GB2540030A

GB2540030A - A microwave resonator and a microwave filter

Info

Publication number: GB2540030A
Application number: GB1608670.4A
Authority: GB
Inventors: Rhodes David
Original assignee: Individual
Current assignee: Individual
Priority date: 2015-05-17
Filing date: 2016-05-17
Publication date: 2017-01-04
Also published as: GB201508457D0; WO2016185196A1; GB201608670D0

Abstract

A microwave resonator comprising a resonator cavity, the cavity wall comprising first and second end faces 4, 5 and a side wall 3, the cavity having a length axis extending from the centre of the first face to the centre of the second face, the cavity having a length A along the length axis between the first and second end faces; the cavity having a width axis normal to the length axis, the width of the cavity from the side wall through the length axis to the side wall on the opposite side of the length axis being C; the resonator further comprising a dielectric body/plate 8 arranged within the cavity spaced apart 9, 10 from the first and second end faces, so defining first and second gaps therebetween; the width of the dielectric plate normal to the length axis being at least 90% of C, the thickness B of the dielectric plate along the length axis being at least 60% of A, and the width of the cavity C being at least four times larger than the length A. The microwave resonator is relatively compact compared to known resonators, in particular it is very thin.

Description

The jWesent i^ a mierowa^ resdnatari More paiSietiiarJyi hot ttoT exettiSively, the present invention relates to a ihicrdvvave resSrtatSr compising a resohatoir cavity defined ly ah eieetftcallY conducting eav^ vVall, the eav comprising first and second spaced apart end f^s: and a side vsfall extending therebetween, the resonator further comprising a dielectric body arranged within the cavity spaced apart from the first and second end faces, the width of the cavity being at least four times larger than the length. In a further aspect the present invention relates to a microwave filter. More particularly, but not exclusively, the preseritih\^nddn;rejals|o^ filter comprisingiapuralipps isi^napifi,|heiespRapfs being eie^ ipiprowave resonaldTSibltefl find application in mobile teiecpmfh in combination wifeothef tusohators they can Prrn raicrowave filters: A rnicrbira comprise signals at a pturailty of different frequencies^ It is desired to filter such a signal to extract the signal at one frequencyi only. Sirniiarly, befete tfansmiMng: a nticrqwayehSighat jt if typicafty desired to pass the signal through a filter so that the signal is tightly constrained to one frequency band only, in roodefn teiecommunicaiions systems different signals are very closely spaced together in frequency and so filters having a very narrow bandpass are required. This in turn typically requires the use of resonators having very high Q values. Such resonator can be difficuit and manufacture and are also large structures.

The present invention seeks to overcome the problems of the prior art.

In a first aspect the present invention provides a microwave fespnatbr com a resonator cavity defined by an electrically conducting cavity wall, the cavity wall comprising first and second spaced apart end faces and a side waii extending therebetween; the cavity having a length axis extending from the centre of the first face to the centre of the second face, the cavity havihg a lengthiAaionglhe tengdi axfeib the cavity having a width axis fiofiTial te tfeie length aiiis, fhS iA'1dth:i0ft;li^^ through the lervgth axis to the side wall on the oppbsTte'Side;df the id axis being C; a dielectric plate arranged within the cavity normal to the length axis; the dieiectric plate being spaced apart from the first and second end faces, so defining first and seeeBdiippthefebetwepn; the widihsolthe dieteettk: plats the thickness B of the dielectric plate along the length axis being at least 60% of A; and, ^vVhereiO:iiS;lsiettea^^

The microwave resonator according to the invention has a very high Q value compared to known microwave resonators of a similar volume at the same frequency. Further, it is relatively cornpacf compared to known resonators, in particular it is very thin. This is of particular importance Th tnode# tsidponris systems where slip Is afea prerniom. : Preldfahly C is at least five times A, ntqre pteferabiy at:least::s&^ ii Preidpbiy nqriTialiio the len^ shape of the dieiectric piatei is the sa me as the;:s^^^ Of the;

Preferably the width of the dielectric plate normal to thO Ishgth axis is at least 95% Of 6,; tfif dielectric plate being spaced apartfromithe side wall,

Preferabiy normal to the length axis the dielectric plate is the same size aa the cavity and abuts the sidewall:.

Pi^rably; δ 70?6 of more preferably at te feore prefembiy: at iea^ h jPreferably the relative permittivity of the dielectric plate is in the range 10 to 90, more preferably 30 to 60^redre preferebiy .

Preferably the cavity has a circular cross section norma! to the length axis;

Preferably the cavity has a fourfold rotational symmetry about the length axis, is preferably square.

Preferably the thickness of the first air gap along the length axis is equal to the thickness of the

Sfi cood a it ga p aloog the (eh|t h^^a

Preferably the dielectric plate has afi aperture extending therethrough, the microwave resonator furthercpreprisihg a through an end plate into the aperture.

Preferably the microwave resonator further comprises at least one electrically conducting tuning plate arranged in one of the first and second gaps and an electrical conductor extending from the tuning plate through an aperture in the end face oroximate to the tuning plate.

Preferably the microwave resonator further comprises a switch connected in series with the electrical conductor, preferably a MEMS switch or a Gallium Mitride PIN diode.

Preferably the microwave resonator ferther comprising a microwave filter connected to the reisonator cavib/

Pptefapiijf: resiifiato r furtte fiomprisss a iseura ίτ> (Grdwwe being adapted to provide a migFOwave signal spltiaiie for expitlfi|::pilher the £f^ tbs resonator^

Preferably C is in the range 10 to 60mm, more preferably 20mm to 30mm.

Preferably 8 is m the range 2mm to 6mm, more preferably 4 to 5mm,

In a further aspect of the invention there is provided a microwave fifer eomprising a plural% of Fesonalprs as clainied in any one of claims 1 to 17, the microwave resonators being electrically cpupledlbgstho*'^

Preferably at least some of the resonators are connected togethir in^^ra^^ preferably the coupled resonators share a common side wail, the common side wall having an apet^bte d t Rgthef ethro

In a further aspect of the invention there is provided a method of filtering a microwave signal eathprasjdithe of prowidingiasmici^wave re^ any one of claims 1 toiiepPd, providing a microwave signal to the resonator, the microwave signal having a frequeney suitabfelp^ exciting either the EHom or EH,i* mode of the resonator. lire pfesent ifivention wilt now be described by way of example only and npt: |p;;:3Ry timi!taiiM^^ with reference to the accompanying drawings in which

Figure l{a j a miefoveaft*

FigifTe lib) sbQWS iiig mterewaveiM 1(a) from above;

Figure 1(c) shows an alternative embodiment of a microwave resonator according to the invention;

Figure 2 shows an alternative embodiment of a mscfDwawe resqnator sccofdlng to vei^iieal crojss sectlqo; l^uto? Ifa) to 3(c) show of retoftantheight tor a resonator according to the invention showing the desired modes and hotod^

Figure 4 shows the electric fielil witoin there^ cavity of a resonator according to the invention inW!ertltoii cross seetfeh

Figure 5 shows the magnetic field In the dielectric plate of the resonator according to the invention m a plane normai to the length axis; iFigdre S Shows the rn^lhitodi |dhtout^ tot; the rnegneti cavity of a resonator atodrdihg to thd in^ Vertkal cross section;

Figure 7 shows and end face of a microwave resoftetor ;aceording to ihe invention;

Figure S shows a mtcrowave filter according to the invention further comprising a filter and microwave source; and,

Figure 9 shows two microwave resonators of a filter according to the invention coupled together.

Shown in figure 1(a) is a microwave resonator 1 according to the invention. The microwave resonator 1 cpppises a resonator cavity 2 defined by an eiectricaily conducting; cavity wail S/Th^^^ cavity wall 3 comprises first and second spaced apart end faces 4,5 and a side wall 6 exteniinl tferebetweerii A )i©ntth:axis;# e}(ten^^ of the first end face 4 to the centre of the second end face 5.

The ienphiO^fcavity 1 along the length axis 7 is A.

Arranged in the resonator cavity 2 is a dielectric plate 8. The dieiectric piate 8 has a thickness B along the length axis 7, The dielectric plate S is spaced apart If secorid end faces 4,5 as shown. 8 IS at least 60% of A, more pre&rafoiy at ol^ rpore ρ more preferably at ieasl 90% of A.

Betweenihe piate S and t^ end face 4 is a first gap 9,:Betweeh tbe·plate S and the seitond fae i tka second gap iO. Preferably the gaps 9,10 are fiUed with air. In this embodiment the thickness of each gap 9,10 along the length axis 7 is the same, in alternative embodiments one gap 9,10 is larger than the other 9,io. T|e dieiectric of the dieiectric plate S is typically a high dielectric Constant, low loss ceramic. The reiative permittivity of the dieiectric plate 8 is typically in the range 10 to SQ, more preferably 30 to 6|, morepreferably 45

Fsgui^ i|y:«bowS; the;i^^ 1 of figure l(a{) from above. In this embodiment the first andisecondiend faces 4,5 are cireUJar and the iaie wall 6 has a circular cross section normal to the length agis 7. Thecrds&seGtioh of the 2 is constant along the length axis 7, T^he idffeSeiiiFit plate S Is the same size and shape as the cross section qi|he ea#y 2 aptiijalipts the cavity side wali 6 as shown. The width C of the cavity 2 is the same as the diameter of the piate 8.

For this cavity C is at ieast four times A, more preferabiy at feast six times A. jn practice the dteiectric plate S is pbsitibPbd ih the Cawty 2 by firstly heating the cavity 2 so that it expands siightly. The plate 8 is then inserted into the cavrtv 2. The cavity 2 is then allowed to cooi and contract, so fixing the piate 8:ih place. ligure 3(c} shows a further embddiment of a mictdwdyie; resonator 1 from above. In this gfdbmiiitnertt the cross section of the cavity 2 normal to the length axis 7 is square. The dteiectrie plate S is also square. Unlike the embodiment of figure l(bjl as one rotates about the length axis T the distance between opposite sides of the cavity 2 through the length axis 7 varies. The width of the cavity 2 is the maximum value of this distance which in this case is the disiaPce across the squ^ shown Similarly, the width of the dielectric piate 8 is the distance between opposite comers of the plate 8

In alterrvativepmbodiipehts lhe plate S and the erosSiSgctidb of the cavity 2 have othg shape of the pj||e $ Is thg i shape of the cross soj^iOh bf;the cavity 1. The plate S apdl: cross secttori of the cavity 2 have a fourfofd ratatibnai sfmmstfy about the length axis 7, that is to say if one rotatesth® plate 8 or Cavity 2 by SCI degrees Sbidbt the tei^^ orientation is indistingoishai^elhom itsodginaf fa:fie erpbolimeht olh|u^ Ihe plate S is slightiy smaller than the cross section of the cavity 2 so resulting in a small air gap 11 betvreen the pMte 8 and cavity side wail 6. Preferably the width of the dielectric piate S norma! to the length axis 7 is at least 90% of C, more preferabiy at least 95% of I.·:

Shown in figure ? ts a further embodiment of a microwave resoBator j accordIn this embodiment an aperture 12 extends through the dielectric plate 8, A tuning screw 13 extends through the first end face 4 into the aperture 12. By turning the tuning screw 13 one can adjust;the freq;Meocies tifthe tesonartt rnodesOf

By way of specific example, the resonator 1 is the resonator of figures l{a} and 1(b). The width of the plate 8 (and hence the cavity 2) is 24mm and the thickness 8 of the plate 8 is 4mm. More generally, the width C is m the range 10 rnm to 60mm, more preferably 20mm to 30mm. Preferably 8 is in the range 2rrim to 6mm, more preferably 4mm to .6rrm.

Shown in figure 3(a) is a plot of the resonant frequencies pf different rgsonaiRt :modes:;:0f the; resonator 1 described above as the dimension A is reduced 4o a level where ^ the dominant modes. Figure 3(b) is a table of the resonarrt frequencies of the different modes as a function of cavity length A, Figure 3(c) is a table of Q values at resonance for the different modeSa^^ a functiort of ceyity ienglh i^. For this prticulgr rniiSowave cesohator ;! the Fe|idh; of pptpjiar Inierest; is M bebyeen 4.5mm aiid SiSmim as; is; expSined in more detail; below, it should be appreclited that sit is not the absdiiM; values of the dimensions that are important, rather the relative values. If all the dimensions of the resonator 1 were scaled by the same scaling factor the resOBatpr I woOid operate ih the satne manner but at a differeot frequency. These particular dimensions are chosen as with these dimensions the resonator 1 shows particulariy desjmhle characteristics in the 3<5Hz to 4GHz range commonly employed in modern telecommunications Systems

Asicah^^ b^ seen from figure 3{a} for A between 4,5mm and 5.5mm and between 3 and 4 GHz the dominant mode is the IHoi^ mode, in known low loss microwave resonalbrs the mode used is the; Tli)M mode:. The fesohatbr l according to the invention is therefore smaller than known microwave MSoftatOtSv Further, in known microv/ave resonators the ΤΕοιδ mode is not the dominatil mode : Which Cab comuse of the resonator.

The ΕΗοιή m<ide has a further adi/aniage in lermMfThe to the field structure to perform efeetromc tuning of the resonstdr. Shiown in figure 4 is the electric field at resonaoee through a cross seEtidn of the resonator 1. The figure is not drawn to scale to as to enable the relative strengths of the field in different regions of the resonator 1 to be observed l‘he field is (Kjostant with rotation aboutthe length axis 7,

The electric field strength in the air gaps 9Λ0 is considerably larger than in the dielectric 8 arid d^O much larger proximate to the length axis T.^The magnitude is illustrated bythe nuFnberofisrroVtfs|^

The magnetic field is at right angles to the electric held. Its variation vvithih the cavit^^^ lijgures I· and 4 again not tfewn to scale. Figure 5 shews the dlTectidri^ of the magnetic held H through the; centre of the dte|ectFiC |jJate 8 length axis 7. The ihsilifetic field has a rotational symmetry about the length axis 7. The strength is zero in the centre and reaches a peak about two thirds bf the distance to the side wall 6 before starting to reduce Figure 6 iilustrates the contours of magnetic field strength la the resonator 1.

For a dielectric resonator 1 with a high Q factor (ie low loss) the loss in the resonator i as aw is dominated by the resistive loss in the surrounding metai. Thus, for the ΕΗοϋ mode most of the loss is in the metal of the side wall 6. To reduce resonant %quency wlh ife sfriaife in Q the air gap 9,10 can he reduced on either one or both sides of the dielectric plate 8. Due to the streng^ ef the E field proximate to the length axis 7 perturbations in the thickness of the air gap 9,10 proximate to tne length axis 7 yylll pppyide^^^ frequency, iince ihe maygh^^ is Very weak in this area then the reduction in Q, will be minimised,

Showfi in figure 7 is an end face 4 of a further embodiinent Of a resonator df according to the invention. Proximate to the end face 4 is a plurality of electrically conducting tuning pfefces 24. The tuning plates 24 are arranged SMbstantially paraliel to the end face 4;arid proa^ axis 7. Igtendingife each turiing plate 14 is ah eleclHcaI cdridUriOr TS. Each electrical conductor 15 extends through an associated aperture 15a in the end ptete 4 as shown. Connected to the electrical conductor IS is a switch 16 vidfiefe can be Switehod and dosed CPriligMratiOriSi The 16 tO the turifrig plate 14 is connected to an electrical tpicailpsn earth 17 0r t© face; 4 pii&xirrtaie t© the iuhit|| dilate ί4< When the

Switch Ϊ©;ΐδ: ίβ ΐίί6 ;0ρ©η ϊ»«1ί|μ^ When the switch 16 is in the ctos®©of the tuning piate 14 is at earth or the same potential as tibi end face 4; This effectively reduces the size of the air gap 9 so changing the resonant frequency of the resonator 1 in the EHoj^ mode. By switching on various combinations of tuning plates 14 one can change the resofiant frequency of the resonator 1 between many different resonant frequencies with minimaIdegradation,

When changing the resonant frequency q|j|ie resonator llhe ei^etaie ch^ will transfer the rnajonty of the sloced enei^y^^^^a^ with the originai resonanee^^^t^^ new resonant freqqehcy vyil tninipnal loss of energy* Tie mitred aaeondinf to the::inye»|iiqn: cantiferelbTeiieisiyitChedbi^ than linowH tnlcm TO; further intinitnise loss far the resonator the switches 16 are typically MEftlS svyittiies; Such switches 16 have a very high on to off impedance change. Aiternativeiy semiconductor switches particularly Gallium Nitride PIN diodes can be used.

Typically a metal input connection pad 18 is connected to the first end face 4. A signal line 19 Is connected to the connection pad IS- The microwave signal to be filtered is provided on the signal tine 19. An output connection pad 20 is connected to either the first or second end faces 4,5. A furtherδίΙόΙΙ lih© 21 is connected to the output connection pad 20 The filtered microwave signai is; received from the output signal line 21. As the mode is symrneM^ length axis the angular separation betweeri Ihe input and:out^ pads about t^ can have any v^lue.

An alternative method of providing a fnicrowave signal to be filtered to the microwave resonator 1 is by means of magnetic coupling. The signal iine;:19; VVhich provides theSffiietO signal is curved proximate to the dielectric plate 8. The field p;Berated ;by the Curved wife e0u|;te& ;^ signal to the resonator cavity 2.

Returning to figure 3(a) as can be seen when A is in the range about 4.5mm to 5rnm the EHn& mode is sufficientiv separated in frequency from the other modeSildSbe suiKessfbliy for narroWbani levy less fitteiiS, T^e Q is 50% higher than the ΙΙΐίΐή mode and S0% higher in frequehey. Tfiis mode haea aih^ field variation around the length ads Ji Hence it is a dual mode les^ each mode ordhog^ each otfien

Vdhen the resonator 1 is used in the EHu* mode the output cormection pad 20 is typically arranged Such that the af^ular separatiO the connection pads 18,20 about the length axis 7 is about 90 degrees. A coupling screw 32 extends through the end face 4 at a point mid^way in angular separatidh about the length axis ? between the input and output connetAion pads 18,20.

The tuneable resonator 1 may further comprise a fitter (not shown) connected to the input or output connection pad 18,20, The passband of the filter includes the frequency of the mode of the resonator 1 one wishes to excite (typically the EHou mode or EHr„i mode) but does not include the respnant frequencies of the other modes.

Shown:i:n:;|igtjfe 8¾¾ a fdrtheTembqdlhieatiO according to the invention,

Connected to an end faceid df ldeTeSdnator TiS iA;ra The microwave source 23 ip adapted to provide a miqroWawe signal of a frequency suitable for exciting one of the EHow or ΕΗ^; modes of the resonator 1.

In the particular example shown, the microwave resonator 1 is being empioyed as a dual mode; resonator 1 (ie EHjija mode) and so compriseE input and output connection pads 18,20 and a Cduplihiscfewi22 arrange as previousiy describe

In an alternative embodiment the source 23 is connected to the end face 4 through a filter as described above. In a further alternative embodiment the filter is connpcted'tq the output 21 of the resohatiof 1, in a iartSier aspect of the jnveotion there is provided a microwave filteF S& CGTO^ such resonators 1 electrically coupled together, at least some of which are connected together in cascade. All of the resonators 1 may be smgle mode resoriators. Ml of ^e rftso^ mode resonators The resonators 1 may lea rnttarg of sirigiea00d resoh^otsi

Figure 9 shows two of the microwave resonators 1 of the filter 30 coupied together. The two resonators 1 share a common side watt 6. The common side wall 6 has at) aperture 31 therein through v/hich the resonators i couple together. Preferably the dielectric plates S of the two resonators l are dimensioned such that they touch as shown.

Claims

iiiisis: 1. resaftator 9; respnator cavity- defined by an iiectrteaiiy cpndlitetii^ cayHy wa!i, the: caydy; wall qompfisjag fir^ and secd^^ end lates aad a side wad emending the cavity having a length axis extending from the centre of the first face to the centre of the second face, the cavity having a length A along the length axis between the first and sMOhd Infi fecesj the cavity having a width axis normal to the length axis, the width of the cavity from the side wait through the length axis to the side wail on the opposite side of the length axis being C; a dielectric plate arranged within the cavity norma! to the length axis; the dielectric plate being spaced apart from the first and second·end fecfSi So definm^^^^ and seebndigapsitheim the width of the dielectric plate normal to the length axis being at least 90% of C, thgihlclioessipf the dielectrl®; ptatedibngthe:te wherein C is at least four times A.
2. A mtcrov.'ave resonant cavity as claimed in claim 1, wherein C is at least five tjmess^^ preferably at least six times A-
3. A microwave resonafit cavity as elairned ip ailherjpf claims I or I whei^Sn: normalife tfit length axis the shape of the dielectric plate is the same as the shape of the Cavity*: |i A microwave resonant cavity as claimed in any one of claims 1 to 3, wherein the width of the dielectric plate norma! to the Ifength axis is ati feast 95% of C, the dielectric plate bemg spaced apart from the side wall
5. A microwave resonator as claimed in darm 3, wherein normal to the length axis the dielectric plate is the same siie as the cavity and abuts the side wall, & A mferowaxre tesooator as claimed in any onebi dalims i to S^iWlitresrf B is^ more: preferably at least 80% of A, more prefeiabiy at Ifeiast SO^: '"$i .A; micmwaSei^Sbhator as· claimed; fhvaby bhe? of'^iairh-s' ϊ;· tb·;; Whereftl:; the: irelatsVe; permittivity of the dielectric plate is in the range 10 to 90, more preferably 30 to 60, more preferabiyAS, 8; A microwave resonator as claimed in any one of claims 1 to 7, wherein the cayity has^a circular cross section normal to the length axis,
9. A microwave resonator as claimed in any one of claims 1 to 8, wherein the cavity has a %otfeld rptalioos I symmetry about the leng^^ M A mlcmwaye as claimed In any one of cbirns 1 to; wherein the tbiclct^^ of the first etr gap along the length axis is equal to the thidmess^O^ air gap aior^ the lerigtha^fe·
11. A mierowayo resonator as claimed in any orie of claims 1 to 10, vvhereirt the dieledtic;pf has an aperture extendihg ihereihroMSh, the mic resortator further €bmprisrh|^^ tunirtf scmw extehdihg tbroagit the ersd 12* A mierowave resonator a in any one of claims 1 to 11, further comprising at least one electrically conducting tuning plate arranged m one of the first and second gaps and an eiectrieal raijiiaiaQf; turitmg pMe thp^gfi an aperture in the end fat^ praximatetg tfee tahingi .
13. A microwave resonator as claimed in claim 12, further comprising a switch connected in series with the electrical conductor, preferably a MEMS switch or a Gallium Nitride PIN diode, 14, ft mlerpwaye resonator as claimed in any one of claims 1 to 13, further coftiprisifig: a microwaye|iiter connected to th^ eayity, 1¾ ft mioiowawo iresonator as ctaimed Iri arty on® of elalms l to 14, further comprising a microwave microwavei source being adapted to provide a microwave signal suitable for exCitiR|'either the or EHiii mode of the resonator. ISi A microwave resonator as claimed in any one of claims 1 to 15, wherein C is in lbesrange^^ 10mm to 60mm, more preferably 20mm to 30mm, 17; A microwave resonator as claimed in any one of claims 1 to 16, wherein B is in the range 2mm to 6mm, more preferably 4 to Smm. 16ΐ A microwave filler comprling a piurflity of i®s®na^ in any one #claims 1 to 17, themicmwawerespnators being etectd^^^ 19 A tpicrowave filter as claimed in claim 18, wherein at least some of the resonators ate; innnected't^ ea scade. 2¾. ft ffliierowave f iter as cJsii^ ciaims if a r ss? w the CDiup share a; eomrriKse side «aih the cemmoR side Mail hawing an aj^rtere esitendihg therethrSiijp. 21; A method of filtering a microwave signal comprising the steps of poviding a microwaweitesonatoriag in ahf oae of cSairnslld-lJiiaftd;^ providing a microwave signal to the resonator, the mlerpwave· slgnai^^ ireeMeney suitafaie for exciting either the ΕΗοιδ or ΕΗ^,δ mode of the resopatar* ft miofpvwaveitesonatorsobstaniaiiyas: 23 ft microwave filter substantially as hereinbefore described;