US 3818389 A
Two interdigital filters are combined in a single structure. The structure consists of a series of adjacent interdigited elements; each filter including one or more appropriately tuned high Q elements and an input coupling element at one end of the high Q elements. Both filters share a common, intermediately located, output coupling element which is connected to a load. Where the dual filter is used in a mixer circuit the individual input signals may be applied to the input coupling elements at the extreme ends of the interdigital structure and a mixing diode may be mounted at the end of the common output coupling element.
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Description (OCR text may contain errors)
United States Patent [191 Fisher DUAL INTERDIGITAL FILTER FOR MICROWAVE MIXER  Inventor: Reed Edward Fisher,
Parsippany-Troy Hills Township, Morris County. NJ.
 Assignee: Bell Telephone Laboratories,
Incorporated, Murray Hill, NJ.
 Filed: Sept. 20, 1973  Appl. No.: 398,966
52 us. Cl 333/73 R, 321/69 w, 333/73 s, 333/73 w, 333/84 M, 333/97 R 51 Int. Cl HOlp 1/20, HOlp 3/08, H02m 5/30  Field of Search... 333/73 R, 73 s, 73 w, 82 R, 333/82 B, 83 R, 84 M, 84 R, 9.7 R, 98 R;
321/69 R, 69 w, 69 NL  References Cited UNITED STATES PATENTS 3,169,227 2/1965 Closson 321/69 NL [451 June 18, 1974 6/l967 Bolljahn et al .r 333/73 R 5 7] ABSTRACT Two interdigital filters are combined in a single structure. The structure consists of a series of adjacent interdigited elements; each filter including one or more appropriately tuned high Q elements and an input coupling element at one end of the high Q elements. Both filtersshare a common, intermediately located, output coupling element which is connected to a load. Where the dual filter is used in a mixer circuit the individual input signals may be applied to the input coupling elements at the extreme ends of the interdigital structure and a mixing diode may be mounted at the end of the common output coupling element.
7 Claims, 3 Drawing Figures PAIENTEU MBIHH 3.818.389
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PATENTEB JUN I FIG. 3
lINPUT snei anrz EEIIII lOUTPUT DUAL INTERDIGITAL FILTER FOR MICROWAVE MIXER BACKGROUND OF THE INVENTION This invention relates to microwave circuitry, and more particularly to microwave frequency filters of the interdigital type.
Microwave frequency apparatus often includes bandpass filters, and principles set forth by G. L. Matthaei, in interdigital Band-Pass Filters, IRE Transactions an M77, November 1962, p. 479 define a class of devices referred to as interdigital filters. These structures utilize a series of resonant elements arranged in a comblike pattern and the elements are tuned so that the structure exhibits a prescribed frequency characteristic over a selected band. These filters are compact and may be manufactured without critical tolerances.
One use for microwave filters is in frequency converters, especially down-converters for the front-end of superheterodyne receivers. Conventional converters for UHF reception exhibit critical coupling characteristics which makes their manufacture, installation and maintenance less than totally satisfactory. For example, trough-line cavity converters require troublesome adjustments of the links, probes and apertures.
Accordingly, it would be advantageous to construct these frequency converters without the use of the critically coupled filters of the past. To this end, interdigital type filters may be utilized as individual filters in prior art converters.
It is the object of the present invention to improve the interdigital filter structure for converter circuits, and particularly to provide a mixer structure including a signal and pump filter of the interdigital variety. It is more particularly the object of the invention to provide a signal and pump filter for the converter mixer as a single unit which may be manufactured at low cost.
SUMMARY OF THE INVENTION Conventional interdigital filters of this type include an array of elements; one or more appropriately tuned high Q elements, an input coupling element at one end of the array and an output coupling element at the other end of the array. The dual interdigital filter is arranged so that two filters share a single output coupling element. The elements may be conventional cylindrical rods or stripline conductors.
The common output coupling element provides an impedance match suitable for both filters and makes direct connection to a load which may be a mixer diode. Where the structure is used in a down-converter, the diode is most conventionally of the varistor variety and may actually be mounted in the same housing as the interdigital filter. The dual filter structure may, of course, be used with devices such as a varactor diode in an up-converter mode or more generally the structure may be used as a diplexer without any mixing diode.
BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a plan view of a mixer assembly including a rod-type dual interdigital filter in accordance with the present invention;
FIG. 2 is a front section of the assembly of FIG. 1
taken along line 22;
FIG. 3 is a plan view of a mixer assembly including a stripline type dual interdigital filter in accordance with the present invention.
FIGS. 1 and 2 show a single unit microwave mixing assembly including a dual interdigital filter. The top cover 11 has been cut away in FIG. 1 to expose five interdigitated hollow conductive rods-2l, 22, 23, 24 and 25 which may be tubing of, for example, brass or copper. The rods are soldered alternately to two side walls, 13 and 14, and as can be seen in FIG. 2 they are centrally located between the two parallel ground planes formed by the interior conductive surfaces 11A and 12A of cover 11 and bottom wall 12, respectively. Cover 11 and bottom wall 12 are joined to side walls 13 and 14 to form a microwave cavity, and the conductive interior surfaces 13A and 14A of walls 13 and 14, respectively, make continuous electrical contact with ground plane surfaces 11A and 12A. One edge of each side wall 13 and 14 may be permanently fastened to one of the ground planes, such as 12A, and the other edge of each side wall is preferably fastened to the cover 11 by screws 17 conveniently located over the center lines of each of the rods 21 through 25.
Each rod 21 through 25 in the cavity is grounded at one end and ungrounded at the other. Coupling between the rods is established by fringing electrical and magnetic fields. It is noted that since there are no significant electrical fields remote from end rods 21 and 25, end walls 15 and 16 are not required and the entire structure may thus be open-ended.
Electrically, rods 21, 22 and 23 comprise a one-stage, high-loaded Q interdigital filter 31. Rod 21 serves as the input coupling section to the filter. Rod 22 is the high Q resonator which is dimensioned to be approximately one-quarter the wavelength of the center of the desired passband and filter frequency is further tuned by adjusting set screw 18 which capacitively couples resonator rod 22 to grounded wall 13. Rod 23 serves as the output coupling section. The input signal is applied via conventional connector 19, the center conductor of which is insulated from wall 12 and couples the ungrounded end of rod 21 to external circuitry.
Rods 23, 24 and 25 constitute a one-stage, highloaded Q interdigital filter 32. Rod 24 is the high Q resonator and is tuned by set screw 20. Rod 23 serves as the output coupling section of filter 32. The input signal, which is applied to the ungrounded end of input coupling rod 25, is shown illustratively as emanating from diode 45 which is representative of a multiplier diode such as the last diode in a frequency multiplier chain. This arrangement is suitable where the input to filter 32 is a locally generated signal. In this case the generating oscillator and other multiplier diodes or transistors in the chain would be mounted external the cavity, most conveniently on an external extension of the bottom wall 12. The extension, not shown, could also provide a mounting surface for other externally located components. One end of diode 45 is connected to a metal disc 49 affixed to the ungrounded end of rod 25. The other end of the diode is connected to the externally mounted components through wall 14 by connector 40 which provides an insulated path via shaft 43 through the wall. The connector may also provide a bypass capacitor formed by wall 14 and conductive plat 46 separated by dielectric layer 47.
Rod 23 located intermediate to the high Q resonator rods 22 and 24 is the output coupling section of both filters 31 and 32. The output signal from the ungrounded end of this common output coupling rod is applied to a load, which may be external but is shown illustratively as mixing diode 35 mounted within the single dual filter assembly. The diode is conveniently connected to a metallic disc 39 which is affixed to the end of the rod 23. The output of the diode is passed through side wall 14 via any convenient connection, such as shaft 33 of connector 30, which insulates the connection from the wall. As shown in FIG. 1, a bypass capacitor formed by the wall and metallic plate 36, separated by dielectric layer 37, is included in the connector.
In the mixer assembly of FIG. 1, a local oscillator (pump) signal generated at the output of diode 45 is filtered by filter 32 which serves as a pump filter. Simultaneously, an input signal coupled from a signal source, such as an antenna, through connector 19, is applied to filter 31 which serves as a signal filter. The two filtered signals from the common output section formed by rod 23 are mixed by diode 35. The loaded Qs of the two filters are high enough to prevent unwanted coupling of both signal power to the multiplier diode 45 and local oscillator power back to the signal source.
This arrangement is assumed to be a down-converter for use in the front end of a superheterodyne receiver and mixing diode 35 would thus be a varistor type diode. However, the structure may also be used as an upconverter, in which case a varactor type diode would be used.
The dual filter structure may, of course, be utilized as a diplexer without the mixing diode 35 and alternative connective arrangements may be used in place of connectors 30 and 40. In addition, the input signal to filter 32 could be produced by means other than a multiplier chain and, hence, diode 45 is not required. Though the rods are shown soldered to walls 13 and 14, ground must be provided only at microwave frequencies, and the required ground could alternatively be achieved by a capacitive connection, such as is disclosed in U.S. Pat. No. 3,348,173 issued Oct. 17, 1967 to G. L. Matthaei et al.
Filters 31 and 32 may also be multiple-stage interdigital filters and their interdigital elements may be other than hollow rods. These alternatives are illustrated in FIG. 3 which shows a layout of a single-unit microwave mixer assembly including a stripline type dual interdigital filter consisting of seven planar stripline conductor strips or bars arranged to form two multiple-stage filters 61 and 62 sharing a common intermediately located stripline conductor element 54. Filter 61 includes an input coupled element 51 and two high Q resonant sections 52 and 53. Filter 62 includes an input coupling section 57 and two high Q resonant sections 55 and 56. Appropriate tuning screws 63 are provided for resonator elements 52, 53, 55 and 56 and standard connections are made between input coupling sections 51 and 57 through the exterior walls via connectors 58 and 59,
respectively. The output from the common coupling section 54 is applied to a load which may be externally located but is shown as diode 70. The diode is mounted within the cavity but is connected to external circuitry by means of connector assembly 71.
In all cases it is to be understood that the abovedescribed arrangements are merely illustrative of a small number of the many possible applications of the principles of the invention. Numerous and varied other arrangements in accordance with these principles may readily be devised by those skilled in the art without departing from the spirit and scope of the invention.
What is claimed is:
l. A microwave structure comprising, a pair of parallel conductive members being electrically connected, a plurality of parallel conducting elements linearly arranged in an interdigital pattern between said parallel conductive members, the ends of adjacent ones of said elements being affixed to opposite ones of said conductive members and extending toward the other of said conductive members but terminating short thereof, first and second means for coupling signal energy respectively into the two extreme end elements of said plurality of elements, the elements between one of the end elements and an intermediate one of said elements being dimensioned to form with said one end element and with said intermediate element a filter adapted to pass a first preselected band of frequencies, said elements between the other of the end elements and said intermediate element bein dimensioned to form with said other end element an with said intermediate element a filter adapted to pass a second preselected band of frequencies, and means for coupling energy in both the first and second preselected bands of frequencies from said intermediate element.
2. A microwave structure as claimed in claim 1 wherein said means for coupling energy from said intermediate element includes means mounted at the termination of said intermediate element for mixing the energy associated with both said first and second preselected band of frequencies.
3. A microwave structure as claimed in claim 1 wherein said first means for coupling energy into one end element includes a multiplier diode mounted at the termination of the one end element.
4. A microwave structure as claimed in claim 1 wherein said plurality of parallel conducting elements are hollow conductive rods, each having its affixed end grounded at microwave frequencies to one of said conductive members.
5. A microwave structure as claimed in claim 1 wherein said plurality of parallel conducting elements are planar conductive strips, each having its affixed end grounded at microwave frequencies to one of said conductive members.
6. A microwave structure as claimed in claim 1 further including a pair of conductive grounded planes, each plane connected to both of said conductive members to form a conductive boundary about said plurality of parallel conducting elements.
7. A microwave structure comprising, a conductively bounded microwave cavity, a plurality of parallel conducting elements, each element having one end grounded to the cavity and having the opposite end ungrounded, said elements being arranged in an interdigital pattern so that opposite ends of adjacent elements are so grounded, first and second means for coupling signal energy respectively into the two extreme end elements of .said plurality of elements, the elements between one of the end elements and an intermediate one of said elements being dimensioned to form with said one end element and with said intermediate element a filter adapted to pass a first preselected frequency band, said elements between the other of the end elements and said intermediate element being dimensioned to form with said other end element and with said intermediate element a filter adapted to pass a second preselected frequencyband, and means for coupling both the first and second preselected frequency bands from said intermediate element.