TRIPLE MODE DIELECTRIC LOADED BANDPASS FILTERS

Abstract: A triple mode dielectric loaded bandpass filter (28) has at least one cavity (30) resonating in three independent ortho­ gonal modes. The triple mode cavity (30) can be mounted adjacent another cavity (32) which may be either a single, dual or triple mode cavity. Any reasonable number of cavities can be used as well as various arrangements of cavities. Inter-cavity coupling is achieved through an iris (34) having two separate apertures (36, 38) that together form a T-shape. The cavities are planar mounted. The filter (28) is designed for use in the satellite communication industry and results in substantial savings in weight and size when compared to previous filters.

Description: This invention relates to a triple mode dielectric loaded bandpass filter. In particular, this invention relates to a bandpass filter having one or more cascaded dielectric loaded waveguide cavities resonating in three independent orthogonal modes, simultaneously. Dielectric loaded triple mode cavities can be used in combination with dual or single mode cavities. In the Fall of 1971, in COMSAT Technical Review, Volume 1, pages 21 to 42, Atia and Williams suggested the possibility of cascading two triple-mode waveguide cavities to realize a six-pole elliptic filter. However, Atia and Williams were unable to achieve the suggested results. In "A true elliptic function filter using triple-mode degenerate cavities", Tang, et al., IEEE MTT-32, No. 11, November, 1984, pp. 1449-1453, there is described a six-pole triple-mode filter that uses a new inter-cavity iris structure that can control three inter-cavity mode couplings simultaneously. The filter is shown in Figure 3 of this paper but the filter is not a planar filter and describes only a two-cavity filter having two triple-mode cavities with two coupling screws per cavity. In EP-A-0 064 799 naming Fiedziuszko as inventor, there is described a miniature dual-mode dielectric-loaded cavity filter. It is an object of the present invention to provide a triple mode bandpass filter wherein each cavity contains a dielectric resonator. It is an object of one embodiment of the present invention to provide a triple mode bandpass filter where cavities resonating in a triple mode are mixed with cavities resonating in a dual or single mode. The present invention is directed to a triple mode function bandpass filter of the general type disclosed in the above-mentioned IEEE MTT-32 paper, which has at least one cavity resonating in three independent orthogonal modes, said filter having an input and output for transferring electromagnetic energy into and out of said filter, the or each cavity having two coupling screws and three tuning screws mounted therein, said coupling screws coupling energy from one mode to another and each of said tuning screws controlling the resonant frequency of a different mode. The present invention is characterised by the or each triple mode cavity having a third coupling screw mounted therein, and the or each triple mode cavity having a dielectric resonator mounted therein, there being an intercavity coupling iris between adjacent cavities when the filter has more than one cavity. Preferably, the filter is a planar filter and the dielectric resonator is planar mounted. A preferred embodiment of the invention is described in the following drawings: Figure 1 is a perspective view of a triple mode bandpass filter having one cavity; Figure 2 is a perspective view of a triple mode function bandpass filter using an aperture on an iris for input and output coupling; Figures 3A, 3B and 3C are schematic views showing field patterns for TM₀₁₁ and HE₁₁₁ modes that can be used with the filter of the present invention; Figure 4 is a graph of a simulated response of an asymmetric three-pole filter with one transmission zero; Figure 5 is a perspective view of a five-pole dielectric-loaded bandpass filter having two cavities; Figure 6 is a graph showing the measured transmission and return loss response of the five-pole filter shown in Figure 4; Figure 7 is a perspective view of a six-pole dielectric-loaded bandpass filter having two cavities; Figure 8 is a graph showing the simulated response of the asymmetric six-pole bandpass filter of Figure 6 with four transmission zeros; Figure 9 is a side view of an iris used for inter-cavity coupling in the five-pole and six-pole filters shown in Figures 4 and 6; Figure 10 is a perspective view of a four-pole dielectric-loaded bandpass filter having two cavities; Figure 11 is a schematic perspective view of a linear arrangement of cavities; Figure 12 is a schematic perspective view of a variation in an arrangement of cavities where an input cavity and an output cavity are adjacent to one another; and Figure 13 is a schematic perspective view of a further arrangement of cavities where an input and an output cavity are not adjacent to one another. Referring to the drawings in greater detail, in Figure 1, a triple-mode function bandpass filter 2 has one waveguide cavity 4 resonating in three independent orthogonal modes. The cavity 4 has a dielectric resonator 6 mounted therein. Preferably, the filter 2 is a planar filter and the dielectric resonator 6 is planar mounted as shown in Figure 2. The filter 2 can be made to resonate in a first HE₁₁₁ mode, a second TM₀₁₁ mode and a third HE₁₁₁ mode. The filter 2 is not restricted to these modes and can operate in any two HE 11(N+1) modes and a TM 01N mode, where N is a positive integer. Input and output energy transfer is provided by coaxial probes 8, 10 respectively. The probes 8, 10 couple electric field energy parallel to the direction of the probe into and out of the first HE₁₁₁ and the third HE₁₁₁ modes respectively. Input and output coupling can be provided in other ways as well. For example, as shown in Figure 2, energy can be coupled into and out of a particular cavity 13 by means of magnetic field transfer through apertures 28, 25 located on irises 27, 23 respectively. The dielectric resonator 6 used in the filter 2 has a high dielectric constant, a low-loss tangent and a low temperature drift coefficient value. The frequency at which the dielectric resonator resonates for a particular mode is directly related to the diameter/length ratio of the dielectric resonator 6. A diameter/length ratio was calculated for the dielectric resonator 6 so that the HE₁₁₁ mode and the TM₀₁₁ mode resonate at the same frequency. The resonator 6 used in the filter 2 is planar mounted on a low-loss, low dielectric constant support 14. In Figures 3A, 3B and 3C, the electrical and magnetic field patterns about the resonator 6 are shown. The electrical field patterns are depicted with a solid line with an arrowhead thereon and the magnetic field patterns are depicted with a dotted line. Figure 3A is a perspective view of the resonator 6, Figure 3B is a top view and Figure 3C is a front view of said resonator. The electrical field patterns of the second TM₀₁₁ mode are shown in Figure 3A while the electrical field patterns of the HE₁₁₁ mode are shown in Figures 3B and 3C. From Figure 3A, it can be seen that the TM₀₁₁ mode has a maximum electrical field strength normal to a surface 12 of the resonator 6. From Figures 3B and 3C, it can be seen that the HE₁₁₁ mode has a maximum electrical field strength parallel to the surface 12 of the resonator 6. By the proper use of coupling screws, a third HE₁₁₁ mode having an electrical field parallel to the dielectric resonator surface 12 and perpendicular to both the first HE₁₁₁ mode and the second TM₀₁₁ mode can be made to resonate in the cavity 4. There are three coupling screws 16, 18, 20 that are located at a 45 o angle from the maximum electrical field in the filter 2. A metallic coupling screw is a physical discontinuity which perturbs the electrical field of one mode to couple energy into another mode. As previously stated, the input probe 8 couples electrical field energy to the first HE₁₁₁ mode parallel to the direction of said probe 8. Coupling screw 16 couples energy between the first HE₁₁₁ mode and the second TM₀₁₁ mode. Coupling screw 18 couples energy between the second TM₀₁₁ mode and the third HE₁₁₁ mode. Coupling screw 20 couples energy between the first HE₁₁₁ mode and the third HE₁₁₁ mode. Output probe 10 couples electrical field energy from the third HE₁₁₁ mode in a direction parallel to said probe 10. A tuning screw is located in the direction parallel to the maximum electrical field strength of a particular mode and is used to control the resonant frequency of said mode. When a tuning screw approaches the dielectric resonator surface 12, it effectively increases the electrical length of the dielectric resonator, thereby resulting in a decrease of the resonant frequency. For filter 2, the tuning screws 22, 24, 26 control the resonant frequencies of the first HE₁₁₁ mode, the second TM₀₁₁ mode and the third HE₁₁₁ mode respectively. The filter 2 produces an asymmetric response where only one transmission zero exists. In general, transmission zeros are created when feed back couplings are implemented. In filter 2, the coupling screw 20, which couples energy between the first HE₁₁₁ mode and the third HE₁₁₁ mode provides a feed back coupling which results in a three-pole asymmetric response with one transmission zero. A simulated response of this asymmetric response is illustrated in Figure 4. In Figure 5, there is shown a further embodiment of the invention in which a five-pole elliptic bandpass filter 28 has two cavities 30, 32. The cavity 30 resonates in a triple mode and the cavity 32 resonates in a dual mode. Since the cavity 30 is essentially the same as the cavity 4 of the filter 2, the same reference numerals are used for those components of the cavity 30 that are essentially the same as the components of the cavity 4. The cavity 30 contains a dielectric resonator 6 that is mounted on a low-loss, low dielectric constant support 14. The resonator 6 is planar mounted within the planar cavity 30. The cavity 30 resonates in a first HE₁₁₁ mode, a second TM₀₁₁ mode and a third HE₁₁₁ mode in a manner similar to the cavity 4 of the filter 2. The cavity 32 resonates in two HE₁₁₁ modes. The cavity 30 is the input cavity to the filter 28 and an input probe 8 couples electrical field energy to the first HE₁₁₁ mode parallel to the direction of said input probe. Energy from the first HE₁₁₁ mode is coupled to the second TM₀₁₁ mode due to the perturbation of fields created by the coupling screw 16. Energy in turn is coupled from the second TM₀₁₁ to the third HE₁₁₁ mode by means of the coupling screw 18. Coupling screw 20 provides a feed back coupling between the first and third HE₁₁₁ modes. The magnitude of the feed back coupling depends upon the penetration of the coupling screw 20 within the cavity 30. Located between the cavity 30 and the cavity 32 is an iris 34 having apertures 36, 38 positioned to couple energy between the adjacent cavities 30, 32. The apertures 36, 38 are normal to one another, each aperture being symmetrical about an imaginary centre line of said iris 34, said centre line being parallel to an axis of the resonator 6. Aperture 38 on iris 34 provides a means by which energy is coupled from the third HE₁₁₁ mode in cavity 30 to a fourth HE₁₁₁ mode in cavity 32 through magnetic field transfer across said aperture. Energy from the fourth HE₁₁₁ mode to a fifth HE₁₁₁ mode is through coupling screw 40. Both the fourth HE₁₁₁ mode and the fifth HE₁₁₁ mode resonate in the cavity 32. Energy output from the cavity 32 is through an output probe 42 in a direction parallel to said probe. The output probe 42 of cavity 32 is similar to the output probe 10 of cavity 4 of Figure 1. A second feed back coupling is provided through the aperture 36 of the iris 34. This feed back coupling occurs between the first HE₁₁₁ mode and the fifth HE₁₁₁ mode by means of electrical field energy coupling across aperture 36. The cavity 32 has a dielectric resonator 44 mounted therein on a low-loss, low dielectric constant support 46. The length and height of the aperture 36 relative to top surfaces 48, 50 of the dielectric resonators 6, 44 respectively determines the magnitude of the second feed back coupling. The two feed back couplings together create the three transmission zeros of the measured isolation response of the filter 28 as shown in Figure 6. The return loss of the filter 28 is also shown in Figure 6. The resonant frequency of the first and third HE₁₁₁ modes in cavity 30 is controlled by tuning screws 24, 22 respectively. Tuning screw 26 controls the resonant frequency of the second TM₀₁₁ mode in cavity 30. The resonant frequency of the fourth and fifth HE₁₁₁ modes in cavity 32 is controlled by tuning screws 52, 54 respectively. By increasing the penetration of the tuning screws 22, 24, 26, 52, 54 the resonant frequency of each of the five modes can be decreased. In Figure 7, there is shown a further embodiment of the invention in which a six-pole elliptic bandpass filter 56 has two adjacent cavities 58, 60, each of said cavities resonating in a triple mode. The same reference numerals will be used in Figure 7 to describe those components of the cavities 58, 60 that are similar to the components used in cavities 30, 32 of Figure 4. The cavities 58, 60 of the filter 56 function in a very similar manner to the cavity 30 of the filter 28. The cavity 58 is the input cavity and resonates in a first HE₁₁₁ mode, a second TM₀₁₁ mode and a third HE₁₁₁ mode. The input probe 8 couples energy into the cavity 58. The cavity 60 is the output cavity and resonates in a fourth HE₁₁₁ mode, a fifth TM₀₁₁ mode and a sixth HE₁₁₁ mode. Energy is coupled out of the filter 56 through output probe 42 that is mounted in a cavity 60. Transfer of energy from the first HE₁₁₁ mode to the second TM₀₁₁ mode in the cavity 58 is through coupling screw 16. Transfer of energy from the second TM₀₁₁ mode to the third HE₁₁₁ mode is through coupling screw 18. Transfer of energy from the third HE₁₁₁ mode in the cavity 58 to the fourth HE₁₁₁ mode in the cavity 60 is through aperture 38 on iris 34. Transfer of energy from the fourth HE₁₁₁ mode to the fifth TM₀₁₁ mode is through the coupling screw 62. Transfer of energy from the fifth TM₀₁₁ mode to the sixth HE₁₁₁ mode in the cavity 60 is through coupling screw 64. Resonant frequencies of modes one to three in cavity 58 are controlled by tuning screws 24, 26, 22 respectively. Resonant frequencies of modes four to six in cavity 60 are controlled by tuning screws 52, 54, 66 respectively. The filter 56 produces a six-pole elliptic bandpass response with four transmission zeros. The transmission zeros are created by feed back couplings between the first and sixth HE₁₁₁ mode (i.e. the M₁₆ coupling value) and between the second and fifth TM₀₁₁ modes (i.e. the M₂₅ coupling value). These two inter-cavity feed back couplings are achieved through aperture 36 on iris 34. In Figure 8, there is shown the simulated response of a six-pole elliptic bandpass filter constructed in accordance with Figure 7 with four transmission zeros. Since the maximum field points of the first and sixth modes occur at a different location from that of a second and fifth modes, by varying the vertical position and the length of the aperture 36, the two feed back couplings can be controlled independently. In Figure 9, there is shown a side view of the iris 34 with apertures 36, 38. While the filter will still function if the apertures 36, 38 are moved vertically to a different position relative to one another from that shown in Figure 9, the position shown in Figure 9 is a preferred position. If desired, the apertures 34, 36 could be positioned to intersect one another. However, the apertures 36, 38 must always be located so that they are symmetrical about an imaginary centre line of said iris 34, said centre line being parallel to an axis of said dielectric resonator. In the iris 34 shown in Figure 9, the imaginary centre line extends vertically across the iris 34 midway between side edges 68. Referring to Figure 10 in greater detail, there is shown a further embodiment of the invention in which a four pole elliptic bandpass filter 70 has two adjacent cavities 58, 72. Cavity 58 resonates in a triple mode and cavity 72 resonates in a single mode. The same reference numerals will be used in Figure 10 to describe those components of the cavities 58, 72 that are similar to the components used in cavities 58, 60 of Figure 7. The cavity 58 of the filter 70 functions in an identical manner to the cavity 58 of the filter 56 as shown in Figure 7. The cavity 58 is the input cavity and resonates in a first HE₁₁₁ mode, a second TM₀₁₁ mode and a third HE₁₁₁ mode. The input probe 8 couples energy into the cavity 58. The cavity 72 is the output cavity and resonates in a fourth HE₁₁₁ mode. Energy is coupled out of the filter 70 through the output probe 42 that is mounted in the cavity 72. Transfer of energy from the first HE₁₁₁ mode to the second TM₀₁₁ mode in the cavity 58 is through coupling screw 16. Transfer of energy from the second TM₀₁₁ mode to the third HE₁₁₁ mode is through coupling screw 18. Transfer of energy from the third HE₁₁₁ mode in the cavity 58 to the fourth HE₁₁₁ mode in the cavity 60 is through aperture 38 on iris 34. A feed back coupling is provided through the aperture 36 of the iris 34 between the first HE₁₁₁ mode and the fourth HE₁₁₁ mode by means of electrical field energy coupling across said aperture. Resonant frequencies of modes one to three in cavity 58 are controlled by tuning screws 24, 26, 22 respectively. The resonant frequency of the fourth mode in cavity 72 is controlled by tuning screw 52. While the filters shown in Figures 5, 7 and 10 are described as resonating in HE₁₁₁ and TM₀₁₁ modes, it should be understood that a filter embodying the present invention can be made to operate in any HE 11(N+1) mode and TM 01N mode, where N is a positive integer. Also, the filters shown in Figures 5, 7 and 10 have only two cavities. A filter embodying the present invention could be constructed with any reasonable number of cavities and triple mode cavities can be cascaded with other triple, dual or single mode cavities to form even or odd order filter functions. In Figures 1, 5, 7 and 10 input and output couplings are achieved with coaxial probes. In a variation of these filters, input and output coupling can be achieved with a ridge waveguide structure operating in a TE₀₁ mode in an under cut-off condition. While the filter embodying the present invention as described in Figures 5, 7 and 10 is a two-cavity filter having one triple mode cavity and either one dual mode cavity, a second triple mode cavity or one single mode cavity, respectively, the filter embodying the present invention is not restricted to the filters shown in the drawings. Virtually any reasonable combination of cavities can be used. For example, a filter embodying the present invention could have two triple mode cavities with a dual mode cavity being located between the two triple mode cavities. As a variation thereof, a three-cavity filter could have an L-shaped configuration with a triple mode cavity located at an angle on the L-shape partially between another triple mode cavity and a dual mode cavity. As a further variation, a four cavity, twelve-pole filter can have a square configuration, with each cavity being a triple mode cavity. In Figures 11, 12, 13, there is shown a number of variations in the arrangement of cavities for a filter embodying the present invention. In all of these three figures, for ease of illustration, all of the component parts of each of the cavities, other than the cavities themselves, have been omitted. In Figure 11, there is shown a filter having six linearly arranged cavities 80, 82, 84, 86, 88, 90. In this arrangement, cavities 80, 90 are end cavities and can be triple mode cavities. But cavities 82, 84, 86, 88 are interior cavities. Interior cavities cannot be triple mode cavities, (without undesirable design changes to the cavity walls) because the interior cavities have only two exposed walls that are normal to one another in which appropriate tuning and coupling screws can be mounted. In Figure 12, the same cavities have been re-arranged in two parallel rows so that cavities 80, 82, 84 are adjacent to cavities 90, 88, 86 respectively. It should be noted that, in this arrangement, the cavities 80, 90 are side by side. If the cavity 80 is the input cavity and cavity 90 is the output cavity, further flexibility can be achieved in the operation of the filter as coupling could be made to occur between the input and output cavities. In Figure 13, the cavities are again re-arranged in two parallel rows except that the cavities 80, 82, 84 are arranged side by side with the cavities 86, 88, 90 respectively. In this arrangement, if cavity 80 is the input cavity and cavity 90 is the output cavity, no coupling would occur between the input and output cavities. Whenever a filter embodying the present invention has more than two cavities in a single row, only the two end cavities of each row will have three exposed walls that are arranged orthogonal to one another in which tuning and coupling screws can be mounted for operating the cavity in a triple mode. In that case, for an interior cavity to operate in a dual mode, it will be desirable to locate one set of coupling screws and tuning screws of the interior cavity so that they are parallel to a centre axis of the dielectric resonator 6 of that cavity. A filter constructed embodying the present invention can achieve weight and size reductions of approximately one-half. This is very important when the filter is used for satellite communications. For example, it is possible to design a filter with a K th order, K being a multiple integer of 3, the filter having only K/3 cavities. Also, improved thermo stability can be achieved with the filters embodying the present invention relative to known triple mode or dual mode filters. In dielectric-loaded waveguide filters, the cavity dimensions are not critical thus, the thermal properties of the filter will be determined mainly by the thermal properties of the dielectric resonators.

Claims: A triple mode function bandpass filter (2) having at least one waveguide cavity (4) resonating in three independent orthogonal modes, said filter (2) having an input (8) and output (10) for transferring electromagnetic energy into and out of said filter (2), the or each cavity having two coupling screws and three tuning screws (22, 24, 26) mounted therein, said coupling screws coupling energy from one mode to another and each of said tuning screws controlling the resonant frequency of a different mode, characterised by the or each triple mode cavity having a third coupling screw (16, 18, 20) mounted therein, and the or each triple mode cavity (4) having a dielectric resonator (6) mounted therein, there being an intercavity coupling iris between adjacent cavities when the filter has more than one cavity. A bandpass filter as claimed in claim 1, characterised in that the filter (28) has at least two cavities and is a planar filter, and the dielectric resonator (6) is planar mounted. A bandpass filter as claimed in claim 1 or 2, characterised in that the filter operates in two HE 11(N+1) modes and a TM 01N mode, where N is a positive integer. A bandpass filter as claimed in claim 3, characterised in that the dielectric resonator (6) is mounted on a low-loss, low dielectric constant support (14). A bandpass filter as claimed in claim 2, characterised in that the inter-cavity coupling iris (34) has appropriate apertures (36, 38) positioned to couple energy between adjacent cavities, each of said cavities having a dielectric resonator (6) mounted therein. A bandpass filter as claimed in claim 5, characterised in that there are at least two triple mode cavities (58, 60) adjacent to one another, each triple mode cavity being located to have at least three exposed orthogonal walls. A bandpass filter as claimed in claim 5, characterised in that there is at least one single mode cavity (72) adjacent to said triple mode cavity (58).

Inventors: BEGGS, BRUCE CAMERON, SFERRAZZA, JOSEPH, SIU, DAVID, TANG, WAI-CHEUNG
Applicants: COM DEV LTD.
Application year: 1986
Main classification: H01P1
IPC codes: H01P1