A tunable filter

Abstract

A tunable filter. The present invention arranges a resonant column in the cavity of the filter to form a structure close to a coaxial resonant cavity, and at the same time, at least two axially arranged slots surrounding the resonant column are arranged on the side wall close to the coaxial structure, and a dielectric sheet is extended into the resonant cavity through the slot to adjust the passband frequency range of the tunable filter. The present invention is suitable for the lower frequency bands of microwave point-to-point communication, and the volume of the filter can be reduced by loading the resonant column and quasi-coaxial cavity loading technology. Band cross-coupling is achieved by matching the double-layer dielectric sheet structure arranged in the cavity. In the entire tuning range of the tunable filter, the transmission zero point achieved by cross-coupling can basically remain unchanged with the passband distance, the passband bandwidth of the filter can basically remain unchanged, and at the same time, the filter echo can also be maintained at a good level. The present invention uses cross-coupling to achieve transmission zero points, which can greatly increase the out-of-band suppression of the filter.

Description

Technical Field

[0001] The invention relates to the field of adjustable microwave devices, and in particular to an adjustable filter. Background Art

[0002] In recent years, due to the development of communication systems, there has been a widespread demand for tunable filters in the microwave frequency band. Companies such as Huawei, ZTE, NEC, and Ericsson have conducted in-depth research in this field. NEC started research in this area as early as 2005 and published relevant research results in 2010.

[0003] At present, the main methods used in the microwave field are as follows: 1. The medium rotation method proposed by Japan's NEC Corporation to achieve tunable filters (patent number: WO2010150815A1), using a piece of high dielectric constant medium as the tuning mechanism, and rotating the medium piece by a stepper motor to rotate an axis to achieve an adjustable effect. 2. Huawei's tunable filter (WO2016095165A1) uses screw debugging to achieve tunable filters. At that time, the screw method itself had problems such as insufficient stability, electromagnetic leakage, and production difficulties.

[0004] The various solutions given above are all based on rectangular waveguide resonant cavities, and the frequency band range of the filter is adjusted through different loading methods. The tunable filter implemented by the above method is generally large in size and suitable for higher microwave frequency bands. For point-to-point communication systems in the 6G or 7 / 8GHz frequency bands, if the above method is still used, the size of the tunable filter / duplexer is too large, making the system too bulky. Summary of the invention

[0005] The present invention aims at the deficiencies of the prior art and provides an adjustable filter. The present invention specifically adopts the following technical solutions.

[0006] Firstly, to achieve the above-mentioned purpose, an adjustable filter is proposed, which comprises: a cavity, comprising a plurality of mutually connected and arranged, at least one side wall of each of the cavities being provided with at least two slits; a resonant column, the number of which is the same as the number of the cavities, and each of the resonant columns is respectively arranged between the slits of each of the cavities; a dielectric sheet, which passes through each of the slits and extends into the cavity in accordance with the number and size of the slits, so as to adjust the passband frequency range of the adjustable filter.

[0007] Optionally, the above-mentioned adjustable filter further includes a base plate and a motor; the base plate is arranged below the cavity and fixedly connected to the cavity; the motor is fixedly arranged above the base plate, and the driving shaft of the motor is connected to the dielectric sheet, driving the dielectric sheet according to the required passband frequency range, and adjusting the depth of the dielectric sheet extending into the gap.

[0008] Optionally, in the above tunable filter, the dielectric constant εr of the dielectric sheet is greater than 4. It can be set between 4-10.

[0009] Optionally, the above-mentioned tunable filter comprises a pair of tunable filters, one for the transmitting end of the system where the tunable filter is located and the other for the receiving end of the system, and the two tunable filters are arranged on both sides above the same base plate, and a circulator is also arranged in the base plate.

[0010] Optionally, in the above-mentioned tunable filter, the dielectric sheets of each tunable filter are interconnected as a whole, and each dielectric sheet of the tunable filter is connected to the same motor, and the motor synchronously drives all the dielectric sheets of the tunable filter to synchronously adjust the depth of each dielectric sheet extending into the gap.

[0011] Optionally, in the above-mentioned tunable filter, a cross-coupling window and / or a cross-coupling metal rod are further provided between the two cavities; the cross-coupling window passes through the side wall of the cavity and is provided between two non-adjacent cavities, and the direction of the cross-coupling window is parallel to the direction of the resonant column, providing positive cross-coupling. The larger the window, the greater the coupling, and the closer the transmission zero point is to the passband; conversely, the smaller the window, the farther the transmission zero point is from the passband. The cross-coupling metal rod extends from the same side of the dielectric sheet parallel to the resonant column, between the two cavities or into the cross-coupling window, and the depth of the cross-coupling metal rod is greater than the depth of the dielectric sheet, providing negative cross-coupling.

[0012] Optionally, the above-mentioned tunable filter further includes a tuning screw, which is respectively arranged in the resonant column in each of the cavities, and the insertion depth of the tuning screw relative to the resonant column can be adjusted separately.

[0013] Optionally, in the above-mentioned tunable filter, when the dielectric sheet in each cavity is arranged in the same direction as the resonant column, the tuning screw extends from the opposite side of the dielectric sheet into the cavity and extends in the direction of the resonant column.

[0014] Optionally, in the above-mentioned tunable filter, when the dielectric sheet in each cavity is arranged opposite to the resonant column, the tuning screw extends into the cavity from the inside of the resonant column along its axial direction.

[0015] Optionally, in the above-mentioned tunable filter, the number of dielectric sheets arranged in the cavity at both ends of the tunable filter is increased, the length of the dielectric sheet extending into the cavity is reduced, and the dielectric sheets in the cavity are arranged to form a resonant rod surrounding the inside of the cavity. Beneficial Effects

[0016] The present invention utilizes the cavity of the filter, and arranges a resonant column inside it to form a structure close to the coaxial structure. At the same time, at least two slits arranged axially surrounding the resonant column are arranged on the side wall close to the coaxial structure, and the dielectric sheet is extended into the resonant cavity through the slit to achieve the adjustment of the passband frequency range of the tunable filter. The present invention is applicable to the 6 / 7 / 8GHz frequency band, and can also be extended to other frequency bands by selecting corresponding materials and optimizing the structure to achieve similar tunable filtering functions. The volume of the filter can be reduced by loading the resonant column, and the double-layer dielectric sheet structure arranged in the cavity can achieve band cross-coupling. In the entire tuning range of the tunable filter, the transmission zero point achieved by cross-coupling can basically remain unchanged with the passband distance, and the passband bandwidth of the filter can also remain basically unchanged. At the same time, the filter echo can also be maintained at a good level. The present invention uses cross-coupling to achieve the transmission zero point, which can greatly increase the out-of-band suppression of the filter.

[0017] Furthermore, the present invention is directed to a microwave duplex system, and the tunable filter structure is respectively provided at the transmitting end and the receiving end thereof, so that the two tunable filters share the same bottom plate, and a high-isolation circulator is provided on the bottom plate, and the loading effect of the dielectric sheet with a high dielectric constant and a low loss tangent and the resonant column in the filter cavity is matched, so that the adjustment of the duplex frequency band range is realized with a more compact structure. The present invention can realize the adjustment of the passband transmission zero point in the microwave frequency band with a lower frequency. The drive of the motor is calculated according to the filter characteristics, and the dielectric sheet is pushed or pulled out by the motor, and the passband of the filter is selected as needed. The passband range of the filter is adjusted to cover a wider frequency band range, and the effective transmission and reception of the passband signal is realized in a wider frequency band range.

[0018] Other features and advantages of the present invention will be set forth in the description which follows, and in part will be apparent from the description, or may be learned by practice of the present invention. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] The accompanying drawings are used to provide a further understanding of the present invention and constitute a part of the specification. Together with the embodiments of the present invention, they are used to explain the present invention and do not constitute a limitation of the present invention. In the accompanying drawings:

[0020] Figure 1 It is a schematic diagram of the overall structure of the adjustable filter of the present invention;

[0021] Figure 2 is a schematic diagram of one side of a cavity in a tunable filter of the present invention;

[0022] Figure 3 yes Figure 2 Schematic diagram of the other side of the cavity in the tunable filter shown;

[0023] Figure 4 yes Figure 2 The frequency response curve of the adjustable filter is shown;

[0024] Figure 5 is a schematic diagram of one side of a cavity in another tunable filter of the present invention;

[0025] Figure 6 yes Figure 5 Schematic diagram of the other side of the cavity in the tunable filter shown;

[0026] Figure 7 yes Figure 5 The frequency response curve of the tunable filter is shown.

[0027] In the figure, 1 represents the base plate; 2 represents the transmitting end; 3 represents the receiving end; 4 represents the motor; 5 represents the cavity; 6 represents the gap; 7 represents the cross-coupled metal rod; 8 represents the dielectric sheet; 51 represents the end resonant column; 52 represents the middle resonant column; and 53 represents the tuning screw. DETAILED DESCRIPTION

[0028] In order to make the purpose and technical solution of the embodiment of the present invention clearer, the technical solution of the embodiment of the present invention will be clearly and completely described below in conjunction with the drawings of the embodiment of the present invention. Obviously, the described embodiment is a part of the embodiment of the present invention, not all of the embodiments. Based on the described embodiment of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0029] It will be understood by those skilled in the art that, unless otherwise defined, all terms (including technical and scientific terms) used herein have the same meaning as those generally understood by those skilled in the art in the art to which the present invention belongs. It should also be understood that terms such as those defined in common dictionaries should be understood to have meanings consistent with the meanings in the context of the prior art, and will not be interpreted with idealized or overly formal meanings unless defined as herein.

[0030] The meaning of "and / or" in the present invention means that both situations where each exists alone or both exist at the same time are included.

[0031] The meaning of "inside" and "outside" in the present invention refers to the direction pointing to the resonant column arranged in the cavity itself, which is inside, and the opposite direction is outside; it is not a specific limitation on the device mechanism of the present invention.

[0032] The term “connection” as used in the present invention may mean a direct connection between components or an indirect connection between components via other components.

[0033] The meaning of "up" and "down" in the present invention refers to that when the user is facing the filter, the direction from the bottom plate to the cavity or the motor is up, and the opposite direction is down, rather than a specific limitation on the device mechanism of the present invention.

[0034] Figure 1 The invention discloses an adjustable filter which can be used in a 6G or 7 / 8GHz frequency band microwave communication system. To match the transmitting end 2 and receiving end 3 of the system, the adjustable filter includes two similar structures, one connected to the transmitting end signal path and the other connected to the receiving end signal path. The two adjustable filters are arranged on both sides above the same base plate 1 and supported by the base plate. A circulator is also arranged in the base plate 1, and the circulator has a high degree of isolation, which is used to achieve isolation between the transmitting end and the receiving end.

[0035] refer to Figure 2 , each of the above-mentioned tunable filters comprises:

[0036] The cavity 5 comprises a plurality of cavities 5 arranged and connected to each other, and at least one side wall of each cavity 5 is provided with at least two slits 6;

[0037] The number of resonant columns is the same as the number of the cavities 5 , and each of the resonant columns is respectively arranged between the gaps 6 of each of the cavities 5 ;

[0038] The dielectric sheet 8 is matched with the number and size of the slots 6 and extends into the cavity 5 through each slot 6 to adjust the passband frequency range of the tunable filter.

[0039] The cavity 5 can be specifically configured as a metal resonant cavity. In the present invention, a protruding metal column is added as a resonant column in the metal resonant cavity. Providing a resonant column is equivalent to loading a capacitor in the metal resonant cavity, thereby reducing the size of the metal resonant cavity. The higher the height of the metal column, the larger the loaded capacitor, and the more the size of the metal resonant cavity is reduced.

[0040] In order to achieve a tuning range greater than 10%, the present invention inserts a dielectric sheet with a larger dielectric constant εr into the resonant cavity. Since the wavelength of the electromagnetic wave in the medium is (λ0 is the wavelength of the electromagnetic wave in a vacuum, εr is the dielectric constant of the medium), the resonant frequency of the resonant cavity can be reduced by selecting a dielectric sheet with a larger dielectric constant. Generally, a dielectric sheet with a dielectric constant εr between 4 and 10 can be selected. If the dielectric constant is too small, the adjustment range is too small. If the dielectric constant is too large, the tuning is too sensitive and difficult to control.

[0041] In order to adjust the passband frequency range of each filter, each of the adjustable filters is also respectively provided with a motor 4 to control the insertion depth of the dielectric sheet. Figure 1The motor 4 is fixedly arranged above the bottom plate 1, and the driving shaft of the motor 4 is connected to the dielectric sheet 8, and the dielectric sheet is driven according to the required passband frequency range to adjust the depth of the dielectric sheet extending into the gap 6. Figure 1 The structure is used for a duplex system, wherein the two filters can be controlled by two motors to control the insertion depth of the dielectric sheets therein. The two motors can be respectively arranged at the upper middle part of the bottom plate 1, and the driving shaft of each motor can extend to the two filters on both sides of the bottom plate. The dielectric sheets of each filter can be arranged as an integral structure through the same connecting rod or through a plurality of interconnected connecting rods. For example, the dielectric sheets can be arranged as an integral comb-shaped structure, and at least two dielectric sheets are inserted on both sides of the resonance column of each resonant cavity. The dielectric sheets of the same adjustable filter are connected to the same motor 4, and the motor synchronously drives all the dielectric sheets of the adjustable filter, and synchronously adjusts the depth of the dielectric sheets extending into the gap 6 to ensure that the distance between the start and end frequencies of the filter passband remains unchanged, and only the frequency band of the filter passband is moved to the high frequency or low frequency direction according to the insertion depth of the dielectric sheet.

[0042] A metal resonant column is arranged in each resonant cavity of the above-mentioned filter. The metal resonant column can be realized by a square, circular or elliptical column. Due to the structure of the resonant column, each cavity structure of the above-mentioned filter is set to be close to a coaxial cavity. It can greatly reduce the size of the resonant cavity by loading. Compared with the ordinary standard waveguide filter, this structure close to the coaxial cavity can reduce the size of the filter by more than 1 / 3 of the original. Among them, the height of the metal resonant column in each resonant cavity of the same filter is basically the same, and the cross-sectional size is kept consistent as much as possible, so that the electric field distribution in each resonant cavity can be basically consistent, ensuring that when the same motor pushes the dielectric sheet, the disturbance components applied to each resonant cavity by each dielectric sheet are basically the same, thereby realizing one motor tuning one filter.

[0043] For further reference, Figure 2 or Figure 6As shown, in order to achieve cross-coupling, a zero point is introduced in the passband range of the filter to achieve stronger out-of-band suppression. The above filter can also set a cross-coupling window 71 and / or a cross-coupling metal rod 7 between the corresponding two cavities as needed. The cross-coupling window 71 runs through the side wall of the cavity 5 where it is set, and is set between two non-adjacent cavities 5. The depth direction of the cross-coupling window 71 is parallel to the direction of the resonant column, providing positive cross-coupling. The larger the cross-coupling window 71, the greater the coupling, and the closer the transmission zero point is to the passband. On the contrary, the smaller the window, the farther the transmission zero point is from the passband. The cross-coupling metal rod 7 extends from the same side of the dielectric sheet parallel to the resonant column, between the two cavities or into the cross-coupling window 71. The depth of the cross-coupling metal rod 7 is greater than the depth of the dielectric sheet, providing negative cross-coupling. Thus, cross-coupling is introduced within the frequency band through the above structure, and a zero point is formed within the frequency band, further improving the out-of-band suppression of the filter.

[0044] The cross-coupling metal rod 7 extends between the cavities or into the cross-coupling window 71, and the principle of providing negative cross-coupling is that when there is a metal rod inserted very deeply in the coupling window, it is equivalent to loading a large capacitor in the window, and when the capacitor is large enough, the coupling will be changed to negative coupling. When the cross-coupling spans 3 resonant cavities, the transmission zero point formed by the negative coupling is on the left side of the filter passband; when the cross-coupling spans 4 resonant cavities, the negative coupling forms two transmission zero points on the left side of the filter passband.

[0045] refer to Figure 3 As shown, the filter will inevitably have processing errors during the processing, and this structural deviation will cause deviations in the frequency domain characteristics of the filter. This deviation can be offset by adding a tuning screw 53 on the basis of the above structure. The tuning screw 53 includes a plurality of tuning screws, each of which is respectively arranged in a resonant column in each of the cavities 5, and the insertion depth of the tuning screw 53 relative to the resonant column can be adjusted separately.

[0046] exist Figure 3 or Figure 6 In the structure shown, when the dielectric sheet 8 in each cavity 5 is arranged opposite to the resonant column, the tuning screw 53 can be arranged to extend into the cavity 5 from the inside of the resonant column along its axial direction.

[0047] In other implementations, when the dielectric sheet 8 in each cavity 5 is arranged in the same direction as the resonant column, the tuning screw 53 extends from the opposite side of the dielectric sheet 8 into the cavity 5 toward the resonant column.

[0048] refer to Figure 5The structure of the filter of the present invention is that the cavity adopts a structure close to that of a coaxial cavity, so the relationship between the depth of the dielectric sheet inserted into the resonant cavity and the frequency is not a linear relationship. When the depth of the dielectric sheet inserted into the resonant cavity is large, the phenomenon of insufficient coupling between the input and output ports will occur. In order to overcome this phenomenon, the number of dielectric sheets 8 arranged in the cavity 5 at both ends of the adjustable filter can be appropriately increased. At the same time, the length of the dielectric sheets in the cavities at these two ends extending into the cavity is reduced, and the dielectric sheets in the end cavity 5 are arranged as resonant rods surrounding the inside of the cavity. Therefore, when the dielectric extends less into the cavity, the dielectric sheets in the cavities at the input and output ends of the filter will not extend into the cavity first. Only when other dielectrics extend deeper, the two dielectric sheets at the input and output ports will be inserted into the cavity to compensate for the coupling amount of the input and output parts of the filter.

[0049] by Figure 2 Taking the cavity structure of the filter shown as an example, a symmetrical zero-point filter is realized with a 6-cavity two-zero-point adjustable structure. The tuning structure of the filter is realized by two layers of comb-tooth-shaped dielectric sheets. The comb-tooth-shaped dielectric sheet is driven by a stepper motor to simultaneously change the entry depth in the filter resonant cavity to achieve frequency tuning. The deeper the dielectric sheet enters the cavity, the lower the frequency. During tuning, the dielectric sheet adjusts the resonant frequency of the resonant cavity while adjusting the cross-coupling of the filter to keep the distance from the out-of-band zero point to the passband of the filter unchanged, and the passband bandwidth of the filter can also remain basically unchanged, and the filter echo is well maintained. The use of cross-coupling to realize the transmission zero point can greatly increase the out-of-band suppression of the filter while keeping the filter order unchanged, and at the same time improve the in-band insertion loss of the filter.

[0050] To compensate for the processing error, the above-mentioned resonant cavity can also refer to Figure 3 Load each tuning screw in this way. Figure 2 or Figure 3 For example, the dielectric sheet and the resonant column inserted in the cavity are located in opposite directions. In this state, the tuning screw is set to be inserted into the resonant cavity from the middle of the resonant column. For the case where the dielectric sheet and the resonant column are located in the same direction, the tuning screw can generally be inserted into the resonant cavity from the cover plate opposite the resonant column.

[0051] exist Figure 2 In the implementation shown, the transmission zero of the filter structure is formed by adding a metal rod inserted deep into the coupling window at the negative coupling on the left side of the filter passband, thereby forming a transmission zero on the left and right sides of the filter passband. The metal rod is always in the same direction as the inserted dielectric sheet to achieve a substantially unchanged position of the zero relative to the filter passband.

[0052] Figure 2 as well as Figure 3The filter shown in the figure has two transmission zeros realized by negative coupling between the second resonant cavity and the fourth resonant cavity. In the figure, reference numeral 8 indicates a dielectric sheet used for tuning in the filter. The dielectric sheet adopts a dielectric sheet with a high dielectric constant and a low loss tangent. The dielectric sheet is divided into two layers, upper and lower. In actual use, the two layers of dielectric sheets are installed on the same metal aluminum block, and all the dielectric sheets thereon are pushed simultaneously by a motor 4. In the figure, 5 indicates the cavity of the filter; 6 indicates a gap for each dielectric sheet to be inserted into the resonant cavity, and the length and width of the gap are less than 1 / 2 of the wavelength of the highest working frequency band; 7 is a metal rod for generating negative cross coupling between the second resonant cavity and the fourth resonant cavity. The metal rod needs to be on the same side of the tuning sheet to ensure that the zero point generated by the negative coupling and the passband position of the filter remain relatively unchanged.

[0053] This filter can be used at the receiving end and Rx end of the system. Figure 3 The figure shows the reverse view of the tunable filter. In the figure, 51 represents the end resonant column in the end cavity, and 52 represents the middle resonant column in the middle cavity. Both 51 and 52 can provide loading. In the filter, by adjusting the size of the resonant cavity, the height of the six resonant columns is kept consistent as much as possible, and the diameter is basically kept consistent, so that the electromagnetic distribution in each resonant cavity is basically consistent, ensuring that each resonant cavity can be tuned synchronously.

[0054] 53 is a tuning screw used to compensate for processing errors, and there is a tuning screw at the center of each resonant column.

[0055] Figure 4 The frequency response curve of the 6th-order two-zero adjustable filter is given. Through the frequency response test, it can be seen that when the insertion depth of the dielectric sheet changes from 1 mm to 5 mm, Figure 2 as well as Figure 3 The center frequency of the filter shown is moved from approximately 7.9 GHz to 7.45 GHz. Over the entire tuning band, the echo is well maintained and the bandwidth of the filter remains essentially unchanged.

[0056] And in Figure 5 as well as Figure 6In the filter shown, the filter is set to an asymmetric transmission zero. Taking the 7th order 1 zero tunable filter shown in the figure as an example, a cross-coupling is set between the third resonant cavity and the fifth resonant cavity in the filter to form a transmission zero (the transmission zero can also be achieved through cross-coupling between other resonant cavities according to the filter parameter requirements). Specifically, the structure opens a window between the third resonant cavity and the fifth resonant cavity, then a positive cross-coupling is formed, and the transmission zero is on the right side of the filter passband; when a deeper metal rod is inserted into the window, a negative cross-coupling is formed, and the transmission zero of the filter is on the left side of the filter passband. Like the tunable filter described above, this tunable filter needs to set the resonant column size in each resonant cavity to be consistent as much as possible.

[0057] In some cases, due to structural problems, the metal resonant columns of the input and output resonant cavities and the metal resonant columns of other resonant cavities cannot maintain the same diameter, such as Figure 5 In this case, when the tuning range is wide, the electromagnetics in the input and output resonant cavities are not completely synchronized with the tuning of other resonant cavities. In this case, if the filter needs to be set with a wide tuning range, the input and output ports are often not coupled enough, which seriously deteriorates the echo performance. To improve this situation, Figure 6 In the two end cavities of the filter, the present invention adds a tuning dielectric sheet inserted into the cavity, respectively, for improving input and output coupling. The depth of the tuning dielectric sheets at these two locations is less than that of other dielectric sheets. When the dielectric sheets are not inserted deeply, the two dielectric sheets are not inserted into the cavity of the filter. Only when the other dielectric sheets are inserted deeply into the cavity of the filter, the two dielectric sheets are inserted into the cavity of the filter. Due to the improvement of the coupling characteristics of the filter input and output, it is ensured that the tunable filter has better performance in a wider tuning range.

[0058] refer to Figure 7 The test results shown in the figure show that under this structure, when the filter's stopband start frequency moves from 7.38 to 7.81, the filter has good echo characteristics in the entire tuning frequency band, and the filter's bandwidth remains basically unchanged during the tuning process.

[0059] The above is only an embodiment of the present invention, and its description is relatively specific and detailed, but it cannot be understood as limiting the scope of the patent of the present invention. It should be pointed out that for ordinary technicians in this field, several modifications and improvements can be made without departing from the concept of the present invention, which all belong to the protection scope of the present invention.

Claims

1. A tunable filter, characterized in that: include: The cavity (5) comprises a plurality of cavities arranged and connected to each other, and at least one side wall of each cavity (5) is provided with at least two slits (6); Resonance columns, the number of which is the same as the number of the cavities (5), and each of the resonance columns is respectively arranged between the gaps (6) of each of the cavities (5); A dielectric sheet (8) which, in accordance with the number and size of the slots (6), passes through each slot (6) and extends into the cavity (5) to adjust the passband frequency range of the tunable filter; A cross-coupling window and / or a cross-coupling metal rod (7) is also provided between the two cavities; The cross-coupling window runs through the side wall of the cavity (5) and is arranged between two non-adjacent cavities (5); the direction of the cross-coupling window is parallel to the direction of the resonant column, providing positive cross-coupling; The cross-coupling metal rod (7) extends from the same side of the dielectric sheet in parallel with the resonant column, into between the two cavities or into the cross-coupling window, and the depth of the cross-coupling metal rod (7) is greater than the depth of the dielectric sheet, providing negative cross-coupling; The number of dielectric sheets (8) provided in the cavity (5) arranged at both ends of the tunable filter is increased, the length of the dielectric sheet extending into the cavity is reduced, and the dielectric sheets in the cavity (5) are arranged to form a resonant rod surrounding the interior of the cavity.

2. The tunable filter according to claim 1, characterized in that: It also includes a base plate (1) and a motor (4); The bottom plate (1) is arranged below the cavity (5) and is fixedly connected to the cavity (5); The motor (4) is fixedly arranged above the base plate (1); the driving shaft of the motor (4) is connected to the dielectric sheet (8), and the dielectric sheet is driven according to a required passband frequency range to adjust the depth of the dielectric sheet extending into the gap (6).

3. The tunable filter according to claim 2, characterized in that: The dielectric constant εr of the dielectric sheet is greater than 4.

4. The tunable filter according to claim 3, characterized in that: The dielectric sheets of the tunable filter are interconnected as a whole, and the dielectric sheets of the tunable filter are all connected to the same motor (4), which synchronously drives all the dielectric sheets of the tunable filter and synchronously adjusts the depth of each dielectric sheet extending into the gap (6).

5. The tunable filter according to any one of claims 1 to 4, characterized in that: It also comprises a tuning screw (53), which is respectively arranged in the resonant column in each of the cavities (5), and the insertion depth of the tuning screw (53) relative to the resonant column can be adjusted separately.

6. The tunable filter according to claim 5, characterized in that: When the dielectric sheet (8) in each cavity (5) is arranged in the same direction as the resonant column, the tuning screw (53) extends from the opposite side of the dielectric sheet (8) into the cavity (5) and extends in the direction of the resonant column.

7. The tunable filter according to claim 5, characterized in that: When the dielectric sheet (8) in each cavity (5) is arranged opposite to the resonant column, the tuning screw (53) extends from the inside of the resonant column along its axial direction into the cavity (5).

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