrate is covered with conduction sheets that make the construction similar to the triple format. Manufacturing techniques that exist on printed circuit boards or low-temperature ceramics can be used to create a series of wall post waves. This format is naturally suitable for post-waveguide filter design.
Double mode filter
The dual-mode filter is a resonant cavity filter type, but in this case each cavity is used to provide two resonators using two modes (two polarizations), thus halving the filter volume for a particular sequence. Increasing the size of these filters is a major advantage in aviation and aerospace aviation applications. High quality filters in this application can require many cavities that occupy significant space.
Filter dielectric resonator
The dielectric resonator is a part of the dielectric material inserted into the waveguide. They are usually cylindrical because these can be made without machines but other forms have been used. They can be made with a hole through the center used to secure them to the waveguide. There is no field in the center when the circular mode TE 011 is used so that the hole has no adverse effect. Resonators can be fitted coaxial to the waveguide, but they are usually mounted transversely along the width as shown in Figure 8. The latter arrangement allows the resonator to be adjusted by inserting the screw through the waveguide wall to the central hole of the resonator.
When dielectric resonators are made of high permittivity, such as one barium titanate, they have an important space-saving advantage compared to cavity resonators. However, they are much more vulnerable to fake mode. In high power applications, a metal layer can be built into the resonator to conduct heat because the dielectric material tends to have low thermal conductivity.
Resonators can be combined together with iris or impedance transformers. Alternatively, they can be placed in a side housing such as a stub and are combined through a small gap.
Enter filter
In inserting filters one or more metal sheets are placed longitudinally along the waveguide as shown in FIG. 9. This sheet has a hole hollowed inside to form a resonator. The air dielectric gives this resonator a high Q . Some parallel inserts can be used in the same waveguide length. A more compact resonator can be achieved with thin sheets of dielectric material and metallic printing instead of holes on metal sheets with lower resonator costs Q .
Filter the end line
Finline is a different type of waveguide technology in which waves in thin strips of dielectrics are constrained by two strips of metallization. There are a number of possible topological settings of the dielectric and metal strips. Finline is a variation of slot-waveguide but in the case of finline the entire structure is flanked by a metal shield. It has the advantage that, like a hollow metal waveguide, no power is lost by radiation. The finline filter can be made by printing metallisation patterns on the dielectric sheet and then inserting the sheet into the E-plane of the hollow metal waveguide as much as it does by inserting the filter. Metal waveguide forms a shield for waveguide finline. The resonator is formed by making a pattern on the dielectric sheet. A more complicated pattern than a simple insertion filter in figure 9 is easily achieved because the designer does not need to consider the effect on mechanical support for removal of metals. This complexity does not add to production costs because the number of required processes does not change as more elements are added to the design. Finline designs are less sensitive to manufacturing tolerances than insert filters and have wide bandwidth.
Evanescent-filter mode
It is possible to design filters that operate entirely internally in evanescent mode. This has the advantage of saving space because the waveguide filters, which often form the housing filters, need not be large enough to support propagation of dominant modes. Typically, the evanescent mode filter consists of a waveguide wavelength smaller than the waveguide that feeds the input and output ports. In some designs this can be folded to achieve a more concise filter. The tuning screw is inserted at certain intervals along the wavelength that produces equivalent equalized capacitances at those points. In newer designs, screws are replaced with dielectric inserts. This capacitor resonates with the previous length of the evanescent waveguide mode which has an inductor equivalent circuit, resulting in a filtering action. The energy of various evanescent modes is stored in the field around each of these capacitive discontinuities. However, the design is such that only the dominant mode reaches the output port; other modes decay much faster among the capacitors.
Wavy waveguide filter
Wavy waveguide filters , also called ridged-waveguide filters , consist of a number of mountains, or teeth, which periodically reduce the height of the internal waveguide as shown in figures 10 and 11. used in applications that simultaneously require a wide passband, good passband matching, and a wide stopband. They are basically a low-pass design (above the usual limit of the cutoff frequency), unlike most other forms that are usually band-pass. The distance between the teeth is much smaller than the typical distance//4 between other filter design elements. Typically, they are designed with image parameter methods with all identical ridge, but other filter classes such as Chebyshev can be achieved in exchange for complexity of manufacture. In the picture design method the same circuit circuit is modeled as a half-section LC cascade. Filters operate in the dominant TE 10 mode, but fake mode can be an issue when it exists. In particular, there is little stopband attenuation from TE 20 and TE 30 mode.
Iron-waffle filter
The waffle-iron filter is a variant of wavy waveguide filters. It has properties similar to those filters with the additional advantages that falsify TE 20 and TE 30 mode is pressed. In an iron-waffle filter, channels are cut through longitudinal mountains below the filter. This leaves the matrix of the tooth protruding internally from the top and bottom surfaces of the waveguide. This tooth pattern resembles a waffle iron, that's the name of the filter.
Waveguide stub filter
A stub is a short waveguide that connects to multiple points in the filter at one end and short-circuits at the other end. Open-circuited stubs are also theoretically possible, but implementations in waveguide are not practical because electromagnetic energy will be launched out of the open end of the stub, resulting in high losses. The stub is a resonator, and equivalent elements are equivalent to the LC resonance circuit. However, through a narrow band, the stub can be seen as an impedance transformer. Short circuits are transformed into inductance or capacitance depending on the stub length.
A waveguide stub filter is created by placing one or more stubs along the length of a waveguide, usually? g /4 are separate, as shown in Figure 12. The ends of the stub are blanked off to their short-circuits. When short-circuited stubs are? g /4 the length of the filter will be a band-stop filter and the stub will have an equivalent element with a series circuit resonance circuit connected with series parallel lines. When stubs are located? g /2 length, the filter will be a band-pass filter. In this case the equation of lumped elements is a series circuit of LC series in series with the line.
Absorption filter
Absorption filter removes energy in an undesirable frequency internally as heat. This differs from the conventional filter design where the unwanted frequency is reflected back from the input port of the filter. This kind of filter is used if it is not desirable for power to be sent back to the source. This is the case with high power transmitters where the reverse power can be high enough to damage the transmitter. The absorption filter can be used to eliminate emissions of false transmitters such as harmonics or fake sidebands. A design that has been used for some time has a slot gap in the wall feed waveguide periodically. This design is known as leaked wave filter . Each slot is connected to a smaller gauge which is too small to support the frequency propagation in the desired band. Thus the frequency is not affected by the filter. The higher frequencies in the unwanted band, however, are ready to propagate along side guides that end with a suitable load at which the power is absorbed. This load is usually a wedge piece of microwave absorber. Another more compact filter absorption design uses a resonator with dielectric lossy.
Devices like filters
There are many filter applications whose design purpose is something other than a certain frequency rejection or forwarding. Often, a simple device intended to work only on a narrow band or just one spot frequency will not look like a filter design. However, the broadband design for the same item requires more elements and design takes on the properties of the filter. Among the more common applications of this type in waveguide are impedance matching networks, directional couplers, power dividers, power combiners, and diplexers. Other possible applications include multiplexers, demultiplexers, negative resistance boosters, and time-delay networks.
Impedance matching
The simplest method of impedance matching is a stub that matches a stub. However, a stub will only result in a perfect match on one particular frequency. Therefore this technique is only suitable for narrow band applications. To expand the multiple bandwidth can be used, and the structure then takes the form of a stub filter. The design results as if it were a filter except that different parameters are optimized. In the frequency filters are usually optimized parameters are stopband rejection, track attenuation, transition steepness, or compromise between the two. In a matching network, the optimized parameter is an impedance match. Device functions do not require bandwidth restrictions, but the designer is still forced to choose bandwidth due to the structure of the device.
Stubs are not the only usable filter format. In principle, any filter structure can be applied to impedance matching, but some will produce a more practical design than others. A frequently used format for impedance matching in a waveguide is a flattened impedance filter. An example can be seen in the duplexer [e] as illustrated in figure 13.
Co-driver and power combiners
Directional couplers, power splitters, and power combiners are all essentially the same type of device, at least when implemented with passive components. A directional coupler divides a small amount of power from the main line to the third port. Stronger devices are combined, but instead identical, can be referred to as a power divider. One that pair exactly half power to the third port (3 dB coupler) is the maximum coupling that can be achieved without reversing the port function. Many power splitter designs can be used in reverse, in which they are combined forces.
A simple form of directional coupler are two parallel transmission channels that are combined together during?/4 long. This design is limited because the length of the electric coupler will just be?/4 on one particular frequency. The coupling will be maximum at this frequency and fall on both sides. Similar to the impedance matching case, this can be improved by using several elements, resulting in a filter-like structure. An analog waveguide of this combined path approach is a directional bethe-hole coupler in which two parallel waveguides are stacked on top of each other and a hole is provided for the coupling. To produce wideband design, several holes are used along the guide as shown in figure 14 and the filter design is applied. Not only does the design of combined paths suffer from narrow bands, all simple designs of the waveguide coupler depend on frequency in several ways. For example racemaker-couplers (which can be implemented directly in a waveguide) work on a completely different principle but still rely on a certain exact length in terms.
Diplomaer and duplexer
Diplexer is a device used to combine two signals that occupy different frequency bands into one signal. This is usually to enable two signals to be transmitted simultaneously on the same communication channel, or to allow transmission on one frequency when receiving on another. (The specific use of a diplexer is called a duplexer.) The same device can be used to separate the signal again at the end of the channel. The need to filter to separate signals while receiving is clear enough but also necessary even when combining two transmitted signals. Without filtering, some power from source A will be sent to source B instead of the combined output. This will have a detrimental effect of losing some of the input power and loading the A source with the output impedance from source B causing a mismatch. These problems can be solved by using 3 dB directional coupler, but as described in the previous section, wideband design needs filter design for directional couplers as well.
Two broadcast narrowband signals can be diplexed by combining the output of two appropriate band-pass filters. Steps should be taken to prevent filters from pairing each other when they are in resonance which will cause their performance degradation. This can be achieved at the right distance. For example, if the filter is an iris-coupled type then the slice closest to the filter junction of filter A is placed? gb /4 of the junction where? gb is the guide wavelength in passband filter B. Similarly, the nearest slice of filter B is placed? ga /4 of the intersection. This works because when filter A resonates, filter B is in the stopband and is merely loosely coupled and vice versa. Alternative setting is to filter each filter to the main waveguide at a separate intersection. A decoupling resonator placed? g /4 of the intersection of each filter. This can be in the form of short stubs that are tuned to the resonant frequency of the filter. This setting can be extended to multiplexers with a number of bands.
For diplexers associated with passgaous passbands, appropriate accounts of crossover filter characteristics should be considered in the design. The most common case of this is where the diplexer is used to divide the entire spectrum into low and high bands. Here a low-pass and high-pass filter is used instead of a band-pass filter. The synthesis techniques used here can be applied to narrowband multiplexers and largely eliminate the need for decoupling resonators.
Direction filter
Source of the article : Wikipedia
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