US2749523A - Band pass filters - Google Patents

Description

`lune 5, 1956 M. DlsHAL. 2,749,523

BAND PASS FILTERS Filed Dec. 1; 1951 2 sheets-sheet 1 ATTENUAT/ON l I I UUTPl/T INVENTOR M/L TON /5H/4L June Y5, 1956 M. DlsHAL BAND PASS FILTERS 2 Sheets-Sheet 2 Filed Dec. l 1951 INVENTOR MILTON DSHAL ATTORNEY Unite 19 Claims. (Cl. 333-73) This invention relates to filters, and more particularly `tcYV'I-IF, UHF, andmicrowave band pass filters of the cavity resonatortype having high rates of cnt-olf.

It 1'has been understood by'those skilled in the art for Amany years that filter networks whose transfer impedance or admittance have both poles and zeros can be designed to producehigher rates of cut-ofi than is possiblewith Acomparable networks having only poles and no zeros. For instance, 11i-derived .filter configurations are ,capable of `greater rates of cut-off than comparable constant-k configurations. When operating in the VHF, UHF, Vand microwave region, it has been further Vunderstood that band passfilters are usually required to supply'srnall percentage 'pass bands, vand to successfully lmeet this requirement, the ratios between the reactances, as meas- 'ured at the mid frequency, of all the various elements in the 'configuration must conform to rigid tolerances. An equivalent concept is embodied in the statement Vthat'the resonant frequency of each resonator and the coefficients of coupling between resonators must .conform to rigid tolerances.

Heretofore, *it has :been `attempted to provide VHF, UHF, and microwave band pass filters having a high rate of'cut-oif due to zeros in the transfer characteristic by utilizingthe old .wellknown configurations and' concepts of standard filter theory. Thishas resulted incircuits having both a configuration and required reactance ratios, `that are not practicably obtained.

It `is `one of the objectives lof the present invention to *provide :a band -pass filter having both a Vconfiguration `and the required reactanee ratios which are practicable to obtain and having provisions to vary the resonant "frequency of :such a configuration 'to accommodate a v-rangeof frequencies in the VI-EF, UHR'andtmicrowave v region. By ernp`loying coaxial orl cavity resonatorswhich -are fanalogous -to I a t low frequency resonant 'circuit with :eah'resonator correctly Ycoupledto the `following resonator =eitherfma`gnetieally 'or electrically, I provide'al'band pass Efilter-operable in Vvthis VHF, UHFfand microwave 7region. 7The-'dimensions `of the cavity resonator areso '-s'elected -thatitw'ill resonate 'at Va desired frequency, or `means maybe provided'to produceresonanceatthe'desired'frequency 'The dimensions of the couplingfopeny"ings 'between adjacent resonators produces Vthe'zcolrrect 'eoe'iiicient of coupling between adjacent resonators. 'Therefore,byfproperly adjusting the =resonators resonant `frequency andfthe coefficients of coupling 'between' adjatserttiresonators `and lthe coupling of the generator :land loadtovthe first andlast'resouators, respectively,a Vtrans- .ferzcharacteristi'c .is Iobtained having complex frequency :poles '.Another object o'f this inventionis'to'provide aifilter lsystemfliavin g iboth poles and zeros Tin 'the transfer Schar- .,aeteristictotproducea-:highrate'of entr-off. VThis-object iis t obtained by s carrying :the `aforenlentioned Lfilterinetwork nonesstep :further to :introduce vzeros rand therebytprovi'de Leadiigh nate of1;cutolf.similar;to thatzobtainedwithnnmderived filter configuration. These zeros or innitefat- States Patent 2,749,523 Patented June 5, 1956 ICC '2 tenuation points areprovided by two coupling lines -between alternate cavities of a filter configuration. Thus, inaddition to the regular coupling connection between adjacent resonators, alternate reasonators to which coupling lines are connected have a pair of probe type'couplings for `each'pair of coupling lines which .connect to them. Due to the fact that it is -possible to get oppositely phased inductive probes, there are fourdfferent cornbinations that can be used to make up the above mentioned two pairs of probes and their coupling lines. The required electrical length ofthe V4coupling lines between alternate resonators will depend upon the probe .combinations employed in `each of `the alternate resonators. In general, including'the end effects of the probes, the coupling lines ywill be some multiple of a vquarterwvavevlength long. The basic requirement for the probe combinations and their'associated coupling lines is'that, 'one line shall produce between the two resonators a knet capacitive type coupling which is essentially constant over the frequency band for which the filter is to be used, and the other line shallproduce between the two resonators a net'constant inductive type coupling `and `the .two types of coupling shall be essentially equal in magnitude. The plurality of cavity resonators may be any desired number with the restriction that'the aforementioned coupling lines must couple alternate cavities in the 'manner indicated and no coupling lines may by-pass energy past othercoupling lines. An explanation of Athe production of zeros is that at some frequencies there is a 180 phase Idierence between the 'net current injected into 'the of this invention and the manner of attaining them will be best understood by referenceto the following description of'enibodiments of the invention taken in conjunction with the accompanying drawings, wherein:

Fig. 1 is a longitudinal cross-section view of a band pass v'filter operable inthe VHF, UHF, and microwave regions inaccordance with the principles of this invention;

`Fig. y2 is a cross-Section view `partiy in elevation 'along line 2 2 onFigjl;

Fig. 3 shows l'the vresponse curves 'for the Vband pass filter, onewith'and'the other without zeros; and

VFigs.'4, A'5, 6, and 7 are longitudinal cross-'Section views 'of band-passfilterseach `showing a different embodiment of the iinvention.

'Referring to Figs. 1 and 2 of the drawing, a band pass -filter capable-of 'operating in the VHF, UHF,and.micro -Wave Vregions isshown as-comprising five coaxial resonators 1, 2, 3 4, and 5 having tuning slugs 6, 7,'8, 9.and 'l0 ther-ein. Cavity resonator ll'v/ith its tuning slug 6 will lelectric .'field present therebetween.

`be described inA detail inrelation `to its configuration which willtsuflice as 'adescription for cavity resonators 2,v 3, 4, land v5 since all resonators herein employed are similar.

4Resonator 1 is considered coaxial due to the inclusion offtun'inglslug '6in the cavity. Tuning slug 6 behaves similar Zto thecenter `conductor of a coaxial line while the 'walls of'resonatorl behave as the outer conductor pendicular-'relationwith the lines of force of the electric ifield. .To enable Vthe :establishment of fields in such arelaltion, it isinecessary :to have 'the dimensions of the cavity correctiforzafdesired frequency. vFor example, a filter operating at ..1'5001rnc. could employ l1/2 cubes, the

walls being approximately 1%16" thick and the slugs should be approximately B/s in diameter. The dimensions of the cube are important as those skilled in the art will realize, while the thickness of the walls and diameter of the slugs are variables that will depend on the skineffect requirement and desired amount of coupling from the resonator at the operating frequency. Tuning slug 6 is adjustable for frequency selection and aperture 11, which mates with aperture 12, sets the coeiicient of coupling to the adjacent resonator, the approximate design equations for coefficient of coupling between adjacent resonators being B W3 ab fo Coaxial resonators 1, 2, 3, 4, and 5 may be held together by soldering, bolt and nuts, or screws, or by some other suitable means. if desired, the walls of the several cavities may be made integral. Coupling between adjacent coaxial resonators 1, 2, 3, 4, and 5 is accomplished by apertures, such as apertures 1I. and 12 in the walls of coaxial resonators 1 and 2. The arrangement shown in Fig. l of the drawing is such that there is magnetic and electric coupling progressing alternately from resonator 1 to resonator 5 of the filter. This arrangement of coupling by alternate fields reduces the direct feedthrough of frequency, on say the magnetic field, which would tend to reduce the skirt selectivity of the filter. in other words, by alternating the adjacent resonator coupling between magnetic and electric fields, it is necessary for the energy in the magnetic field to be transferred to the electric field by a resonator before it can pass on to the next resonator so that the energy must pass selectively from one cavity to another giving an overall optimum selectivity. Using the same line of reasoning, it can be seen that the input aperture 13 and the output aperture 14 should follow the same pattern. It is conceivable that the location of apertures 13 and 14 could be such as not to follow this pattern with the only result being that far out on the skirts of the response the attenuation may not be as great as possible.

The location of the strongest electric field is in the region 1S near the end of tuning slug 6 and the strongest magnetic field is in the region 16 far from the end of tuning slug 6. To obtain maximum results from the filter, it is helpful to locate the adjacent coupling apertures, such as 11 and 12, and apertures 13 and 14 in these regions of maximum field strength. However, the performance of the filter is not dependent upon locating the apertures in the region of the strongest fields as long as coupling means are located adjacent to the fields. Apertures 11 and 12 when located adjacent the magnetic field may be oval or rectangular in shape with the longer dimension in parallel relation with magnetic lines of force, and when such apertures are located adjacent the electric field, they may be circular in shape although other shapes may be employed.

A filter as described to this point is one which contains poles, but no zeros, much like a constant-k configuration in low frequency work. The response curve of such a VHF, UHF, and microwave band pass filter will be seen in Fig. 3 curve A. It will be noticed that this arrangement alone is not as good as may be desired since the rate of cut-off as shown by the slope may not be as sharp as required. Coupling separately alternate resonators as shown in Fig. l accomplishes this object of my invention, resulting in curve B, Fig. 3. A coupling line 17 with its end probes 21 and 21a is used to produce a resultant inductive coupling between resonators 1 and 3, while a coupling line 18 with its end probes 22 and 22a ISvndiaeent p produces capacitive coupling between resonators 1 and 3.

coupling lines 17, 18 and 19, 20 will depend upon probe L combinations therein, but in general will be some multiple of a quarter wavelength long.

One of the four different combinations of coupling probes and line lengths is shown in resonators 1 and 3 of Fig. 1 wherein the magnetic probes 21 and 21a, connected to the one-quarter wavelength long coupling line 17, and the electric probes 22 and 22a, connected to the one-quarter wavelength long coupling line 18, are in such a physical arrangement that each current injected into resonator 3, due to the voltage across resonator 1 is shifted and -{9G, respectively, with respect to the voltage across resonator 1. This is one of the fundamental requirements that must be satisfied by a pair of coupling lines and their end probes. Another fundamental requirement is that one of the aforementioned 90 currents increases in magnitude in direct proportion to frequency, and the other 90 current decreases in magnitude in direct proportion to frequency.

There is in addition a fundamental requirement upon the adjacent or intermediate resonator couplings which may be stated as follows: The phase relation between the net current injected into resonator 3 by the alternate resonator coupling and the net current injected into resonator 3 by the intermediate resonator coupling must be such that at the desired frequency of infinite attenuation they are opposing in phase. Thus, at these frequencies where the phase of the waves are out of phase, by proper adjustment of the amount of the adjacent and intermediate coupling, it is possible to completely cancel both the magnetic and electric propagation, thereby producing zeros or inserting infinite attenuation points, such as 23 on curve B, Fig. 3. The preceding equation gave the approximate required value for the adjacent couplings in order to produce a desired pass band width, and the below equation `gives the approximate required value for the alternate couplings The second infinite attenuation points 24 on curve B are produced in a similar manner, by employing coupling lines 19 and 2f) cooperating with coaxial resonators 3, 4 and S wherein the various phase relations are identical. Through the frequency band which the filter is operative, the capacitive type coupling coefficient of coupling line 18 and the inductive type coupling coemcient of coupling line 17 are essentially constant and equal in magnitude.

Fig. 4 in the drawing illustrates another embodiment of my invention wherein the individual cavity resonators are identical with the coaxial resonators 1, 2, 3, 4, and 5 discussed above. The difference between the embodiments is the arrangement whereby the adjacent coupling apertures 25, 26, 27, 28 are situated alternately in an electric field region and a magnetic field region, with both the input aperture 29 and output aperture 30 of the filter being situated adjacent a magnetic field region. The results obtained with this arrangement are substantially identical with that of the aforementioned arrangement with possibly the exception that the attenuation far from the pass band is slightly decreased due to the adjacent coupling aperture 28 and the output aperture 30 being in the region of the same energyfield for reasons previously mentioned. This apparent disadvantage of the coupling arrangement in coaxial resonator 31 may be readily overcome by shifting aperture 30 vertically in order that it may couple from the electric field rather than the magnetic field thus reducing the possibility of direct feedthrough of unwanted frequency on the magnetic field. Voltage in coupling lines 32, 33 and 34, 35 in cooperation with the voltage passing through resonators 2 and 4, respectively, produces the points of infinite attenuation as discussed with reference to Figs. 1 and-2 of the drawing.

:arr-49ans .The combinations of. magnetic probes '.36 fandi36a .and :electric probes :.37 .and 37 a with ftheinrespective .coupling 4lines Y 33 Vand 62, :are .physically arranged Vto satisfy the phase requirements. as set `Vforth -in t the embodiment shown in diig. lfby having-.theflength ofv coupling :lineiSZHthesame as the length of couplingtlineS 'Ewith ,the Ytot-al length :of reachline being2some4odd .tmultipleY-.of .a quarter iwave- -.length.

v;Still another .fembodiment of my .invention .is .shown :in Pig. 5Y of .the drawing. This `variation-operates Von the -..same principle :as 4the `previous embodiment as -to the .development of zeros. .The cavity resonators-arerectan -ygular .waveguides.38 39, 40, 41, and 42-which :operate tin the TEon'mode. ,Physical dimensions of .theindividual waveguide resonator-s are approximately 0707A along, '10T/.07x wide, -where )t :is the .free:spacewavelength, and .theY height .being .chosen to -satisfy the requirement .of electric fieldstrength. .As is .known vthe-electric .field fis concentrated .betweenpointsAS ,and 44 .in ia .typical fwave- .guide :resonator tof this type :while .the.magnetic field .ris l '..circulardn cross section, concentric withthe concentrated electricv eld.

.In the arrangement indicated, 'the tinput tothe filter -is by. means of .loop 45 .through aperture"46 in the wall adjacent to themagnetic field of waveguide `resonator 38. `The voltage'oscillates betweenthe magnetic :eldf'and the electric `field -with part of 4the electric .field being .coupled to waveguide-resonator 39 through-circular .aperture-44. .A .resulting :electric .and magnetic field oscillation results in waveguide resonator 39 and part of the magneticfleld is coupled to waveguide resonator 40 via a rectangular slit aperture ltS-through thewalltherein. This -alternating process from electric to :magnetic coupling continues progressively through the filter until .the selected .energy is coupled out ofthe system through aperture .-49 .in'fthe v .electric field of waveguide resonator'42. The zerosl in '.the transfercharacteristic are produced-as heretofore .ex- Epllained in connection with Fig. '1 by employingcoupling Llines '50, 51,152, and`53 and the associated probes. With the probes as shown, lines `50 and'51 must be an identical odd number of quarter Wavelengthsvlong Lines 52 and 53 .must also be ,an identical `odd number fof quarter` wavelengthslOng.

A further embodiment of my inventionis shown in Fig. 6 which illustrates three adjacent waveguideresonators'SA, 55, and 56, the dimensions of which are discussed A.in V'coninection Vwith.=Fig. A5, with adjacent coupling .provided Yby quarter wavelength coaxial lines 58 and 59. v

Production of zeros is accomplished by employing the third of the four coupling probe combinations hereinbefore mentioned. As shown because of the opposite sense of the coupling loops 64 and 64a, the coupling lines 62 and 63, with their associated probes 64 and 64a and 65 and 65a, must differ in length by a half wavelength. The extra half wavelength may be placed in the coupling line 62 connecting the magnetic probes. This arrangement then establishes, in accord with the fundamental phase requirements outlined in the discussion of Fig. 1, the requirement that coupling loops 61 and 60 be oppositely bent as shown.

A fourth coupling combination may be achieved by placing the required extra half wavelength in the coupling line 63 connecting the capacitive probes. This will require one coupling loop 60 or 61 to be reversed from the position shown to establish the proper phase relation as outlined in the discussion of Fig. 1 which allows achievement of the desired filter having a high rate of cut-off.

Fig. 7 illustrates another embodiment wherein waveguide resonators 64, 65, and 66 are adjacently coupled by quarter wavelength waveguides 67 and 68. Alternate resonator coupling is provided through coupling lines 69 and 70 in cooperation with magnetic coupling probes 71 and 71a and capacitive coupling probes 72 and 72a. The physical arrangements of these magnetic probes 71 and 71a are in the opposite sense with reference to their slt) V.respectivecapacitivetzcouplinggprobes 72 and 72a, therefore, the length of the coupling linesf69 and i70=rnust differ .rinflengthfbyone-half -wavelangthttoproperly attain the A.aforementioned filter results. Because vadjacent coupling Iwaveguides V67 and 68 are bothof the positive -mutual inductance type, it is necessary to add the required extra half wavelength in athecouplingfline 70 joining the capaci- Y.tive probes 72 ,and .72a.in order that the required 'filter `energy. intothe .series-of resonators, output terminal .means to-remove energyfromtsaidtseries of. resonators, saidseries of resonators .including two ,groups, one `of said groups .consistingfof .the .odd ones of .saidseries of resonators and :the other of said groupsconsisting'of ,the 'even ones of isaid-vscriesof resonators, and .apferiodic means coupling ,themember-s of oneof.said,,groups in tandem.

{2.A band'passfilter according toclaim 1, wherein said lmeanscouplingsaid resonators in series includes walls of .said resonators having-.apertures therein, certain of .said .apertures being .configured .anddisposedfor coupling with `.thevelectric -field .andfothervofasaid apertures being con- :figuredV and disposed .for 4.coupling with the .magnetic field of. .said resonators.

3. A band pass lfilter .accordingto Vclaim 2 lwherein .saidaapertures r coupled with lthe electric field are circular .andsaidapertures coupled withthe magnetic .field are .elongated inta directionlparallel to ,the magnetic lines of ...force 4. IA band pass filter according vto .claim .2, wherein the energy inreach `of? said .resonators :is coupled between .the electriciand magnetic .fields-and :said .means coupling .saidresonators series couples ithe energyV progressively .between vthelelectric and:magnetc.fields of said series of resonators.

5. `A band,pass.filteraccording-to claim 2, wherein said Lmeanscoupling said y.resonators in series .includes coaxial .transmission linesxhaving allengthequal to=odd multiples .of one-quartenwavelength.

s .6. ,A.bandp.ass filter .according-.totclaiml lwherein said means coupling said resonators in series includes waveguide couplers having a length equal to odd multiples of one-quarter wavelength.

7. A band pass filter according to claim l, wherein said input terminal means is disposed to be coupled with the electric field of the first of said series of resonators and said output terminal means is disposed to be coupled with the magnetic field of the last of said series of resonators.

8. A band pass filter according to claim 7, wherein said input terminal means comprises a probe extending into the first of said series of resonators for capacitive coupling to the electric field therein while said output terminal means comprises a loop extending into the last of said series of resonators for inductive coupling to the magnetic field therein.

9. A band pass lter according to claim 7, wherein said input terminal means comprises a loop extending into the first of said series of resonators for inductive coupling to the magnetic field therein while said output terminal means comprises a probe extending into the last of said series of resonators for capacitive coupling to the electric iield therein.

l0. A band pass filter according to claim l, wherein said aperiodic means includes a pair of coupling lines, one of said lines having capacitive probes and the other of said lines having inductive loops for capacitive and inductive coupling of energy between the members of said one of said groups.

11. A band pass filter according to claim 10, wherein said coupling lines include coaxial coupling lines having a length equal to odd multiples of one-quarter Wavelength long.

12. A band pass filter according to claim 1, wherein said resonators are coaxial resonators including a cylindrical tuning slug.

13. A band pass filter according to claim 1, wherein said resonators are rectangular waveguide resonators.

14. A band pass filter comprising five coaxial resonators each including a cylindrical tuning slug, means coupling said resonators in series, input terminal means to introduce signal to said series of resonators, output terminal means to remove signal from said series of resonators, said series of resonators including two groups, one of said groups consisting of the odd ones of said series of resonators and the other of said groups consisting of the even ones of said series of resonators, and aperiodic means coupling the members of one of said groups in tandem.

15. A band pass filter according to claim 14, wherein each of said resonators encompassa magnetic field and electric field in coupling relation with each other and said means coupling said resonators in series are so disposed with respect to the magnetic and electric fields of said resonators that the energy traverses said series of resonators alternately on the electric and magnetic fields of said resonators, said input terminal means being disposed in coupling relation with the electric field of the first of said series of resonators and said output terminal means being disposed in coupling relation with the magnetic field of the last of said series of resonators.

16. A band pass filter according to claim 15, wherein said aperiodic means comprises two pairs of coaxial coupling lines having a length equal to an odd multiple of one-quarter wavelength, one pair of said coupling lines coupling the first and the third resonators while the other pair of said coupling lines couple the third and fth resonators, one coupling line of each pair of coupling lines having capacitive coupling probes and the other coupling line of each pair of coupling lines having inductive coupling probes for magnetic and electric field energy coupling between the members of said one of said groups.

17. A band pass filter comprising five rectangular waveguide resonators, means coupling said resonators in series, input terminal means to introduce signal to said series of resonators, output terminal means to remove signal from said series of resonators, said series of resonators including two groups, one of said groups consisting of the odd ones of said series of resonators and the other of said groups consisting of the even ones of said series of resonators, and aperiodic means coupling the members of one of said groups in tandem.

18. A band pass filter according to claim 17, wherein each of said resonators encompass a magnetic field and an electric field in coupling relation with each other and said means coupling said resonators in series are so disposed with respect to the magnetic and electric fields of said resonators that the energy traverses said series of resonators alternately on the electric and magnetic fields of said resonators, said input terminal means being disposed in coupling relation with the electric field of the first of said series of resonators and said output terminal means being disposed in coupling relation with the magnetic field of the last of said series of resonators.

19. A band pass filter according to claim 18, wherein said aperiodic means comprises two pairs of coaxial coupling lines having a length equal to an odd multiple of one-quarter wavelength, one pair of said coupling lines coupling the first and the third resonators while the other pair of said coupling lines couple the third and the fifth resonators, one coupling line of each pair of coupling lines having capacitive coupling probes and the other coupling line of each pair of coupling lines having inductive coupling loops for magnetic and electric field energy coupling between the members of said one of said groups.

References Cited in the file of this patent UNITED STATES PATENTS 2,177,761 Wheeler Oct. 31, 1939 2,293,384 De Cola Aug. 18, 1942 2,418,469 Hagstrum Apr. 8, 1947 2,419,557 Friis Apr. 29, 1947 2,566,087 Lerbs Aug. 28, 1951 2,649,576 Lewis Aug. 18, 1953 OTHER REFERENCES relied on.

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