Dual mode cavity filter assembly operating in a TE22N mode

A microwave cavity filter is configured for operation in the dual TE22N mode to realize a very high Q factor at very high frequency ranges. The microwave filter is formed from using one or more cylindrical cavities in which two orthogonal field polarizations of the TE22N mode are excited and coupled together by means of a coupling element. Different combinations of inter-cavity irises provide for both direct and cross-coupling of aligned field polarizations in adjacent cavities, as required, to realize complex filter functions. The irises may be formed in either a side or end wall of the cavities for both collinear and planar mount configuration. Negative mode coupling also allows for transmission zeros to be realized on either side of the filter passband.

FIELD

Embodiments described herein relate generally to microwave resonator filters and, more particularly, to dual mode microwave resonator filters exhibiting low loss at very high frequency ranges.

INTRODUCTION

A microwave filter is an electromagnetic device that can be tuned to pass energy within bands of frequencies encompassing resonant frequencies of the filter, while substantially suppressing inter-band frequencies. The resulting bandpass characteristic of the microwave filter can be described by one or more different performance criteria. For example, insertion loss describes the amount of signal loss exhibited in the microwave filter's passband, rejection (or “isolation”) describes the amount of signal attenuation exhibited in the filter's stopband, return loss relates to the ratio of signal power incident on and reflected from the filter, loss variation (sometimes referred to as “ripple”) describes the flatness of the passband, and group delay is related to the phase characteristics of the filter throughout the passband.

One commonly used performance characteristic of microwave filters is the so-called quality (“Q”) factor of the filter. The Q factor of a microwave resonator can be related to the proportion of energy stored by the resonator in relation to its losses. For a microwave filter realized using one or more resonators, the Q factor also provides a relation between the passband and centre frequency of the filter, as well as being related to both the insertion loss and pass-band flatness exhibited by the realized microwave filter. Generally, microwave filters having higher Q factors tend to have lower insertion loss and steeper roll-off in the transitional band between the filter's passband and the stopband, which result in a more square-shaped passband response. In contrast, filters having lower Q factors tend to exhibit increased insertion loss and a more gradual transitional band roll-off, which both decreases efficiency and increases inter-channel distortion (for example, if the filter is being deployed in a channel multiplexer). For at least these reasons, high Q factor filters may be preferably used in some telecommunications applications where excessive inter-channel distortion can be undesirable or is not permitted. Waveguide (hollow cavity) and dielectric resonator filters are two examples of generally high Q factor microwave filters. Depending on the application, Q factors on the order of about 8,000 to 16,000 can be realized using hollow cavity and dielectric resonator topologies.

SUMMARY OF THE INVENTION

In one broad aspect, some embodiments provide a microwave resonator assembly comprising: a cavity defined by an electrically conductive cylindrical enclosure in which electromagnetic energy radiated into the cavity resonates in a plurality of resonance modes comprising a dual TE22Nmode, N greater than or equal to one; an input port provided in the cylindrical enclosure for radiating a first TE22Nmode having a first polarization into the cavity; and a discontinuity formed within the cavity for electromagnetically coupling the first TE22Nmode with a second TE22Nmode having a second polarization orthogonal to the first polarization.

In another broad aspect, some embodiments provide a microwave resonator filter comprising: a plurality of cavities including at least a first cavity and a second cavity located adjacent to the first cavity, each of the first cavity and the second cavity defined by a corresponding electrically conductive cylindrical enclosure in which electromagnetic energy radiated into that cavity resonates in a plurality of resonance modes comprising a dual TE22Nmode, N greater than or equal to one; and at least one coupling element for radiating electromagnetic energy between the first cavity and the second cavity, the at least one coupling element configured to electromagnetically couple a first TE22Nmode resonating in the first cavity with a fourth TE22Nmode resonating in the second cavity, and a second TE22Nmode resonating in the first cavity with a third TE22Nmode resonating in the second cavity, the first and fourth TE22Nmodes having a first polarization and the second and third TE22Nmodes having a second polarization orthogonal to the first polarization.

These and other aspects are set forth herein.

It will be understood that reference to the drawings is made for illustration purposes only, and is not intended to limit the scope of the embodiments described herein below in any way. For convenience, reference numerals may also be repeated (with or without an offset) throughout the figures to indicate like or analogous components or features.

DETAILED DESCRIPTION OF THE INVENTION

Microwave resonator filters are commonly designed to operate in the TE11Nor TE011mode for high Q factor applications because, at lower frequency ranges, such as the C band (4-8 GHz) or the Kuband (12-18 GHz), the TE11Nor TE011modes can offer better performance than other resonance modes. For example, low loss filters having Q factors up to about 16,000 are realizable using the TE11Nor TE011modes. Quality factors up to and exceeding those realizable using the TE11Nor TE011modes of the same or higher order can be also achieved by designing the microwave filter to operate in higher order resonance modes, such as the TE22Nmode. However, for microwave filters designed for the C or Kubands, the realized TE22Nmode filter tends to be larger and bulkier as compared to the TE11Nor TE011modes. In certain telecommunications applications, such as satellite or spacecraft installations, where size and weight can be important design constraints, the additional weight and bulk incurred by the TE22Nmode filter may represent a significant overall cost. Often at the lower C and Kuband frequencies, Q factors higher than 16,000 are unnecessary.

Depending on the application, however, at higher frequency ranges, such as the K band (18-27 GHz), microwave resonator filters realized using higher order resonance modes can begin to offer competitive design considerations. Although TE22Nmode filters remain generally larger and bulkier, the size penalty between the higher order and lower order mode filters usually preferred at lower frequencies is not as dramatic at the higher K band frequencies. Given that the TE22Nmode can achieve comparable or even superior Q factors, for higher frequency band applications, the superior Q factor offered by the TE22Nmode may be traded off against the size penalty incurred relative to the TE11Nor TE011modes. For example, Q factors of about 25,000 are realizable in 20 GHz, TE22Ntype filters.

The described embodiments provide a microwave resonator filter that operates in the dual TE22Nmode to realize a very high Q factor at very high frequency ranges. The microwave resonator filter can comprise one or more cylindrical cavities in which two orthogonal field polarizations of the TE22Nmode can be excited and coupled together using a suitably located coupling element. Different combinations of inter-cavity irises provide for both direct and cross-coupling of aligned field polarizations, as required, to realize complex filter functions, such as elliptical or Chebyshev functions, as well as other functions. Negative mode coupling also allows for transmission zeros to be realized on either side of the filter passband.

Referring initially toFIGS. 1A and 1B, there is shown a microwave resonator assembly50in perspective and top views. The microwave resonator assembly50is formed using a cylindrical enclosure52, which can be constructed out of a suitable metal or other electrically conductive material. For example, the cylindrical enclosure52can be constructed out of aluminum, which is commonly used for spacecraft and other telecommunication applications due to its comparatively lightweight. As an alternative to aluminum, conductive materials having lower co-efficients of thermal expansion, including nickel-steel alloys such as INVAR, can be used to form the cylindrical enclosure52to obviate or at least reduce the need for temperature compensation devices to be incorporated into the microwave resonator assembly50. However, whether aluminum or nickel-steel alloy is used, temperature compensative devices may be used. The nickel-steel alloys also tend to be denser, more expensive and more difficult to machine than aluminum. In some cases, the conductive properties of the cylindrical enclosure52can be improved by adding a thin coating of silver, for example, or some other metal having better conductive properties than the base metal used to form the cylindrical enclosure52.

The cylindrical enclosure52includes a cylindrical sidewall54extending between opposing end walls56and58and is hollow, as illustrated inFIG. 1A, thereby defining a cavity60in the interior space of the cylindrical enclosure52. Any suitable technique for forming the cylindrical enclosure52may be used. For example, the cylindrical sidewall54and end wall56can be formed or shaped into a unitary piece of metal, with the opposing end wall58formed as a separate piece and attached to the cylindrical sidewall54after the fact. As will be appreciated, a metallic weld or alternatively mechanical fasteners (e.g., screws) can be used for this purpose. In the latter case, a mounting flange or lip (not shown) can also be incorporated into the sidewall54adjacent to where the connection is made with the end wall58. Screw holes (not shown) aligned with corresponding screw mounts in the flange can also be bored or otherwise formed in the end wall58for making the mechanical connection. Of course, other techniques for forming the cylindrical enclosure52may also be apparent.

As illustrated inFIGS. 1A and 1B, the cylindrical enclosure52has a circular cross-section defined by a radius (R)62(FIG. 1B) extending outwardly in a transverse plane in all directions from a longitudinal axis64of the cylindrical enclosure52. Alternatively, the cross-section of the cylindrical enclosure52can also be some pseudo-circular shape, such as an octagon or higher-degree polygon, which exhibits 90-degree radial symmetry and thereby approximates the boundary conditions presented by a perfectly circular cross-section. In such alternative configurations, the cross-section of the cylindrical enclosure52can be characterized by an effective radius, as opposed to a true radius, (i.e., which approximately defines the shortest distance between the longitudinal axis64(FIG. 1B) and any point on the inner sidewall54). As used herein throughout, the term ‘cylindrical’ should be understood as including both circular pseudo-circular geometries, as noted above.

Input port66is provided in the cylindrical enclosure52for radiating electromagnetic energy into the cavity60from an external waveguide section68or coaxial cable (not shown). Different structures can also be utilized for realizing the input port66, as will be appreciated. In the embodiment explicitly shown inFIGS. 1A and 1B, input port66is formed as an aperture (or iris) extending completely through the sidewall54to form a continuous volume between the external waveguide section68and the cavity60. With this arrangement, electromagnetic waves transmitted along the waveguide section68are coupled into the cavity60due to field interactions between the electromagnetic energy inside the cavity60and the incident electromagnetic wave. Alternatively, a coaxial coupler, comprising an outer cylindrical conductor separated from an interior conductive probe by a dielectric mounting plate, or some other suitably configured electromagnetic probe can be used to couple electromagnetic energy into the cavity60.

It should be appreciated that the designation of an “input” port is somewhat arbitrary and made only for the sake of clarity. Depending on the particular application to which the resonator assembly50is used, the input port66could instead be used as an output port for radiating stored electromagnetic energy out of the cavity60to the external waveguide section68. However, in the event that the resonator assembly50is used to realize a non-symmetrical filter (containing distinct “input” and “output” ports), the designation of input port66as such will be followed throughout. It should also be appreciated that the input port66may be used to couple the cavity60with some microwave component other than external waveguide section68, such as a second cavity located adjacent to the first cavity60, and thereby used to radiate electromagnetic energy between the two adjacent cavities60, as in a multi-cavity microwave resonator filter.

Referring now toFIG. 2, electromagnetic energy radiated into the cavity60ofFIGS. 1A and 1Bcan be excited into an infinite number of different resonance modes, each of which is characterized by a corresponding resonant frequency and is supported by the particular geometry of the cavity60. In general, microwave filters are designed to operate in only one particular resonance mode, which defines a frequency range of operation for the filter, for example in terms of a centre frequency and bandwidth. Other unwanted (or spurious) modes appearing in the cavity and characterized by other resonant frequencies, therefore, represent an effective limit on the operational range of the filter. In addition to the TE11Nand TE011modes commonly used in lower frequency telecommunications applications, the cylindrical shape of the cavity60also supports the TE22Ndual resonance mode. As will be appreciated, the third co-efficient index, “N”, indicates the repetition rate (in terms of half wavelengths) of the resonance mode's electromagnetic field pattern in the axial direction and can be any integer greater than or equal to one. The cylindrical geometry of the cavity60supports all TE22Nmodes, although as a practical matter, the TE221mode may be preferred to other higher modes for its larger spurious free range as compared to higher TE22Nmodes. A mode chart can be consulted for a complete listing of resonance modes supported by the cavity60.

Due to the 90-degree radial symmetry of the cavity60, two distinct TE22Nmodes may be excited in the cavity60. Thus, the TE22Nmode can be referred to as a dual mode to reflect the fact that two electromagnetic resonators having the same resonant frequency are supported simultaneously by one physical cavity. Relative to the first TE22Nmode70(leftmost field pattern shown inFIG. 2), the second TE22Nmode72(field pattern shown inFIG. 2) has the same electromagnetic field pattern, but an orthogonal polarization. As will be appreciated and as used herein throughout, two modes are referred to as being ‘orthogonal’ modes, if for a perfectively symmetrical cavity, the respective E and H field components of the two modes are oriented 90-degrees relative to one another at all points within the cavity. As the two TE22Nmodes70and72are “orthogonal” to one another, they naturally co-exist within the cavity60without substantial field interactions, so that electromagnetic energy excited in one of the TE22Nmodes70and72is contained within that given mode and, in the absence of a discontinuity or coupling element formed within the cavity60, would not leak over into the other “orthogonal” mode.

Using the two characterizing vectors74and76to establish a reference angular position within the cavity60, the second TE22Nmode72is 45-degrees offset from the first TE22Nmode70in the transverse plane to the longitudinal axis64of the cavity60. (In other words, a 45-degree angle is formed between the two characterizing vectors74and76). The choice of the two characterizing vectors74and76is somewhat arbitrary because, owing to the 90-degree radial symmetry of the first and second TE22Nmodes70and72, any one of 4 different vectors (shown inFIG. 2) can be selected for each TE22Nmode70and72to serve as the characterizing vector. In either case, the set of 4 vectors are oriented 90-degrees offset from one another. For sake of clarity, reference will simply be made to the characterizing vectors74and76, which can be any of the vectors illustrated inFIG. 2.

Referring back toFIGS. 1A and 1B, electromagnetic energy radiated into the cavity60through the input port66will be excited into one of the two TE22Nmodes70or72(shown inFIG. 2), if the incident electromagnetic wave is radiated at or near to the resonant frequency of the TE22Ndual mode. Which of the two orthogonal polarizations is excited within the cavity60can depend on the particular mechanism of input coupling and the angular position of the input port66in relation to the two characterizing vectors74and76, as will be explained in more detail below. The other of the two TE22Nmodes70or72not directly coupled to the input port66is simultaneously excited within the cavity60by forming at least one discontinuity within the cavity60at a corresponding location within the cavity60, where each of the TE22Nmodes70and72have non-zero field components. For example, coupling of the two TE22Nmodes70and72is accomplished using one or more coupling screws78projecting through the sidewall54(or alternatively end walls56or58) into the interior of the cavity60. Alternatively, other structures that disturb the radial symmetry of the cavity60can be used to provide intra-cavity coupling between the two orthogonal TE22Nmodes70and72, including deformations (convex or concave) formed in the sidewall54, dielectric blocks mounted within the cavity60or other dielectric boundary conditions, and the like. The term “discontinuity” is understood to encompass each of the above-noted disturbances to the 90-degree radial symmetry of the cavity60.

Tuning screws82and80, which like the coupling screws78project through the sidewall54into the interior of the cavity60, are used for making fine adjustments to the resonant frequencies of the first and second TE22Nmodes70and72, respectively. The location of the tuning screws82and80within the cavity60determines which of the two orthogonal TE22Nmodes70and72are affected. For example, the tuning screw82is used to adjust the resonant frequency of the first TE22Nmode70(defined by characterizing vector74) and has comparatively less effect on the resonant frequency of the second TE22Nmode72(defined by characterizing vector76). On the other hand, the tuning screw80, which is located at a 45 degree angular offset from the tuning screw82is used to adjust the resonant frequency of the second TE22Nmode72, while having comparatively little effect on the resonant frequency of the first TE22Nmode70. The tuning screws82and80therefore provide relatively independent tuning of the first and second TE22Nmodes70and72and can be used, for example, to compensate for resonant frequency shifting caused by other components of the resonator assembly50, such as input port66, coupling screws78, etc.

The resonator assembly50also includes at least one coupling element for radiating electromagnetic energy out of the cavity60(e.g., into an adjacent cavity to realize a multi-cavity filter having 4 or more poles). In the embodiment explicitly shown inFIGS. 1A and 1B, the resonator assembly50includes radial iris84and radial irises86. As will be explained in more detail below, the angular position of the radial irises84and86in relation to the characterizing vectors74and76determines which of the two TE22Nmodes70and72are predominantly coupled. As shown, the radial iris84couples the first TE22Nmode70, due to its angular position within the cavity60, while providing substantially less coupling of the orthogonal TE22Nmode72. Moreover, the two radial irises86(which are each located at a 45-degree angular offset from the radial iris84) achieve the opposite effect of coupling the second TE22Nmode72predominantly while providing substantially less coupling of the orthogonal TE22Nmode70. The size and location of the radial irises84and86also determine the amount of coupling between aligned modes in adjacent cavities, as will be explained in more detail below.

Although not explicitly illustrated inFIGS. 1A and 1B, a temperature compensation device can also be included in the microwave resonator assembly50. The temperature compensation device can be used to stabilize the resonant frequency of the TE22Nmodes70and72over a range of different operating temperatures as follows. When the resonator assembly50is subjected to a temperature gradient, the material used to form the cylindrical enclosure52will expand or contract according to its co-efficient of thermal expansion. For example, aluminum has a relatively large co-efficient of thermal expansion as compared to the temperature stabilized nickel-steel alloys. Expansion or contraction of the cylindrical enclosure52causes a corresponding change in the volume of the cavity60defined therewithin. Since the resonant frequency of the dual TE22Nmode is related to the volume of the cavity60, without some form of temperature compensation, that frequency can “drift” about its centre point over the range of operating temperatures as the cavity60expands and contracts.

As will be appreciated, different approaches to providing temperature compensation in the resonator assembly50are possible. For example, a temperature compensation device can be mounted to the exterior portion of end wall56or58, whichever is free and not used for external mounting of the resonator assembly50. The temperature compensation device can comprise a strap or end cap assembly of a comparatively low thermal expansion material coupled to the exterior wall portion, so that as the operating temperature of the resonator assembly50increases, the strap or end cap assembly exerts a force on the end wall56or58to bend or flex the end wall56or58inwardly. The corresponding decrease in cavity volume due to the inward flexing of the end wall56or58counterbalances the corresponding increase in cavity volume due to radial expansion of the cavity60, thereby maintaining an essentially constant cavity volume over the entire operating range of the resonator assembly50. Accordingly, for both planar and stack-up (collinear) configurations having side launch termination (i.e., input/output coupling provided in the sidewall54), the resonator assembly50can accommodate a temperature compensation device to adjust an exposed end wall56or58and, consequently, the axial length of the cavity60in order to compensate frequency drift due to temperature gradients. While the strap or end cap assembly explicitly described above represents one possible temperature-compensating device, still other mechanisms for providing temperature compensation may be apparent.

Referring now toFIG. 3, different locations for the input port66within the cavity60are possible because of the 90-degree radial symmetry of the TE22Ndual mode. Four such locations for the input port66are shown inFIG. 3, spaced 90-degrees apart from each other, at locations within the cavity60having an angular position, in relation to the characterizing vector76, equal to an integer multiple of 90 degrees. As used herein throughout, the term “integer multiple” should be understood as including every whole number multiple, positive and negative, as well as zero. In general, when the input port66is realized using an iris or aperture defined through the sidewall54, the characterizing vector of the coupled TE22Nmode will be offset essentially 45-degrees from the input port66, plus an integer multiple of 90 degrees, regardless of the absolute angular position of the input port66within the cavity60. Thus, each of the four locations for the input port66explicitly shown inFIG. 3would be suitable for exciting the first TE22Nmode70as these locations are 45-degrees offset from the characterizing vectors74shown inFIG. 2. It follows also that by rotating the angular position of the input port66within the cavity60by 45 degrees, relative to one of the locations explicitly shown inFIG. 3, the input port66would be made suitable for exciting the second TE22Nmode72defined by the second characterizing vector76. Of course, it should be appreciated that the terms “first” and “second” are used herein throughout only to distinguish between the two orthogonal polarizations of the dual TE22Nmode.

Referring now toFIGS. 4A and 4B, the 90-degree symmetry of the dual TE22Nmode also results in different possible locations for the coupling screw78as illustrated inFIG. 4A(or79as illustrated inFIG. 4B) to be formed within the cavity60for coupling together the two orthogonal TE22Nmodes70and72. More generally, any electromagnetic discontinuity, such as those described above, can be formed at the locations indicated. To provide good intra-cavity mode coupling, the electromagnetic discontinuity, or discontinuities, should be formed at a location within the cavity60where each of the orthogonal TE22Nmodes70and72have non-zero field components, so that by perturbing the field pattern of the first TE22Nmode70, an appreciable amount of electromagnetic energy will transfer into the orthogonal polarization and thereby indirectly excite the second TE22Nmode72. A single coupling screw78(or79) can be projected into the cavity60at one of the locations indicated, depending on the particular application, if the single coupling screw78or79provides the required amount of mode coupling. However, multiple screws78(such as the two screws78seen inFIGS. 1A and 1B), or other discontinuities, can be included in the resonator assembly50to increase coupling of the two TE22Nmodes70and72as required.

Using the characterizing vectors74and76as reference angular positions, the coupling screw78can be located so as to have an angular position within the cavity60that is substantially intermediate the two characterizing vectors74and76. In a particular case, the coupling screw78can be located at the angular midpoint between the two characterizing vectors74and76, so that the angular position of the coupling screw78bisects the 45-degree angle formed between the two characterizing vectors74and76, 22.5 degrees offset from each respective vector. Although it is not strictly necessary for the coupling screw78to be located at the precise angular midpoint between the two characterizing vectors74and76, for good coupling between the orthogonal TE22Nmodes70and72, the angular spacing of the coupling screw78from each characterizing vector74and76can be more than minimal. A screw or other electromagnetic discontinuity aligned with either of the two characterizing vectors74or76would provide substantially less coupling of the two TE22Nmodes70than does the coupling screw78when positioned intermediate the two characterizing vectors74and76.

It will also be understood that the axial position of the coupling screw78is optimizable and can depend on the axial repetition rate of the dual TE22Nmode field pattern (i.e., the value of “N”), depending on the amount of coupling required for the particular application. Since each increment of “N” represents one half-wavelength in the axial field pattern of the dual TE22Nmode, the order of the TE22Nprescribes certain E-field maxima along the axial length of the cavity60, and based upon which the coupling screw78can be located to provide good coupling. As will be appreciated, the TE221mode has one E-field maximum located at the axial midpoint of the cavity60, the TE222mode has two E-field maxima located at the one and three-quarter heights of the cavity60and, in general, the TE22Nmode has E-field maxima located at odd integer multiples of one-quarter wavelength. The coupling screw78may conveniently be located at these axial positions exhibiting respective E-field maxima, although it is not necessary and other axial locations can provide sufficient coupling as well. Accordingly, the range of suitable locations for the coupling screw78can be generalized to include a plurality of different locations within a wedge of the cavity60, defined by the longitudinal axis64, the two characterizing vectors74and76, and the arcuate portion of the sidewall54subtended between the two characterizing vectors74and76.

Again due to the 90-degree radial symmetry of the dual TE22Nmode, the one or more electromagnetic discontinuities used for inter-mode coupling can be formed at different locations within the cavity60. Eight exemplary locations are illustrated inFIGS. 4A and 4B, which are separated into two sets of four locations each based on the relative sign of the inter-mode coupling that is realized at each respective location. Coupling screws78are spaced 90-degrees apart from each other and at angular positions, in relation to the first characterizing vector74, equal to negative 22.5 degrees plus an integer multiple of 90 degrees. Coupling screws79are also spaced 90-degrees apart from each other but are located at angular positions, in relation to the first characterizing vector74, equal to positive 22.5 degrees plus an integer multiple of 90 degrees. Thus, the set of coupling screws79is 45-degrees offset with respect to the set of coupling screws78. Consequently, for a given polarity of the TE22Nmode70, the corresponding polarity of the TE22Nmode72when excited by the coupling screws78will be opposite to that of the TE22Nmode72when excited by the coupling screws79. (It is noted that the angular positions of coupling screws78and79could equivalently be defined in relation to the characterizing vector76and is defined with reference to characterizing vector78for convenience only.)

Referring now toFIGS. 5A and 5B, one or more different coupling elements can be included in the resonator assembly50for radiating one or both of the TE22Nmodes70and72out of the cavity60. The coupling elements can be provided in either the sidewall54or the end wall58(as illustrated inFIGS. 1A and 1B) in different configurations of the resonator assembly50. In each case, the shape and location (axial and angular) of the coupling element within the cavity60can influence the amount of coupling achieved with respect to each of the two orthogonal TE22Nmodes70and72, Coupling elements formed at certain locations and angular positions within the cavity60also couple one of the TE22Nmodes substantially more than the other orthogonal mode. The iris configurations illustrated inFIG. 5Aprovide relatively more coupling of the first TE22Nmode70defined by characterizing vector74, while those illustrated inFIG. 56provide relatively more coupling of the second TE22Nmode72defined by characterizing vector76.

As seen inFIG. 5A, radial iris84is formed in the end wall58having an angular position equal to an integer multiple of 90-degrees, in relation to the second characterizing vector76. Four such locations for the radial iris84are indicated due to 90-degree radial symmetry in the cavity60, namely at 0, 90, 180 or 270 degrees (and hence at integer multiples of 90 degrees) offset from the second characterizing vector76. Each of the radial irises84has a generally rectangular shape forming an elongated, slot-shaped aperture extending predominantly outwardly from the longitudinal axis64of the cavity60in the radial direction. Thus, the radial iris84can be substantially aligned with the effective radius62but can also have some radial skew or yaw. The radial iris84can have square corners as shown or alternatively can have rounded edges to realize a higher Q factor. The centre of the radial iris84is spaced apart from the longitudinal axis64by a radial distance of approximately 0.728R, where R is the actual or effective radius of the cavity60. As can be seen fromFIG. 2, for example, at this radial distance (and angular position with the cavity60), the first TE22Nmode70has relatively dense E-field lines extending orthogonal to the radial iris84, indicating that a radial iris84having the radial spacing, orientation and angular position shown inFIG. 5Awould provide good coupling of the first TE22Nmode70.

While a radial distance of 0.728R represents one possibility, the spacing for the radial iris84is optimizable to fit the particular microwave application. For example, the relatively strong coupling achieved when the radial iris84is spaced at 0.728R from the longitudinal axis64can make this radial position suitable for wideband applications. Other radial positions spaced apart from the 0.728R point may otherwise be suitable for narrowband applications due to the relatively weaker coupling that can be expected at these other radial positions. Accordingly, a radial spacing greater than about 0.455R may be appropriate for different applications. The length of the radial iris84can also be adjusted as needed when the radial iris84is shifted away from the 0.728R point to compensate for some of the consequent loss of bandwidth. Moreover, depending on bandwidth requirements, the radial iris84can also be located (not shown) at a radial distance of about 0.25R, or more generally between about 0.1 R to 0.4R. This approximate range may be suitable again for some more narrowband applications. As will be appreciated, the radial iris84can also have different shapes other than rectangular, such as a triangle or sector.

In addition to, or in place of, the radial iris84, transverse angular iris88is also suitable for coupling the first TE22Nmode70. Transverse angular iris88is formed in the end wall58having an angular position equal to an integer multiple of 90-degrees, in relation to the first characterizing vector74. Thus, again four different locations for the transverse angular iris88are indicated due to 90-degree radial symmetry in the cavity60, which occur at 0, 90, 180 or 270 degrees offset from the first characterizing vector74. Each of the transverse angular irises88shown have a generally rectangular shape, but elongated now in a direction transverse to the real or effective radius of the cavity60(i.e., in an “angular” or “tangential” direction). The centre of each transverse angular iris88is shown spaced apart from the longitudinal axis64by a radial distance of approximately 0.455R. The relatively dense, orthogonal E-field lines of the first TE22Nmode70(FIG. 2) at these radial and angular positions again indicate their suitability for coupling the first TE22Nmode.

Like the radial iris84, the radial spacing of the transverse angular iris88is also optimizable to fit the particular microwave application. While a radial spacing of 0.455R may be suitable for wideband applications, a radial distance of between about 0.25R and 0.728R for the transverse angular iris88may still be suitable for some narrowband applications. Optionally, the length of the transverse angular iris88can also be adjusted to control the achievable bandwidth. A separate range of radial distances of between about 0.85R and the sidewall54(i.e., greater than 0.85R) may also be suitable for some narrowband applications, due to the relatively weaker coupling that can be expected at these other radial positions in comparison to have 0.455R when the E-field lines of the first TE22Nmode are denser. The transverse angular iris88can be rectangular (as shown) or arcuate in a trajectory tangential to the sidewall54, and can have some angular skew or be substantially orthogonal to the effective radius62. The edges of the transverse angular iris88can also be square or rounded to realize a higher Q factor.

FIG. 5Bshows radial irises86and transverse angular irises90, similar to the radial irises84and88illustrated inFIG. 5A, but at locations within the cavity60that are suitable for coupling the second TE22Nmode72defined by characterizing vector76(as opposed to the first TE22Nmode70defined, by characterizing vector74). Radial irises86are located at an angular position equal to an integer multiple of 90-degrees in relation to the first characterizing vector74, and are therefore 45-degrees offset with the radial irises84. However, like radial irises84as illustrated inFIG. 5Asuitable for coupling the first TE22Nmode70, the radial irises86can be located at a radial distance from the longitudinal axis64equal to any of the distances or ranges discussed above depending on the application and bandwidth requirements of the resonator assembly50. In the exemplary case illustrated, each radial iris86can be centered at a radial distance approximately equal to 0.728R.

The transverse angular irises90shown inFIG. 5Bare located at an angular position equal to an integer multiple of 90-degrees in relation to the second characterizing vector76, which is 45-degrees offset with respect to the transverse angular irises88as illustrated inFIG. 5A. The approximate radial distances and ranges indicated for the transverse angular iris88also apply to the transverse angular irises90, except that transverse angular irises90provide good coupling of the second TE22Nmode72at these locations within the cavity90. The particular radial distance selected for the transverse angular iris90can again depend on bandwidth requirements or other factors. In an exemplary case, the transverse angular iris90can be located at about 0.455R, where R is the effective radius of the cavity60.

Referring now toFIGS. 6A-6F, there are illustrated some exemplary combinations of coupling elements that can be formed in the end wall58for radiating one or both of the TE22Nmodes70and72(as illustrated inFIG. 2) out of the cavity60. It should be appreciated that the examples shown inFIGS. 6A-6Fare illustrative only and not to be understood as representing an exhaustive set of all possible combinations of coupling elements. As can be seen from the example configurations shown, the number and location of each type of coupling element is optimizable to provide different strengths and relative proportions of coupling. In some cases, a single coupling element may be used to couple a given TE22Nmode (either the first TE22Nmode70or the second TE22Nmode72, as the case may be). In other cases, multiple coupling elements can be used simultaneously to provide greater amounts of coupling. As examples only, the set of coupling elements formed in the end wall58can also include all radial irises, all transverse angular irises, or a mix of radial and transverse angular irises, in addition to other shapes or orientations of coupling elements.

The combination shown inFIG. 6Aincludes a radial aperture84together with a pair of radial apertures86located at a 45-degree angular offset (positive and negative, respectively) from the radial aperture84. The radial aperture84(aligned with the characterizing vector76) couples the first TE22Nmode70, while the radial apertures86(an integer multiple of 90-degrees offset from the characterizing vector74) jointly couple the second orthogonal TE22Nmode72. The combination inFIG. 6Bis similar to that shown inFIG. 6A, but now includes a pair of radial irises84together with two pairs of radial irises86arranged diametrically opposed. Again the radial irises84provide coupling of the first TE22Nmode70, while the radial irises86provide coupling of the second TE22Nmode72. The combinations shown inFIGS. 6A and 6Bare two examples of coupling being provided by all radial irises84or86.

InFIG. 6C, a single radial iris84aligned with the characterizing vector76for coupling the first TE22Nmode70is combined with a single transverse angular iris90, which is also aligned with the characterizing vector76and therefore provides coupling of the second TE22Nmode72. InFIG. 6D, four such combinations of a radial iris84and transverse angular iris90are formed in the end wall58, each combination of a radial iris84and transverse angular iris90spaced apart from each other combination within the cavity60by 90-degree angular offsets. Accordingly, each radial iris84predominantly couples the TE22Nmode70and each transverse angular iris90predominantly couples the orthogonal TE22Nmode72.

It is also possible to utilize all transverse angular irises88and90, as shown inFIGS. 6E and 6F. The combination inFIG. 6Eincludes a pair of transverse angular irises90suitable for coupling the second TE22Nmode72, together with two pairs of transverse angular irises88suitable for coupling the first TE22Nmode70. As will be understood, each transverse angular iris88is located an integer multiple of 90 degrees offset in relation to the first characterizing vector74, and likewise for each transverse angular iris90in relation to the second characterizing vector76. The combination of coupling elements shown inFIG. 6Fis similar to that shown inFIG. 6E, but includes only a single transverse angular iris90and a pair of transverse angular irises88.

Referring now toFIGS. 7A and 7B, input coupling into the cavity60can also be accomplished using an input port92as illustrated inFIG. 7Aor port94as illustrated inFIG. 76formed in the end wall56of the cylindrical enclosure52, as an alternative to the input port66formed in the sidewall54. The locations of the input ports92and94are similar to the transverse irises88and radial irises84(FIG. 5A), but formed in the end wall58rather than the end wall56. As shown inFIG. 7A, an input port92can be formed in the end wall56at a location having an angular position, in relation to the first characterizing vector74, equal to an integer multiple of 90 degrees. The input port92is formed out of an elongated iris oriented generally transverse to the effective radius of the cavity60, so that the input port92has predominantly an angular (as opposed to a radial) dimension, and can be spaced apart from the longitudinal axis64by a radial distance again as discussed in relation to the transverse angular irises88. Thus, in some configurations of the resonator assembly50, the centre point of the input port92can have a radial spacing of about 0.455R, where R is the effective radius of the cavity60. But other radial spacings within the ranges discussed above may be suitable as well for different applications.

Now referring specifically toFIG. 7B, input coupling can alternatively be achieved using an input port94formed in the end wall56at a location having an angular position, in relation to the second characterizing vector76, equal to an integer multiple of 90 degrees. The input port94is formed out of an elongated iris oriented in a generally radial direction and spaced apart from the longitudinal axis64by a radial distance, depending on the particular application, falling within one of the ranges discussed above in the context of the radial iris84. In one exemplary configuration, the centre point of the input port94can have a radial spacing of about 0.728R, where R is the effective radius of the cavity60.

Referring now toFIGS. 8A and 8B, one or more tuning elements can be placed within the cavity60at different locations in order to make minor adjustments to the resonant frequencies of one or the other of the TE22Nmodes70and72, or in some cases to both TE22N70and72modes simultaneously. As will be appreciated, the number and location of tuning elements is optimizable and may depend on the particular application or use of the microwave resonator assembly50. At least some of the tuning elements shown inFIGS. 8A and 8Bcan also improve the spurious performance of the microwave resonator assembly50, as will be explained. The tuning elements can be formed using screws or other suitable structures (e.g., rods, wall deformations and dielectric blocks) for causing small perturbations to the electromagnetic field patterns of the TE22Nmodes70and72. For the sake of clarity only, reference may be made primarily to tuning screws.

To provide relatively independent tuning of the orthogonal TE22Nmodes70and72, at least some of the tuning elements can be placed at locations within the cavity60where one of the TE22Nmodes70and72has relatively large field components as compared to the other TE22Nmode, so that the tuning element disproportionately disturbs one of the corresponding field patterns relative to the other. As will be appreciated, the small field perturbation can incrementally adjust the corresponding TE22Nmode's resonant frequency higher or lower, thereby “tuning” the corresponding TE22Nmode to a selected frequency (for example, in order to place the centre frequency of a microwave bandpass filter). Although tuning elements, such as tuning screws, may be utilized to incur fine adjustments to a resonant frequency, there may be a practical limit on the degree to which that resonant frequency can be adjusted. For coarser adjustments, it may be required or preferable to re-design other dimensions of the cavity60, such as its axial length or effective radius62.

The tuning elements shown specifically inFIG. 8Aare suitable for tuning the first TE22Nmode70defined by characterizing vector74. Tuning screws82may project through the side wall54into the interior of cavity60, at a suitable axial height (which may depend on the value of “N”) within the cavity60, and at angular positions equal to an integer multiple of 90 degrees in relation to the second characterizing vector76. The dimensions and penetration depth of the tuning screw82into the cavity60determine its influence on the resonant frequency of the first TE22Nmode70.

Alternatively, or additionally, one or more tuning screws95may be included in the resonator assembly50. The tuning screws95project through the end wall56into the interior of the cavity60, and are placed at locations having angular positions equal to an integer multiple of 90 degrees in relation to the first characterizing vector74. The tuning screws95can also each be spaced from the longitudinal axis64of the cavity60by a radial distance of about 0.455R, where R is the effective radius of the cavity60, or in one of the indicated ranges of radial spacing for the transverse angular iris88as describe with reference toFIG. 5A. As discussed above, within these approximate ranges and at the angular positions shown, the field components of the first TE22Nmode70are relatively dense.

As a further possibility, one or more tuning screws96may project through the end wall56into the interior of the cavity60, at angular positions equal to an integer multiple of 90 degrees in relation to the second characterizing vector76. The tuning screws96can also each be spaced from the longitudinal axis64of the cavity60by a radial distance of between about 0.728R or one of the above-discussed ranges for the radial iris84, as the field components of the first TE22Nmode70are again relatively dense in these regions of the cavity60.

Similar tuning elements are illustrated inFIG. 8B, but at locations within the cavity60that are suitable for tuning the second TE22Nmode72. Accordingly, tuning screws80project into the interior of the cavity60(at a suitable axial height based on the value of “N”), and at angular positions equal to an integer multiple of 90 degrees in relation to the first characterizing vector74. Tuning screws98project through the end wall56, spaced apart from each other by 90-degrees, at angular positions within the cavity60equal to an integer multiple of 90-degrees in relation to the second characterizing vector76. Finally, tuning screws99project through the end wall56or58(not shown inFIG. 8B), spaced apart 90-degrees from each other, at angular positions equal to an integer multiple of 90-degrees in relation to the first characterizing vector74. The tuning screws98and99can have the same radial spacing (or range of spacing) as tuning screws95and96, respectively, shown inFIG. 8A.

A single tuning screw97(illustrated in bothFIGS. 8A and 8B), projecting into the interior of the cavity60at the centre-point of the end wall56or58, aligned with the longitudinal axis64, can also be included in the microwave resonator assembly50. Due to the radial symmetry of the cavity60, the tuning screw97can be used to adjust the resonant frequencies of both the TE22Nmodes70and72simultaneously, with the amount and direction (higher or lower) of the adjustment depending on the dimensions and penetration depth into the cavity60of the tuning screw97. Inclusion of the tuning screw97can additionally improve the spurious performance of the microwave resonator assembly50by pushing the resonant frequencies of adjacent, spurious modes away from the operational dual TE22Nmode. Because the field components of each TE22Nmode70and72are fairly small at the centre-point of the end wall56or58(seeFIG. 2), tuning screw97will have a larger relative sifting of the resonant frequencies of other resonant modes having comparatively large field components at this point. For example, the TM121spurious mode is strong at the centre-point and therefore will be disproportionately affected. Tuning screw97can be included, for example, to supplement to the other tuning elements illustrated inFIGS. 8A and 8Band for improved spurious performance.

Referring now toFIGS. 9A-9C, there is illustrated a microwave resonator filter100in perspective, top and side views. The microwave resonator filter100is realized using microwave resonator assembly50, shown inFIGS. 1A and 1B, to form a multi-cavity structure. By exciting each cavity in the dual TE22Nmode, the microwave resonator filter100realizes 2 poles per cavity for an overall 4-pole filter characteristic. Of course, it should be appreciated that the microwave resonator filter100can be realized using any arbitrary number of cavities, in alternative configurations, to realize additional poles and higher order filters. However many cavities are included, a combination of direct and cross-coupling between adjacent cavities makes it possible to realize elliptic and Chebyshev functions. Transmission zeros are also realizable by designing the filter to incorporate negative mode coupling, either between orthogonal modes excited within a single cavity or between mutually aligned modes resonating in adjacent filter cavities. For brevity some aspects of the microwave resonator filter100described above in the context of the resonator assembly50will not be described again or may be described in less detail.

A first cylindrical enclosure52adefining a first cavity60ais formed out of cylindrical sidewall54a, end wall56aand common end wall158. A second cylindrical enclosure52bdefining a second cavity60bis formed out of cylindrical sidewall54b, end wall56band the common end wall158. Accordingly, the first cavity60ais separated from the second cavity60bby the common end wall158between the first and second cylindrical enclosures52aand52b, so that the first and second cavities60aand60bare adjacent and collinear (i.e., so that the first and second cavities60aand60bshare a common longitudinal axis). While the cavities60aand60bare illustrated inFIGS. 9A-9Cas sharing a common end wall158between the cylindrical enclosures52aand52, alternatively, the cavities60aand60bcan be separated by corresponding adjacent end walls having a small air gap formed therebetween.

Input port66acoupled to external waveguide section68aexcites a first TE22Nmode70within cavity60ahaving a first polarization defined by the first characterizing vector74as illustrated inFIGS. 2,3,4A,4B,5A,5B,6A-6F,7A,7B,8A and8B. The pair of diametrically opposed coupling screws78aprojecting through the sidewall54ainto the interior of the cavity60acouple the first TE22Nmode70excited in the first cavity60awith a second TE22Nmode72also excited in the first cavity60a, the second TE22Nmode72having an orthogonal polarization relative to the first TE22Nmode70. Tuning screws82aand80aoptionally adjust the resonant frequencies of the first and second TE22Nmodes70and72for closer placement to a selected centre frequency of the microwave resonator filter100.

Transverse angular iris90formed in the common end wall158between the first and second cavities60aand60bcouples the second TE22Nmode72excited in the first cavity60awith a third TE22Nmode72excited in the second cavity60b. Simultaneously, radial iris84formed in the common end wall158couples the first TE22Nmode70excited in the first cavity60awith a fourth TE22Nmode70excited in the second cavity60b. The first and fourth TE22Nmodes have mutually aligned polarizations defined by the characterizing vector74, while the second and third TE22Nmodes have mutually aligned polarizations defined by the characterizing vector76.

Within the second cavity60b, the pair of diametrically opposed coupling screws79bprojecting through the sidewall54bcouple together the third TE22Nmode72and fourth TE22Nmode70excited also in the second cavity60b. As will be explained in more detail below, the angular position of the coupling screws79boffset 45-degrees in relation to the coupling screws78aplaced in the first cavity60arealizes transmission zeroes in the microwave resonator filter100. Also, output port66bis used to radiate electromagnetic energy out of the second cavity60bby coupling the fourth TE22Nmode70within the second cavity60bwith the external waveguide section68b. Tuning screws80band82bare optionally included to adjust the resonant frequency of the third TE22Nmode and fourth TE22Nmode to the selected centre frequency of the microwave resonator filter200. As in a symmetric filter, the designation of “input” and “output” ports may be somewhat arbitrary and depend on perspective, the output port66bis substantially similar to the input port66aand can be formed in any of the locations illustrated inFIG. 3(or alternativelyFIGS. 7A-7B).

The particular combination of direct and cross-coupling elements shown inFIGS. 9A-9Crealizes a 4-pole, cross-coupled filter. A general folded path between the input port66aand output port66bis formed by the successive mode coupling provided by the coupling screws78a(first to second), the transverse angular iris90(second to third), and the coupling screws79b(third to fourth). In addition to the general folded path, the radial iris84then provides a cross-coupled path directly between the first TE22Nmode70resonating in the first cavity60aand the mutually aligned fourth TE22Nmode70resonating in the second cavity60b. Accordingly, in the configuration shown, the transverse angular iris90serves as a direct coupling element, while the radial iris84serves as a cross-coupling element. Although it should be appreciated that the function served by these coupling elements may be reversed and depends on their angular position in relation to the characterizing vectors74and76, as herein described. Moreover, the term “direct coupling element” as used herein can refer to any element that provides coupling between two successive modes in the general folded path (e.g., second and third), while the term “cross coupling element” can refer to any element that provides coupling between two non-successive (e.g., first and fourth) modes in the general folded path.

It should also be appreciated that, as an alternative to the cross-coupled filter configuration shown inFIGS. 9A-9C, a general folded filter configuration (without cross-coupling) is also realizable by omitting the cross-coupling element, in this case the radial iris84. With no cross-coupled path directly between the first and fourth TE22Nmodes70, the remaining coupling elements (i.e., coupling screws78a, transverse angular iris90, and coupling screws79b) realize the general folded path between the input port66aand output port66bby coupling the first, second, third and fourth modes successively.

Principles of microwave filter design may be utilized in order to determine the number, type, location and size of the coupling elements included in the microwave filter100. For example, a transfer function for the microwave filter100can be calculated, usually by selecting a filter type (elliptic, Chebyshev, etc.), and then calculating poles and zeros of the transfer function that will realize a specified set of performance criteria, such as insertion loss, return loss, passband ripple, stopband ripple, bandwidth, isolation. Often the specified performance criteria will be interrelated to the order of the microwave filter100, so that either the selected criteria will dictate a minimum required filter order or, alternatively, if the filter order (e.g., 4-poles, 8-poles, etc.) is fixed, constraints may then be imposed on the realizable performance criteria. As will be appreciated, the design process can be iterative requiring multiple formulations until an acceptable transfer function is designed. Design software may be of assistance throughout the process.

After synthesizing the filter transfer function, a variety of different techniques can then be used to realize a physical microwave resonator (e.g., microwave resonator filter100) that exhibits the synthesized transfer characteristics. One such technique involves formulating a coupling matrix (usually designated “M”) from the synthesized transfer function. As will be appreciated, the entries in the coupling matrix M specify the magnitude and sign of coupling required between each resonator included in the microwave resonator filter100to realize the synthesized transfer function. Once the coupling matrix has been formulated, physical dimensions for the microwave filter can be solved that provide the required couplings. Of course, it is possible that not every synthesized transfer function will be physically realizable. For example, cross-coupling between two non-successive resonators (or even between successive resonators) may be required that cannot easily be realized. The physical realization stage of the design process may be iterative as well, and it may be necessary to reformulate the filter transfer function subject to physical constraints as well as performance criteria.

Assuming a realizable transfer function has been synthesized, the coupling elements included in the microwave resonator filter100can be selected and configured to meet the requirements of the coupling matrix M. In terms of coupling the first TE22Nmode70and second TE22Nmode72excited in the first cavity60a, the number and respective sizing of coupling screws78a(as well as angular position) can be varied to meet the requirement. Similarly, in terms of coupling the third TE22Nmode72and fourth TE22Nmode70excited in the second cavity60b, the number and respective sizing of coupling screws79b(as well as angular position) can be varied to meet the requirement. In general, increasing the size and number of coupling elements will increase the amount of coupling provided. Depending on whether transmission zeros are to be created, coupling screws having the same or different polarity of coupling can be used in the cavities60aand60b. In the exemplary configuration shown, the coupling screws have opposite polarities to create transmission zeros.

A similar process can be followed to size the coupling elements formed in the common end wall158for radiating energy between the two cavities60aand60b. The number and relative sizing of radial irises86and/or transverse angular irises90(FIG. 5B) can be varied until the required coupling between the mutually aligned second and third TE22Nmodes72excited in the first and second cavities60aand60b, respectively, is achieved. If cross-coupling between the first and fourth TE22Nmodes70is also prescribed by the coupling matrix M, then the number and relative sizing of radial irises84and/or transverse angular irises88(FIG. 5A) can be varied until the required coupling is realized. The illustrative combinations presented inFIG. 6represent just some of the possible ways in which to realize different amounts and direct and cross-coupling of modes in the microwave resonator filter100. Design software can again be of assistance in the process of sizing the different coupling elements.

The microwave resonator filter100is configurable based on the selection of intra or inter cavity coupling elements to realize two transmission zeros, thereby creating an overall symmetric filter function. Coupling of the first TE22Nmode70to the second TE22Nmode72within cavity60ais achieved using one or more of the coupling screws78, while coupling of the third TE22Nmode72to the fourth TE22Nmode70within cavity60bis achieved using one or more of the coupling screws79, which are 45-degrees offset from the coupling screws78. When coupling screws78are included in cavity60aand coupling screws79are included in cavity60b(or vice versa), the respective couplings in each cavity60aand60bhave opposite polarities, or are disposed in an anti-symmetrical relationship in relation to each other, resulting in the creation of the transmission zeros. On the other hand, transmission zeros can be avoided by placing coupling screws78(or equivalently coupling screws79) in each cavity60aand60b, so that the respective couplings have the same polarity (whether positive or negative) and therefore do not form an anti-symmetrical relationship.

Referring now toFIGS. 10A and 10B, in an alternate configuration of the resonator assembly50, coupling elements are formed in the sidewall54of the cylindrical enclosure52(as opposed to the end well58ofFIG. 10A) to make the microwave resonator assembly50suitable for inclusion in a planar-mounted microwave filter. The configuration of microwave resonator assembly50shown inFIGS. 10A and 10Bis similar in some respects to that shown inFIGS. 1A and 1B. For the sake of clarity, discussion of like or analogous elements may be somewhat abbreviated while differences may be emphasized.

A cavity60is again defined by a cylindrical enclosure52formed out of sidewall54extending between opposing end walls56and58. Input port66couples electromagnetic energy radiated by external waveguide section68into the cavity60, inside which a first TE22Nmode70having a first polarization (defined by characterizing vector74) is excited. At least one discontinuity is formed within the cavity60, for example using coupling screws78or79(not shown inFIG. 10A to 10B), to couple the first TE22Nmode70with a second TE22Nmode72having a second field polarization orthogonal to that of the first TE22Nmode70. Tuning screw82is used to make small adjustments to the resonant frequency of the first TE22Nmode70; tuning screw80serves the same function for the second TE22Nmode72.

However, rather than forming coupling elements in the end wall58for radiating electromagnetic energy out of the cavity60(e.g., into an adjacent cavity for realizing a multi-cavity microwave filter), coupling elements are instead formed in the sidewall54. As illustrated inFIGS. 10A and 10B, when located at angular positions within the cavity60equal to an integer multiple of 90-degrees in relation to the second characterizing vector76, longitudinal iris83couples the first TE22Nmode70predominantly while coupling the second TE22Nmode72to a comparatively less degree. In this respect, the longitudinal iris is similar to the radial iris84(FIG. 5A). Once the input port66fixes the polarization of the first TE22Nmode70, any of four equivalent locations in the sidewall54, spaced 90-degrees apart from each other, can be used to radiate the first TE22Nmode70out of the cavity60using the longitudinal iris83.

Transverse angular iris85is shown inFIGS. 10A and 10Bformed in the side wall54in close proximity to, and at the same angular position as, the longitudinal iris83. At that angular position within the cavity60, transverse angular iris85couples the second TE22Nmode72and couples the first TE22Nmode70. The degree of coupling of the second TE22Nmode72by the transverse angular iris85is greater than the coupling of the first TE22Nmode72. But again due to the 90-degree radial symmetry of the cavity90, the angular position of the transverse angular iris85is not fixed and can equal any integer multiple of 90-degrees in relation to the second characterizing vector76. In this regard, the transverse angular iris85is similar to the transverse angular iris90(FIG. 5B). While it is not strictly necessary for the longitudinal iris83to have the same angular position as the transverse angular iris85within the cavity60, locating these two coupling elements at the same angular position (as will be seen) can facilitate design of a two-cavity, planar mounted microwave filter. Of course, if three or more cavities are included in the microwave filter, then other relative angular positions for the longitudinal iris83and transverse angular iris85may be apparent.

Referring now toFIGS. 11A and 11B, in yet another alternate configuration of the microwave resonator assembly50, the transverse angular iris85shown inFIGS. 10A and 10Bcan be replaced with a second longitudinal iris87located at a 45-degree angular offset, in relation to the longitudinal iris83as illustrated inFIGS. 11A and 11B, plus in some cases an integer multiple of 90 degrees. Accordingly, similar to the radial iris86(FIG. 5B), the longitudinal iris87can be located within the cavity60at an angular position equal to an integer multiple of 90-degrees in relation to the first characterizing vector74. Any of the four locations within the cavity60satisfying this relationship will provide good coupling of the second TE22Nmode72. Although as will be seen, preserving a 45-degree angular between the longitudinal irises83and87can facilitate design of a two-cavity, planar mounted microwave filter.

Referring now toFIGS. 12A and 12B, there is illustrated a microwave resonator filter200in perspective and top views. The microwave resonator filter200is realized using the microwave resonator assembly50, shown inFIGS. 10A and 10B, which through inclusion of sidewall coupling elements is suitable for constructing a planar-mounted, microwave filter. Again by operating in the dual TE22Nmode, the microwave resonator filter200realizes 2 poles in each of two adjacent cavities for an overall 4-pole bandpass characteristic. Of course, additional cavities can be included to realize additional poles in the filter function. A combination of direct and cross-coupling of modes resonating in adjacent cavities makes it possible to realize a variety of different linear filter functions, such as elliptic and Chebyshev filter functions, as well as other functions. Transmission zeros are also realizable through the use of negative mode coupling. For the sake of clarity, discussion of certain aspects shared in common by the two microwave resonator filters100and200may be abbreviated while differences may be highlighted.

A first cavity60ais formed in close lateral proximity to a second cavity60b, so that corresponding adjacent portions of the cylindrical sidewalls54aand54b, as illustrated inFIG. 12A, separate the two cavities60aand60b. In some cases, a small arcuate portion of the cylindrical sidewalls54aand54bcan be shared between the first and second cavities60aand60bto form a common sidewall portion (not shown). However, a small air gap can alternatively be formed between the corresponding adjacent portions of sidewalls54aand54b, provided the inter-cavity separation is relatively short (e.g., to maintain good coupling between the two cavities60aand60b). In this arrangement, the first and second cavities60aand60bhave respective longitudinal axes (not explicitly shown) that are parallel, but non-collinear.

Input port66acoupled to external waveguide section68aexcites a first TE22Nmode70within cavity60ahaving a first polarization defined by the first characterizing vector74. The pair of diametrically opposed coupling screws78aprojecting through the sidewall54acouple the first TE22Nmode70excited in the first cavity60awith a second TE22Nmode72excited in cavity60aand having an orthogonal field polarization relative to the first TE22Nmode70. Tuning screws82aand95aare optionally included to adjust the resonant frequency of the first TE22Nmode70to a selected centre frequency of the microwave resonator filter200. Likewise tuning screws80aand98aare optionally included adjust the resonant frequency of the second TE22Nmode72also to the selected centre frequency.

As shown inFIGS. 12A and 12B, transverse angular iris85couples the second TE22Nmode72excited in the first cavity60awith a mutually aligned third TE22Nmode72excited in the second cavity60b. Simultaneously, longitudinal iris83couples the first TE22Nmode70excited in the first cavity60awith a mutually aligned fourth TE22Nmode70excited in the second cavity60b. Coupling screw79bthen couples together the third TE22Nmode72and fourth TE22Nmode70excited in the second cavity60b, and output port66bis used to radiate electromagnetic energy out of the second cavity60bby coupling the fourth TE22Nmode70with the external waveguide section68b. Tuning screws80band98bare optionally included to adjust the resonant frequency of the third TE22Nmode72to the selected centre frequency of the microwave resonator filter200, as are tuning screws82band95bfor the same purpose in relation to the fourth TE22Nmode70. Screws97aand97bare optionally included to improve the spurious free range of the microwave resonator filter200.

The respective dimensions and axial positioning of the longitudinal iris83and the transverse angular iris85are optimizable to adjust the coupling provided by each iris as specified in the coupling matrix M. For example, the longitudinal axis83can be located at or near a maximum in the axial field pattern of the TE22Nmode (i.e., at an odd multiple of quarter-wavelengths in the axial direction) to provide strong coupling of the first and fourth TE22Nmodes70, but also at other axial positions depending on the application. The transverse angular iris85can then be located vertically adjacent the longitudinal axis83in space remaining in the sidewall54. As shown inFIG. 12B, the transverse angular iris85abuts the end wall58, but other locations are possible as well.

The respective couplings of the longitudinal iris83and transverse angular iris85, as illustrated inFIGS. 12A and 12B, are related to their angular position within the cavity60a(or equivalently within the cavity60b). Referring now toFIGS. 13A and 13B, for example, by undergoing a 45-degree translation relative to the configuration seen inFIGS. 12A and 12B, the longitudinal iris87now couples the second and third TE22Nmodes72, while the transverse angular iris89couples the first and fourth TE22Nmodes70. Intermediate angles between these two extremes are possible as well, in which case the inter-cavity coupling elements would be offset an integer multiple of 90-degrees from some intermediate vectors between the first or second characterizing vectors74and76. At this intermediate angle, each of the longitudinal iris87and the transverse angular iris89would provide some non-negligible coupling of the first and fourth TE22Nmodes70, as well as some non-negligible coupling of the second and third TE22Nmodes72. It should be understood, however, that the angle between the longitudinal iris87and the transverse angular iris89can remain 45-degrees. Depending on the particular application, any offset angle in relation to the characterizing vectors74and76may be prescribed. Accordingly, the relative spacing and angular positions of these coupling elements are optimizable to realize different filter functions in the microwave resonator filter200. Tuning screw95ais optionally included to adjust the resonant frequency of the first TE22Nmode70to a selected centre frequency of the microwave resonator filter200. Likewise, tuning screw98ais optionally included to adjust the resonant frequency of the second TE22Nmode to the selected center frequency. Tuning screw98bis optionally included to adjust the resonant frequency of the third TE22Nmode72to the selected centre frequency of the microwave resonator filter200, and tuning screw95bis optionally included for the same purpose in relation to the fourth TE22Nmode70. Screws97aand97bare optionally included to improve the spurious free range of the microwave resonator filter200.

Referring now toFIGS. 14A and 14B, in an alternative configuration of the microwave resonator filter200, a pair of longitudinal irises83aand87ais used to couple the first and second cavities60aand60b. The resonant modes coupled by each longitudinal iris83aor87a(as well as the relative strengths of these couplings) are related to the angular position of the respective coupling element within the cavities60aand60b. The longitudinal iris83a, being diametrically opposed to the input port66a(and hence an integer multiple of 90-degrees offset from the second characterizing vector76), predominantly but not exclusively couples the first and fourth TE22Nmodes70. Likewise the longitudinal axis87a, being 45-degrees offset from the longitudinal axis83a(and hence an integer multiple of 90-degrees offset from the first characterizing vector74), predominantly but not exclusively couples the second and third TE22Nmodes72. Tuning screw98ais optionally included to adjust the resonant frequency of the second TE22Nmode to the selected center frequency of the microwave resonator filter200, and tuning screw98bis optionally included for the same purpose but to adjust the resonant frequency of the third TE22Nmode72to the selected centre frequency. Screws97aand97bare optionally included to improve the spurious free range of the microwave resonator filter200.

Although not explicitly illustrated, the relative couplings provided by the longitudinal irises83and87would be opposite to that provided by the exemplary configuration shown inFIGS. 14A and 14B. If the longitudinal iris87were instead to be located diametrically opposed to the input port66a, then it would be the longitudinal iris87coupling the first and fourth TE22Nmodes70and the longitudinal iris83coupling the second and third TE22Nmodes72. Again the longitudinal irises83and87can be formed at angular positions equal to an integer multiple of 90-degrees offset from some intermediate vectors between the first and second characterizing vectors74and76, thereby adjusting the relative couplings of each orthogonal mode to suit the application.

Some combinations of the longitudinal iris83with the longitudinal iris87will also realize transmission zeros in the filter characteristic of the microwave resonator filter200. The polarity of the coupling provided by the longitudinal irises83and87can depend on the size of the iris in relation to the free-space wavelength of the resonance modes being coupled together. For example, if the major dimension (i.e., axial length) of the longitudinal iris83or87is less than one half of the free-space wavelength, the resulting coupling will have a certain polarity. But coupling of the opposite polarity will result if the major dimension of the longitudinal iris83or87is greater than one half of the free-space wavelength. By sizing the axial lengths of the longitudinal irises83and87in relation to one half-wavelength, the couplings provided by each respective iris83and87can be made to have opposite polarities and relative magnitudes, as specified by the M matrix, such that transmission zeros are created. For example, the length of one longitudinal iris (e.g.,83) can be less than one half-wavelength, while the length of the other longitudinal iris (e.g.,87) can be larger than one half-wavelength. By adjusting the relative dimensions of the two longitudinal irises83and87, depending on the application, to provide the specified couplings, the transmission zeros can be realized.

In an alternative configuration of the resonator assembly50not explicitly illustrated, the longitudinal irises83and87can be sized to be both smaller or both larger than one half of the free-space wavelength. In either case, both smaller or both larger, the relative couplings provided by the longitudinal irises83and87will have the same polarity, positive or negative. It is not necessary for the longitudinal irises to have the same axial length and can be sized differently, depending on the particular application, to provide different relative couplings. In these configurations, transmission zeros can be created in the microwave filter200instead by the relative angular positions of the coupling screws78and79placed in each cavity60aand60b, as described above with reference toFIGS. 4A and 4B.

Referring now toFIGS. 15A and 15B, in an alternative configuration of the microwave resonator filter200, a single longitudinal iris83is used to provide resonant mode coupling between the first and second cavities60aand60b. Coupling screw91aplaced in cavity60aprovides coupling between the first TE22Nmode70and second TE22Nmode72resonating therewithin. Similarly coupling screw91bplaced in cavity60bprovides coupling between the third TE22Nmode72and fourth TE22Nmode70. The coupling screws91aand91bproject through cavity end walls (as opposed to a side wall) at angular positions located substantially intermediate the characterizing vectors74and76, where the TE22Nmodes70and72have non-zero field components. Tuning screw95ais optionally included to adjust the resonant frequency the first TE22Nmode70to a selected center frequency of the microwave resonator filter200. Likewise, tuning screw98ais optionally included to adjust the resonant frequency of the second TE22Nmode to the selected centre frequency. Tuning screw98bis optionally included to adjust the resonant frequency of the third TE22Nmode72to the selected centre frequency of the microwave resonator filter200, and tuning screw95bis optionally included for the same purpose in relation to the fourth TE22Nmode70.

As discussed above, the single longitudinal iris83may provide coupling of the first and fourth TE22Nmodes70simultaneously with coupling of the second and third TE22Nmodes72. However, the relative amounts of each type of mode coupling may generally depend on the angular position of the longitudinal iris83in relation to the characterizing vectors74and76. At the angular position shown explicitly inFIGS. 15A and 15B, the longitudinal iris83(being offset an integer number of 90 degrees from the second characterizing vector76) may predominantly couple the first and fourth TE22Nmodes70. However, some amount of coupling of the second and third TE22Nmodes72excited in the cavities60aand60bwill occur as well.

The sizing and axial positioning of the longitudinal iris83are again two of the free variables through which to control the amount of coupling provided to suit the particular application. However, as there is only the one longitudinal iris83used to couple each pair of mutually aligned TE22Nmodes, the realizable couplings may be somewhat constrained as compared to a filter configuration that utilizes two or more coupling elements. As will be appreciated, the inclusion of additional coupling elements increases the number of free variables, such as relative angular spacing and sizing, which can be optimized in the design process. As a third possible design variable, the angular position of the longitudinal iris83in relation to the characterizing vectors74and76can also be optimized. Thus, although not explicitly shown, the longitudinal iris83can also be translated 45-degrees to be offset an integer number of 90 degrees from the first characterizing vector76. At this alternative angular position, the longitudinal iris83then predominantly couples the second and third TE22Nmodes72. For intermediate couplings, some angular offset between this and the position shown inFIGS. 15A and 15Bcan be selected.

Referring now toFIGS. 16A and 16B, there is illustrated a microwave resonator filter300in perspective and top views. The microwave resonator filter300is formed using a single microwave resonator assembly50and, by operating in the dual TE22Nmode, realizes a 2-pole bandpass characteristic. In the configuration shown, input port66aand output port66bare provided in a single cavity60and lead to external waveguide sections68aand68b, respective. The input port66aexcites the first TE22Nmode70within cavity60and the output port66b, being located 45-degrees offset from the input port66a, is suitable for coupling the second TE22Nmode72. Coupling between the orthogonal TE22Nmodes70and72is provided, for example, using coupling screw91. It should be appreciated however that one or more coupling screws78or79(not shown inFIG. 16Aor16B) could be used alternatively or additionally. Tuning screws95and98are included and used to make small adjustments to the resonant frequencies of the first and second TE22Nmodes70and72, respectively.

Referring now toFIGS. 17A-17D, alternative cavity geometries can be utilized in the resonator assembly50to adjust one or more performance characteristics. Each of the alternative geometries illustrated presents different boundary fields for the TE22Nmode, relative to the cylindrical shape of the cavity60. For example, inFIG. 17A, the cavity160comprises a central cylindrical section161between two inwardly tapered end sections163. The cavity260shown inFIG. 17Bsimilarly comprises a central cylindrical section261, but now includes two outwardly tapered end sections263. Alternatively, as seen inFIG. 17C, the cavity360can comprise central cylindrical361between two puck sections363. Finally, the cavity460shown inFIG. 17Dincludes central cylindrical section461between two end flange sections463.

Two of the performance characteristics that can be varied in the alternative cavity geometries are spurious performance and Q factor. For example, the outwardly tapering end sections263inFIG. 17Band the end flange sections463inFIG. 17D, which each represent an expansion of the corresponding cavity relative to its axial midsection, can offer better spurious performance on the low-frequency side of the passband. On the other hand, the inwardly tapering end sections163inFIG. 17Aand the puck sections363inFIG. 17C, which each represent a narrowing of the corresponding cavity relative to its axial midsection, can offer better spurious performance on the high-frequency side of the passband. The inwardly tapering end sections163and the puck sections363also provide a larger Q factor relative to the cylindrical cavity60.

While the above description provides examples and specific details of various embodiments, it will be appreciated that some of the described features and/or functions admit to modification without departing from the scope of the described embodiments. The detailed description of embodiments presented herein is intended to be illustrative of the invention, the scope of which is limited only by the language of the claims appended hereto.