CA1041619A - Adjustable interdigital microwave filter - Google Patents
Adjustable interdigital microwave filterInfo
- Publication number
- CA1041619A CA1041619A CA236,500A CA236500A CA1041619A CA 1041619 A CA1041619 A CA 1041619A CA 236500 A CA236500 A CA 236500A CA 1041619 A CA1041619 A CA 1041619A
- Authority
- CA
- Canada
- Prior art keywords
- housing
- resonators
- filter
- microwave
- bandwidth
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
- Expired
Links
Classifications
-
- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01P—WAVEGUIDES; RESONATORS, LINES, OR OTHER DEVICES OF THE WAVEGUIDE TYPE
- H01P1/00—Auxiliary devices
- H01P1/20—Frequency-selective devices, e.g. filters
- H01P1/201—Filters for transverse electromagnetic waves
- H01P1/205—Comb or interdigital filters; Cascaded coaxial cavities
Landscapes
- Physics & Mathematics (AREA)
- Electromagnetism (AREA)
- Control Of Motors That Do Not Use Commutators (AREA)
Abstract
TITLE
ADJUSTABLE INTERDIGITAL MICROWAVE FILTER OF CONSTANT BANDWIDTH
ABSTRACT OF THE DISCLOSURE
A rectangular waveguide is operated in the transverse electromagnetic mode as a bandpass filter by utilizing interdigital resonators which are alternately mounted on and perpendicular to two opposing walls of the waveguide. When the length of the resonators are adjustably altered to vary the center frequency, there is a corresponding change in the mutual coupling between adjacent resonators such that the bandwidth of the filter remains substantially constant and independent of changes in center frequency.
ADJUSTABLE INTERDIGITAL MICROWAVE FILTER OF CONSTANT BANDWIDTH
ABSTRACT OF THE DISCLOSURE
A rectangular waveguide is operated in the transverse electromagnetic mode as a bandpass filter by utilizing interdigital resonators which are alternately mounted on and perpendicular to two opposing walls of the waveguide. When the length of the resonators are adjustably altered to vary the center frequency, there is a corresponding change in the mutual coupling between adjacent resonators such that the bandwidth of the filter remains substantially constant and independent of changes in center frequency.
Description
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1 ADJUSTABLE INTERDI~ITAL MICROWAV~ FI~TER OF CONSTANT ~ANDWIDTH
BACKGROUND OF THE INVENTION
6 The invention relates to microwave interdigital bandpass ~ filters cypically used in radio telecommunication systems.
8 Microwave interdigital bandpass filters, and the general 9 mathematical bases therefor, are well known. Such filters utilize the transverse electromagnetic mode relationships which exist between the 11 coupled elements, or resonators~ which extend perpendicularly from the 12 internal housing wall or walls thereof.
13 ~ More specifically, the interdigital filter structure consists 14 of a conductive microwave cavity, such as a waveguide, in which a series of conductive rods (also called resonators) are alternately spaced 16 longitudinally on opposite side walls of the cavity. One end of each rod is 17 grounded by physical connection to the housing wall, and is usually referred 18 to as the short circuit end. The other end of the rod protruding into the 19 housing cavity is ungrounded, and is referred to as the open circuit end.
These rods are made adjustable in length so that their physical length can 21 be made equal to n ~/4, where n is an odd integer and ~ is the wavelength ~ -22 of the center frequency of the filter passband. The rods form a series of 23 resonant elements tuned so that the transmission of the cavity exhibits the 24 bandpass frequency eharacteristic desired. In such a multi-element structure there is a definite coupling of energy between the spatially
1 ADJUSTABLE INTERDI~ITAL MICROWAV~ FI~TER OF CONSTANT ~ANDWIDTH
BACKGROUND OF THE INVENTION
6 The invention relates to microwave interdigital bandpass ~ filters cypically used in radio telecommunication systems.
8 Microwave interdigital bandpass filters, and the general 9 mathematical bases therefor, are well known. Such filters utilize the transverse electromagnetic mode relationships which exist between the 11 coupled elements, or resonators~ which extend perpendicularly from the 12 internal housing wall or walls thereof.
13 ~ More specifically, the interdigital filter structure consists 14 of a conductive microwave cavity, such as a waveguide, in which a series of conductive rods (also called resonators) are alternately spaced 16 longitudinally on opposite side walls of the cavity. One end of each rod is 17 grounded by physical connection to the housing wall, and is usually referred 18 to as the short circuit end. The other end of the rod protruding into the 19 housing cavity is ungrounded, and is referred to as the open circuit end.
These rods are made adjustable in length so that their physical length can 21 be made equal to n ~/4, where n is an odd integer and ~ is the wavelength ~ -22 of the center frequency of the filter passband. The rods form a series of 23 resonant elements tuned so that the transmission of the cavity exhibits the 24 bandpass frequency eharacteristic desired. In such a multi-element structure there is a definite coupling of energy between the spatially
2~ displaced overlapping portions of the rods which affects the transmission 27 characteristics of the filter.
28 Interdigital filters, used in microwave radio systems are 29 designed to operate over a relatively wide range of frequencies so that the passband of the filter can be set for any channel assignment in a particular ~ i . - . . ,, , , ... ., . .
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1 ¦ radio band. For any particular channel assignment the center frequency of Z ¦ the filter fc, can be changed to the appropriate frequency by varying the
28 Interdigital filters, used in microwave radio systems are 29 designed to operate over a relatively wide range of frequencies so that the passband of the filter can be set for any channel assignment in a particular ~ i . - . . ,, , , ... ., . .
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1 ¦ radio band. For any particular channel assignment the center frequency of Z ¦ the filter fc, can be changed to the appropriate frequency by varying the
3 ¦ electrical length of the rods. Whatever the channel assignment, the filter
4 ¦ bandwidth, BW, should remain fixed~ Vnfortunately it is an inherent ¦ disadvantage of such filters that as the center frequency is changed the 6 ¦ bandwidth also changes proportionally. This problem exists because in any 7 ¦ particular filter the bandwidth and center frequency are related to the 8 ¦ coupling coefficient K; BW/fc = K. If K remains constant, for each change 9 ¦ in fc there must be a corresponding change in BW. To correct for this ¦ changing bandwidth problem some filter designs employ added compensating 11 ¦ means to adjust the bandwidth as fc is changed. One technique uses 12 ¦ additional posts placed between each rod to further define a cavity and 13 ¦ modify the shape of the microwave field distribution.
14 ¦ Additionally, the methods of constructing the filter housing ¦ have been susceptible to various limitations. Normally, the housing is 16 ¦ formed of built~up metal plates and spacers, or by casting the desired 17 ¦ configuration. In the former, the plates are screwed together which can 18 ¦ give rise to radio frequency leakage during use. In the alternative, the 19 ¦ plates are soldered together thereby creating a problem with corrosive flux ¦ deposits in addition to the operationally detrimental conductivity of the 21 ¦ solder it6elf. In the latter, i.e. castings, meticulous, and thus 22 ¦ expensive, finishing of the interior surfaces is required while 23 ¦ simultaneously controlling the porosity of such surfaces.
24 ¦ It is therefore, an object of this invention to provide a ¦ microwave interdigital filter for use in the transverse electromagnetic mode26 l which has an adjustable center frequency and a Eixed bandwidth without the 27 ¦ use of additional bandwidth compensating means.
~8 l ¦ In accordance with the invention a microwave interdigital 31 ¦ bandpass filter is provided for operation in the transverse electromagnetic 32 ¦ mode.
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~(;3416~9 1 The resonators which are mounted in contact with and pass 2 through the wall of the housing are, each within the enclosed housing 3 cavity, comprised of two portions. The first and fixed portion extends from 4 the interior of the housing wall and terminates in a plurality of ¦ concentrically arranged, semi-arcuately shaped "fingers" which mechanically 6 ¦ and electrically contact the second resonator portion supported therein and 7 ¦ passing therethrough.
9 ¦ One end of the second resonator portion protrudes into the ¦ housing interior beyond the ends of the first portion finger to comprise the 11 ¦ portion of the resonator that is mutually coupled to analogous portions of 12 ¦ adjoining resonators. The noncoupled length of the second resonator portion 13 ¦ passes through the first portion and through the housing wall where it is 1~ ¦ threaded to accept a locking means such as a threaded nut. A fixed mounting ¦ of the nut on the exterior wall of the housing provides means for axially 16 ¦ adjusting the length of the second resonator portion that protrudes from the -17 ¦ fingers of the first portion. When the length of the second resonator 18 ¦ portion is altered to vary the center frequency of the filter, there is a `
19 ¦ corresponding change in the mutual coupling between adjacent resonators such ¦ that the bandwidth of the filter remains substantially constant.
21 l 22 ¦ An embodiment of the invention will now be described, by way 23 ¦ of example, with reference to the accompanying drawings in which:
24 ¦ FIG. 1 is a perspective view; partially in section, of a ¦ filter embodying the invention;
26 ¦FIG. 2 is a side view in section showing typical resonator 27coupling for a low frequency condition; and 28FIG. 3 is a side view in section showing typical resonator 29coupling for a high frequency condition.
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1iO41619 l This invention relates to an interdigital filter for operation 2 in the transverse electromagnetic mode which is advantageously used as a 3 component in a microwave communication system.
4 More specifically, and with reference to FIG. l, the filter generally comprises a housing 10 and a plurality of resonators lZ. The 6 housing 10 is preferably constructed of a rectangular wave guide which is 7 for~ed from a seamless copper extrusion. By utilizing the extrusion 8 process, the housing is formed with an exceptionally smooth highly 9 conductive dense inner surface which provides an accurate low resistive ground plane for the filter structure.
ll The resonators 12 are mounted, as shown, in and through the 12 walls of the housing 10. Within the housing, the resonators are comprised 13 of a first or short circuit conductive end 14. The short circuit end of 14 each resonator is fixed in position and is not adjustable. This fixed mounting ensures that the current flow in the area of the short circuit end, læ which is the high current region, will not be affected by any variation 17 resulting from a variable mechanical contact.
18 The end of the first resonator portion that is spacedly 19 disposed with respect to the housing walls 18 is formed with a plurality of so-called fingers 20 that are bent radially inwardly at their ends 22.
21 These flexible fingers provide mechanical and electrical contact to the 22 ¦ spaced and movable portion which passes therethrough.
23 The movable portion extends through the respective wall 18 and 24 terminates in a screw thread 24 and slot 26 Eormed thereon external to the housing. The threaded end of the second resonator portion i5 received and 28 ¦ maintained in radial position by a spacer 27 that is secured to the housing 27 ¦ wall and is maintained in an axially adjustable position by a threaded nut 28 1 28. , 29 ¦ Input and output coupling to the filter is accomplished by 3l meana of capacitive probes 30 to which input and output connectors 32 and 34 ~2 -4-'P~ l ~ :; , : ,, ~16~'~
1 are directly attached. These probes 30 provide a capacitive coupling 2 between the connectors and the open c;rcuit ends 16 of the outermost 3 resonators. This coupling, ~hich determines the efficiency of power 4 transfer into and out of the filter, is adjusted by means of threads 40 on the connectors 32 and 34 which are screwed into nuts 42 fixed to the filter 6 housing 10 to thereby alter the relative proximity of the probe to the 7 associated resonator end.
8 During alignment of the filter, the resonators and the 9 capacitive probes are screwably adjusted so that the desired center frequency and return-loss are obtained. The dotted lines of FIG. 2 ll illustrate a mutually coupled length of the resonators that would be used 12 for low frequencies. FIG. 3 on the other hand, illustrates the position of the rods for a higher frequency setting. Clearlyg the amount of coupling l~ between the rods has been reduced since the physical length 'of the rods has been effectively reduced.
16 In FIG. 2 and FIG. 3 it can be seen that the mutually-coupled 17 length decreases as the frequency increases. This causes the bandwidth to 18 decrease as 'the mutually-coupled length decreases. This bandwidth reduction l9 is in proportional opposition to the inherent tendency of an uncompensated filter to exhibit an increasing bandwidth.
21 The described adjustable resonators and capacitive probes have 22 been found to be efficient and easy to adjust over a wide tuning range, 23 e.g., from 1.9 GHz to 2.3 GHz and to inherently provide a constant 24 bandwidth. The amount of physical adjustment provided for is determined by the required tuning range. Additionally, it has been found that minor 26 variations in t'he resonator length adjustment enable fringing and other 27 detrimental effects to be compensated for.
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14 ¦ Additionally, the methods of constructing the filter housing ¦ have been susceptible to various limitations. Normally, the housing is 16 ¦ formed of built~up metal plates and spacers, or by casting the desired 17 ¦ configuration. In the former, the plates are screwed together which can 18 ¦ give rise to radio frequency leakage during use. In the alternative, the 19 ¦ plates are soldered together thereby creating a problem with corrosive flux ¦ deposits in addition to the operationally detrimental conductivity of the 21 ¦ solder it6elf. In the latter, i.e. castings, meticulous, and thus 22 ¦ expensive, finishing of the interior surfaces is required while 23 ¦ simultaneously controlling the porosity of such surfaces.
24 ¦ It is therefore, an object of this invention to provide a ¦ microwave interdigital filter for use in the transverse electromagnetic mode26 l which has an adjustable center frequency and a Eixed bandwidth without the 27 ¦ use of additional bandwidth compensating means.
~8 l ¦ In accordance with the invention a microwave interdigital 31 ¦ bandpass filter is provided for operation in the transverse electromagnetic 32 ¦ mode.
l -2-~'i,`i l . .
" ' ' ' ''" ' : : , ~: :
~(;3416~9 1 The resonators which are mounted in contact with and pass 2 through the wall of the housing are, each within the enclosed housing 3 cavity, comprised of two portions. The first and fixed portion extends from 4 the interior of the housing wall and terminates in a plurality of ¦ concentrically arranged, semi-arcuately shaped "fingers" which mechanically 6 ¦ and electrically contact the second resonator portion supported therein and 7 ¦ passing therethrough.
9 ¦ One end of the second resonator portion protrudes into the ¦ housing interior beyond the ends of the first portion finger to comprise the 11 ¦ portion of the resonator that is mutually coupled to analogous portions of 12 ¦ adjoining resonators. The noncoupled length of the second resonator portion 13 ¦ passes through the first portion and through the housing wall where it is 1~ ¦ threaded to accept a locking means such as a threaded nut. A fixed mounting ¦ of the nut on the exterior wall of the housing provides means for axially 16 ¦ adjusting the length of the second resonator portion that protrudes from the -17 ¦ fingers of the first portion. When the length of the second resonator 18 ¦ portion is altered to vary the center frequency of the filter, there is a `
19 ¦ corresponding change in the mutual coupling between adjacent resonators such ¦ that the bandwidth of the filter remains substantially constant.
21 l 22 ¦ An embodiment of the invention will now be described, by way 23 ¦ of example, with reference to the accompanying drawings in which:
24 ¦ FIG. 1 is a perspective view; partially in section, of a ¦ filter embodying the invention;
26 ¦FIG. 2 is a side view in section showing typical resonator 27coupling for a low frequency condition; and 28FIG. 3 is a side view in section showing typical resonator 29coupling for a high frequency condition.
3~ -3-. :, , .... .... ,: :
1iO41619 l This invention relates to an interdigital filter for operation 2 in the transverse electromagnetic mode which is advantageously used as a 3 component in a microwave communication system.
4 More specifically, and with reference to FIG. l, the filter generally comprises a housing 10 and a plurality of resonators lZ. The 6 housing 10 is preferably constructed of a rectangular wave guide which is 7 for~ed from a seamless copper extrusion. By utilizing the extrusion 8 process, the housing is formed with an exceptionally smooth highly 9 conductive dense inner surface which provides an accurate low resistive ground plane for the filter structure.
ll The resonators 12 are mounted, as shown, in and through the 12 walls of the housing 10. Within the housing, the resonators are comprised 13 of a first or short circuit conductive end 14. The short circuit end of 14 each resonator is fixed in position and is not adjustable. This fixed mounting ensures that the current flow in the area of the short circuit end, læ which is the high current region, will not be affected by any variation 17 resulting from a variable mechanical contact.
18 The end of the first resonator portion that is spacedly 19 disposed with respect to the housing walls 18 is formed with a plurality of so-called fingers 20 that are bent radially inwardly at their ends 22.
21 These flexible fingers provide mechanical and electrical contact to the 22 ¦ spaced and movable portion which passes therethrough.
23 The movable portion extends through the respective wall 18 and 24 terminates in a screw thread 24 and slot 26 Eormed thereon external to the housing. The threaded end of the second resonator portion i5 received and 28 ¦ maintained in radial position by a spacer 27 that is secured to the housing 27 ¦ wall and is maintained in an axially adjustable position by a threaded nut 28 1 28. , 29 ¦ Input and output coupling to the filter is accomplished by 3l meana of capacitive probes 30 to which input and output connectors 32 and 34 ~2 -4-'P~ l ~ :; , : ,, ~16~'~
1 are directly attached. These probes 30 provide a capacitive coupling 2 between the connectors and the open c;rcuit ends 16 of the outermost 3 resonators. This coupling, ~hich determines the efficiency of power 4 transfer into and out of the filter, is adjusted by means of threads 40 on the connectors 32 and 34 which are screwed into nuts 42 fixed to the filter 6 housing 10 to thereby alter the relative proximity of the probe to the 7 associated resonator end.
8 During alignment of the filter, the resonators and the 9 capacitive probes are screwably adjusted so that the desired center frequency and return-loss are obtained. The dotted lines of FIG. 2 ll illustrate a mutually coupled length of the resonators that would be used 12 for low frequencies. FIG. 3 on the other hand, illustrates the position of the rods for a higher frequency setting. Clearlyg the amount of coupling l~ between the rods has been reduced since the physical length 'of the rods has been effectively reduced.
16 In FIG. 2 and FIG. 3 it can be seen that the mutually-coupled 17 length decreases as the frequency increases. This causes the bandwidth to 18 decrease as 'the mutually-coupled length decreases. This bandwidth reduction l9 is in proportional opposition to the inherent tendency of an uncompensated filter to exhibit an increasing bandwidth.
21 The described adjustable resonators and capacitive probes have 22 been found to be efficient and easy to adjust over a wide tuning range, 23 e.g., from 1.9 GHz to 2.3 GHz and to inherently provide a constant 24 bandwidth. The amount of physical adjustment provided for is determined by the required tuning range. Additionally, it has been found that minor 26 variations in t'he resonator length adjustment enable fringing and other 27 detrimental effects to be compensated for.
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Claims (4)
1. A microwave interdigital filter for operation in the transverse electromagnetic mode exhibiting a bandpass transmission characteristic with a tunable center frequency fc and having a constant bandwidth, said filter comprising:
a conductive housing in the shape of an elongated rectangular parallelepiped;
input and output means for coupling microwave signals respectively into and out of said housing; and a plurality of conductive coplanar resonators alternately mounted on and perpendicular to two opposing internal walls of said housing, each of said resonators comprising:
a hollow first portion mechanically and electrically secured to one housing wall and extending into said housing interior;
a second portion movably supported within said first portion and extending into the housing interior beyond said first portion, said second portion having a threaded end extending through the housing wall, and being secured to the housing wall in such a manner to permit a variable adjustment in length of said second portion extending beyond said first portion such that as said length is changed to effect a change in fc a proportional change in mutual coupling occurs thereby maintaining the bandwidth of said filter substantially constant.
a conductive housing in the shape of an elongated rectangular parallelepiped;
input and output means for coupling microwave signals respectively into and out of said housing; and a plurality of conductive coplanar resonators alternately mounted on and perpendicular to two opposing internal walls of said housing, each of said resonators comprising:
a hollow first portion mechanically and electrically secured to one housing wall and extending into said housing interior;
a second portion movably supported within said first portion and extending into the housing interior beyond said first portion, said second portion having a threaded end extending through the housing wall, and being secured to the housing wall in such a manner to permit a variable adjustment in length of said second portion extending beyond said first portion such that as said length is changed to effect a change in fc a proportional change in mutual coupling occurs thereby maintaining the bandwidth of said filter substantially constant.
2. A microwave interdigital filter as in claim 1 wherein said input and output means includes a capacitive probe having a capacitive probe end and a threaded probe end, said capacitive probe end being capacitively coupled to the open-circuited end of said second resonator portion of the outermost resonators, and threaded fastening means secured to the wall of said housing opposite said one of said resonators, said threaded probe end being received by said threaded fastening means for providing axial adjustment of the position of said capacitive probe end with respect to said open-circuited end of said one of said resonators which is effective to adjust the capacitive coupling therebetween.
3. A microwave interdigital filter as in claim 2 wherein each of said hollow first resonator portions comprises a plurality of arcuately disposed fingers in movable contact with said second resonator portion for providing mechanical and electrical contact therebetween.
4. A microwave interdigital filter as in claim 3 wherein said conductive housing is an extruded copper waveguide.
Priority Applications (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA236,500A CA1041619A (en) | 1975-09-26 | 1975-09-26 | Adjustable interdigital microwave filter |
Applications Claiming Priority (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA236,500A CA1041619A (en) | 1975-09-26 | 1975-09-26 | Adjustable interdigital microwave filter |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1041619A true CA1041619A (en) | 1978-10-31 |
Family
ID=4104155
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA236,500A Expired CA1041619A (en) | 1975-09-26 | 1975-09-26 | Adjustable interdigital microwave filter |
Country Status (1)
| Country | Link |
|---|---|
| CA (1) | CA1041619A (en) |
Cited By (2)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| FR2569496A1 (en) * | 1984-08-21 | 1986-02-28 | Murata Manufacturing Co | DIELECTRIC FILTER |
| EP0176966A2 (en) * | 1984-09-27 | 1986-04-09 | Nec Corporation | Bandpass filter with dielectric resonators |
-
1975
- 1975-09-26 CA CA236,500A patent/CA1041619A/en not_active Expired
Cited By (3)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| FR2569496A1 (en) * | 1984-08-21 | 1986-02-28 | Murata Manufacturing Co | DIELECTRIC FILTER |
| EP0176966A2 (en) * | 1984-09-27 | 1986-04-09 | Nec Corporation | Bandpass filter with dielectric resonators |
| EP0176966A3 (en) * | 1984-09-27 | 1988-06-22 | Nec Corporation | Bandpass filter with dielectric resonators |
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