WO2019106596A1 - High frequency selectivity filter for microwave signals - Google Patents
High frequency selectivity filter for microwave signals Download PDFInfo
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- WO2019106596A1 WO2019106596A1 PCT/IB2018/059458 IB2018059458W WO2019106596A1 WO 2019106596 A1 WO2019106596 A1 WO 2019106596A1 IB 2018059458 W IB2018059458 W IB 2018059458W WO 2019106596 A1 WO2019106596 A1 WO 2019106596A1
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- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01P—WAVEGUIDES; RESONATORS, LINES, OR OTHER DEVICES OF THE WAVEGUIDE TYPE
- H01P1/00—Auxiliary devices
- H01P1/20—Frequency-selective devices, e.g. filters
- H01P1/207—Hollow waveguide filters
- H01P1/208—Cascaded cavities; Cascaded resonators inside a hollow waveguide structure
Abstract
The invention concerns a microwave filter (2), that comprises: a first folded circuit (21) including a plurality of first rectangular waveguide resonators (211,212,213) connected in cascade by means of first line couplings (214,215) in rectangular waveguide technology; and a second folded circuit (22) including a plurality of second rectangular waveguide resonators (221,222,223) connected in cascade by means of second line couplings (224,225) in rectangular waveguide technology. The first and second folded circuits (21,22) are designed for operating with a TE10N resonant mode and are transversally coupled on a coupling plane (PC) that is perpendicular to a transversal plane (PT) crossing all the first and second rectangular waveguide resonators (211,212,213,221,222,223). Each first rectangular waveguide resonator (211,212,213) is separated from a respective second rectangular waveguide resonator (221,222,223) by means of a respective metal or metallized wall lying on the coupling plane (PC). Moreover, each first rectangular waveguide resonator (211,212,213) is transversally coupled to said respective second rectangular waveguide resonator (221,222,223) by means of: a respective positive transversal coupling including a respective single slot (23,25) made through said respective metal/metallized wall and centered with respect to the transversal plane (PT); or a respective negative transversal coupling including a respective pair of slots (24) made through said respective wall and symmetrically spaced apart from the transversal plane (PT) by a predefined distance. For each positive transversal coupling, each of the first (211,213) and second (221,223) rectangular waveguide resonators transversally coupled by means of the respective single slot (23,25) is crossed by the transversal plane (PT) at a respective resonator section where magnetic field component coupled by said respective single slot (23,25) is maximum. For each negative transversal coupling, each of the first (212) and second (222) rectangular waveguide resonators transversally coupled by means of the respective pair of slots (24) is crossed by the transversal plane (PT) at a respective resonator section where magnetic field component coupled by said respective pair of slots (24) is null.
Description
HIGH FREQUENCY SELECTIVITY FILTER FOR MICROWAVE SIGNALS
CROSS-REFERENCE TO RELATED APPLICATIONS
This patent application claims priority from Italian patent application no. 102017000137455 filed on 29/11/2017, the entire disclosure of which is incorporated herein by reference .
TECHNICAL FIELD OF THE INVENTION
The present invention relates, in general, to a microwave filter and, more particularly, to a high frequency selectivity filter for microwave signals, such as those used in uplink and downlink satellite transmissions.
STATE OF THE ART
As is known, in satellite transponders, several uplink channels from a single ground antenna need to be separated (demultiplexed) to allow separate routing and processing before power amplification and downlink transmission.
Additionally, downlink channels need to be efficiently combined (multiplexed) into a high-power composite output microwave signal, which is then fed into a satellite antenna system for downlink transmission.
Multiplexers and demultiplexers currently used in satellite transponders (such as those operating at Ku band (i.e., 12-18 GHz) or at higher frequencies) typically include waveguide filters, which may be coupled in different ways depending on specific requirements. Each waveguide filter is generally dedicated to separation of signal frequencies associated with a respective uplink/downlink channel.
Normally, the bandwidth of a typical transponder channel is a small percentage of the central operating frequency and is in close proximity with the adjacent channels .
Thence, the filters must meet stringent requirements in
terms of frequency selectivity in order to avoid adjacent channel interference. To this end, the filter design must exploit a circuit topology that permits the allocation of transmission zeros in the proximity of the lower and the upper edges of the pass-band.
Typically, modern filter design can be divided into two main steps (in this connection, reference can be made, for example, to R. J. Cameron, "Advanced Filter Synthesis" , IEEE Microwave Magazine, Volume 12, Issue 6, pages 42-61, 6 September 2011) .
In particular, in a first design step, the filter topology and the number of resonators are determined to ensure the compliance with given requirements. The first design step typically includes defining an ideal circuit that comprises ideal resonators coupled by impedance inverters .
A circuit topology which is well suited to provide a highly selective frequency response is the so-called symmetric folded circuit. In this respect, Figure 1 schematically illustrates an example of ideal symmetric folded circuit of order six (in Figure 1 denoted as a whole by 1 and hereinafter referred to as "symmetric folded circuit 1") .
As shown in Figure 1, the symmetric folded circuit 1 includes :
• a first input/output port Pi and three first ideal resonators 11 connected in cascade by means of first line couplings (represented as continuous-line segments) ; and
• a second input/output port P2 and three second ideal resonators 12 connected in cascade by means of second line couplings (again represented as continuous-line segments) .
Each first ideal resonator 11 is transversally coupled to a respective second ideal resonator 12 by means of a respective transversal coupling (in particular, a respective impedance inverter represented in Figure 1 as a
respective dotted-line segment) .
The symmetric folded circuit 1 allows the allocation of four transmission zeros that are symmetrically placed below and above the pass-band.
The impedance inverters are characterized by transversal coupling values/coefficients that can be positive or negative. The frequency response of the symmetric folded circuit 1 is uniquely defined by the values of all the coupling coefficients (i.e., line and transversal coupling coefficients) and by the values of the resonance frequencies associated with all the resonators 11 and 12. With proper values of these design parameters, the symmetric folded circuit 1 can provide the selective frequency response shown in Figure 2 (which is centered at 41 GHz) .
As known to those skilled in the art, ideal symmetric folded circuits of lower/higher orders can be defined by reducing/extending the structure of order six shown in Figure 1.
The second step of the modern filter design includes the definition and the optimization of a waveguide structure such that to approximate the electrical response of the ideal circuit. This second design step is rendered hard by the fact that a waveguide structure is made up of distributed elements, which behave differently than the lumped elements used in an ideal circuit. Nevertheless, with a proper selection of the waveguide structure, the two responses can be very close over a wide frequency range that covers the pass-band of interest.
Nowadays, many solutions are known to the problem of finding a waveguide structure such that to approximate the electrical response of a given resonator circuit. Different solutions may present important differences in terms of manufacturing costs, accuracy of the frequency response with respect to the ideal one, power handling, etc. The best trade-off depends on the application.
The most common filters used in multiplexers/demultiplexers of satellite payloads are those based on dual-mode cavities in circular waveguide (in this connection, reference can be made, for example, to A. E. Atia, A. E. Williams, "Dual-Mode Canonical Waveguide Filters" , IEEE Transactions on Microwave Theory and Techniques, Volume 25, Issue 12, pages 1021-1026, December 1977) . The dual-mode configuration is characterized by a compact size, a high unloaded quality factor, and a high flexibility in the realization of transversal couplings with positive and negative signs.
Anyway, depending on the specific application of interest (and, hence, on the related requirements/constraints), also other solutions have been proposed, such as single-mode filters based on rectangular waveguide cavities, wherein the coupling coefficients with different signs are obtained by means of inductive and capacitive couplings in the form of irises (or windows) .
An example of single-mode filter based on rectangular waveguide cavities is provided in T. Shen, H-T. Hsu, K. A. Zaki, A. E. Atia, "Full-Wave Design of Canonical Waveguide Filters by Optimization," IEEE Transactions on Microwave Theory and Techniques, Volume 51, Issue 2, pages 504-511, February 2003. Being folded in the E plane, the configuration according to this paper is well suited for a realization in clam-shell technology. In fact, it can be manufactured as an assembly of two parts obtained by splitting up the structure along its symmetry plane. This solution makes use of different coupling mechanisms (i.e., capacitive and inductive couplings) in order to realize coupling coefficients with different signs. The windows which implement the transversal couplings have small heights (about 1 mm in the six order filter design centered at 4 GHz reported in this paper) .
Another example of single-mode filter based on rectangular waveguide cavities (in particular, on
rectangular waveguide cavities folded in the H plane) is provided in J. A. Ruiz-Cruz, K. A. Zaki, J. R. Montejo- Garai, J. M. Rebollar, "Rectangular Waveguide Elliptic Filters with Capacitive and Inductive Irises and Integrated Coaxial Excitation" , IEEE MTT-S International Microwave Symposium Digest, 2005) . Also in this case, the different coupling signs are obtained using capacitive and inductive couplings .
Additionally, a further example of filters in rectangular waveguide technology is provided in C. Carceller et al . , "New folded configuration of rectangular waveguide filters with asymmetrical transmission zeros", 44th European Microwave Conference, European Microwave Association, pages 183-186, 6 October 2014, which discloses a filter topology for the implementation of asymmetric responses with transmission zeros in rectangular waveguide technology, wherein said filter topology is based on a compact folded E-plane arrangement where adjacent resonators are capacitively coupled through rectangular slots, and non-adjacent resonators are coupled through inductive windows.
Finally, an example of hybrid folded rectangular waveguide filter is provided in US 2016/240905 Al, which discloses a group of rectangular waveguide resonators including first and second resonators that are arranged so that first lateral walls of the first resonator extend in parallel to second lateral walls of the second resonator. The first lateral walls correspond to broad sides of a first cross section of the first resonator perpendicular to a guide direction of the first resonator. The second lateral walls correspond to broad sides of a second cross section of the second resonator perpendicular to a guide direction of the second resonator. The first and second resonators are further arranged so that one of the first lateral walls at least partially faces one of the second lateral walls, and the first resonator is
electromagnetically coupled to the second resonator through a first aperture in the one of the first lateral walls and a second aperture in the one of the second lateral walls.
With respect to the dual-mode filters in circular waveguide, the single-mode filters in rectangular waveguide may present some advantages in term of a lower manufacturing complexity and a reduced number of mechanical parts. However, as previously explained, in the single-mode solutions based on rectangular waveguide cavities, the coupling coefficients with different signs are obtained using inductive and capacitive couplings in the form of irises/windows. Inductive windows are easy to manufacture and can provide a wide range of couplings. On the other hand, capacitive irises provide really strong couplings with small gaps, since the iris itself is a section of the propagation rectangular waveguide. Therefore, capacitive irises do not represent the best solution for the implementation of small couplings coefficients because the corresponding slot would be difficult to make due to its small size. This problem is exacerbated at high frequencies and for filters characterized by a narrow pass-band, because the amplitudes of the transversal coupling coefficients become smaller as the bandwidth decreases.
OBJECT AND SUMMARY OF THE INVENTION
In view of the foregoing, today in the space/satellite sector there is an increasingly felt need for microwave filters with enhanced frequency selectivity capabilities.
Thence, an object of the present invention is that of providing a microwave filter with enhanced frequency selectivity capabilities and with a more compact structure with respect to those of the currently known solutions.
This and other objects are achieved by the present invention in that it relates to a microwave filter, as defined in the appended claims.
BRIEF DESCRIPTION OF THE DRAWINGS
For a better understanding of the present invention, preferred embodiments, which are intended purely by way of non-limiting examples, will now be described with reference to the attached drawings (all not to scale), where:
• Figure 1 schematically illustrates an ideal symmetric folded circuit of order six;
• Figure 2 shows an example of frequency response of the ideal symmetric folded circuit of order six shown in Figure 1;
• Figure 3 shows a single-mode filter in rectangular waveguide technology according to a preferred, non-limiting embodiment of the present invention;
• Figures 4 and 5 show the magnetic field in two couples of resonator cavities coupled, respectively, through a pair of slots (Figure 4) and through a single slot (Figure 5) in the single-mode filter in rectangular waveguide technology shown in Figure 3;
• Figure 6 shows a simulated frequency response of the single-mode filter in rectangular waveguide technology shown in Figure 3; and
• Figure 7 shows an assembly of three parts forming the single-mode filter of Figure 3 according to a preferred, non-limiting mode of carrying out the present invention .
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION
The following discussion is presented to enable a person skilled in the art to make and use the invention. Various modifications to the embodiments will be readily apparent to those skilled in the art, without departing from the scope of the present invention as claimed. Thence, the present invention is not intended to be limited to the embodiments shown and described, but is to be accorded the widest scope consistent with the principles and features disclosed herein and defined in the appended claims.
As previously explained, in the space/satellite sector there is an increasingly felt need for microwave filters with enhanced frequency selectivity capabilities. In response to said need, the Applicant has conceived an innovative microwave filter that comprises:
• a first folded circuit including a plurality of first rectangular waveguide resonators connected in cascade by means of first line couplings in rectangular waveguide technology; and
• a second folded circuit including a plurality of second rectangular waveguide resonators connected in cascade by means of second line couplings in rectangular waveguide technology.
The first and second folded circuits are designed for operating with a TE10N resonant mode and are transversally coupled on a coupling plane that is perpendicular to a transversal plane crossing all the first and second rectangular waveguide resonators.
Each first rectangular waveguide resonator is:
• separated from a respective second rectangular waveguide resonator by means of a respective metal or metallized wall lying on the coupling plane; and
• transversally coupled to said respective second rectangular waveguide resonator by means of
- a respective positive transversal coupling including a respective single slot made through said respective metal/metallized wall and centred with respect to the transversal plane, or
- a respective negative transversal coupling including a respective pair of slots made through said respective metal/metallized wall and symmetrically spaced apart from the transversal plane by a predefined distance.
For each positive transversal coupling, each of the first and second rectangular waveguide resonators transversally coupled by means of the respective single
slot is crossed by the transversal plane at a respective resonator section where magnetic field component coupled by said respective single slot is maximum.
Instead, for each negative transversal coupling, each of the first and second rectangular waveguide resonators transversally coupled by means of the respective pair of slots is crossed by the transversal plane at a respective resonator section where magnetic field component coupled by said respective pair of slots is null.
Preferably, the first and second folded circuits are symmetrical with respect to a rotation of 180 degrees around a symmetry axis defined by an intersection of the transversal and coupling planes (and, hence, lying on both said transversal and coupling planes) .
The following are preferred features of the microwave filter :
• the first folded circuit is symmetrical with respect to a first plane of symmetry parallel to the coupling plane;
• the second folded circuit is symmetrical with respect to a second plane of symmetry parallel to the first plane of symmetry and to the coupling plane;
• said coupling plane is equidistant from said first and second planes of symmetry;
• each first rectangular waveguide resonator includes a respective first rectangular waveguide resonant cavity;
• each second rectangular waveguide resonator includes a respective second rectangular waveguide resonant cavity;
• the first line couplings include first curved rectangular waveguide lines connecting in cascade the first rectangular waveguide resonant cavities; and
• the second line couplings include second curved rectangular waveguide lines connecting in cascade the second rectangular waveguide resonant cavities.
Conveniently, the first and second curved rectangular
waveguide lines have smaller height than the first and second rectangular waveguide resonant cavities, whereby said first and second curved rectangular waveguide lines have smaller characteristic impedance than said first and second rectangular waveguide resonant cavities.
Preferably, the microwave filter is split up, at the first and second planes of symmetry, into three parts that comprise :
• a first part including a first symmetrical half of the first folded circuit;
• a second part including a second symmetrical half of the first folded circuit and a first symmetrical half of the second folded circuit; and
• a third part including a second symmetrical half of the second folded circuit, wherein said first, second and third parts are designed to be stacked on one another to form the microwave filter.
For a better understanding of the present invention, Figure 3 shows a single-mode filter (denoted as a whole by 2) in rectangular waveguide technology according to a preferred, non-limiting embodiment of the present invention. In particular, the single-mode filter 2 represents a preferred mode for carrying out the ideal symmetric folded circuit 1 of order six (with four transmission zeros allocated in the proximity of the pass- band edges) shown in Figure 1 and previously described.
In particular, the single-mode filter 2 includes:
• a first folded circuit 21 symmetrical with respect to a first plane of symmetry Psi;
• a second folded circuit 22 symmetrical with respect to a second plane of symmetry Ps2 parallel to the first plane of symmetry Psi.
In Figure 3, the first and second folded circuits 21 and 22 are shown spaced apart from each other only for the sake of a better understanding of their structures (conveniently, their S-shaped structures) . Nevertheless,
actually, said first and second folded circuits 21 and 22 are transversally coupled to each other on a coupling plane Pc that is parallel to, and equidistant from, the first and second planes of symmetry P si and P s2 .
The first and second folded circuits 21 and 22 are designed for operating with a TE10N resonant mode.
Moreover, the first folded circuit 21 comprises:
• a first rectangular waveguide resonator (or resonant cavity) 211, that is crossed (orthogonally to first folded circuit's path) by a transversal plane PT at a respective transversal waveguide section where (i.e., at which) the longitudinal magnetic field component is maximum (condition for positive transversal couplings); wherein said transversal plane PT is perpendicular to the first and second planes of symmetry P si and P s2 , and to the coupling plane P c ;
• a second rectangular waveguide resonator (or resonant cavity) 212, that is crossed (orthogonally to the first folded circuit's path) by the transversal plane PT at a respective transversal waveguide section where (i.e., at which) the longitudinal magnetic field component is null (condition for negative transversal couplings); and
• a third rectangular waveguide resonator (or resonant cavity) 213, that is crossed (orthogonally to the first folded circuit's path) by the transversal plane PT at a respective transversal waveguide section where (i.e., at which) the longitudinal magnetic field component is maximum (condition for positive transversal couplings) .
The first, second and third rectangular waveguide resonators 211, 212 and 213 are connected in cascade by means of first line couplings in rectangular waveguide technology, in particular:
• a first curved rectangular waveguide line 214 connecting the first and second rectangular waveguide resonators 211 and 212 (specifically, via a curve of 180 degrees on the first plane of symmetry P si (i.e., in the E
plane) ) ; and
• a second curved rectangular waveguide line 215 connecting the second and third rectangular waveguide resonators 212 and 213 (specifically, via a curve of 180 degrees on the first plane of symmetry Psi (i.e., in the E plane) opposite to that one of the first curved rectangular waveguide line 214) .
The second folded circuit 22 comprises a fourth rectangular waveguide resonator (or resonant cavity) 221, a fifth rectangular waveguide resonator (or resonant cavity) 222 and a sixth rectangular waveguide resonator (or resonant cavity) 223. The fourth rectangular waveguide resonator 221 is crossed (orthogonally to second folded circuit's path) by the transversal plane PT at a respective transversal waveguide section where (i.e., at which) the longitudinal magnetic field component is maximum (condition for positive transversal couplings) . The fifth rectangular waveguide resonator 222 is crossed (orthogonally to the second folded circuit's path) by the transversal plane PT at a respective transversal waveguide section where (i.e., at which) the longitudinal magnetic field component is null (condition for negative transversal couplings) . The sixth rectangular waveguide resonator 223 is crossed (orthogonally to the second folded circuit's path) by the transversal plane PT at a respective transversal waveguide section where (i.e., at which) the longitudinal magnetic field component is maximum (condition for positive transversal couplings) .
The fourth, fifth and sixth rectangular waveguide resonators 221, 222 and 223 are connected in cascade by means of second line couplings in rectangular waveguide technology, in particular:
• a third curved rectangular waveguide line 224 connecting the fourth and fifth rectangular waveguide resonators 221 and 222 (specifically, via a curve of 180 degrees on the second plane of symmetry Ps2 (i.e., in the E
plane) ) ; and
• a fourth curved rectangular waveguide line 225 connecting the fifth and sixth rectangular waveguide resonators 222 and 223 (specifically, via a curve of 180 degrees on the second plane of symmetry Ps2 (i.e., in the E plane) opposite to that one of the third curved rectangular waveguide line 224) .
Conveniently, the first, second, third and fourth rectangular waveguide lines 214, 215, 224 and 225 have a first height that is smaller than a second height of the first, second, third, fourth, fifth and sixth rectangular waveguide resonant cavities 211, 212, 213, 221, 222 and 223, thereby resulting in a first characteristic impedance associated with the rectangular waveguide lines 214, 215, 224 and 225 that is smaller than a second characteristic impedance associated with the rectangular waveguide resonant cavities 211, 212, 213, 221, 222 and 223.
More in general, the values of the line coupling coefficients may be conveniently controlled by properly tuning the characteristic impedances (waveguide heights) associated with the line coupling waveguides with respect to those of the resonant waveguides.
Additionally, the lengths of the line coupling waveguides may be advantageously optimized to minimize the frequency spreading (i.e., the variations in frequency domain) of the line coupling coefficients over the operating frequency band.
Conveniently, the first and second folded circuits 21 and 22 include also, each, a respective rectangular waveguide input/output port 216,226 connected to, respectively, the first/fourth rectangular waveguide resonator 211,221.
The first and second folded circuits 21 and 22 are symmetrical with respect to a rotation of 180 degrees around a symmetry axis As defined by the intersection of the transversal and coupling planes PT and Pc and, hence,
lying on both said transversal and coupling planes PT and Pc.
Said first and second folded circuits 21 and 22 are transversally coupled by means of transversal coupling slots (or apertures) lying on the coupling plane Pc, in particular :
• a first transversal coupling slot 23 for a positive transversal coupling of the first and fourth rectangular waveguide resonators 211 and 221, wherein said first transversal coupling slot 23 lies on the coupling plane Pc at the transversal plane PT (conveniently, is centered with respect to said transversal plane PT) ;
• a pair of second transversal coupling slots 24 symmetrically spaced apart from each other with respect to the transversal plane PT for a negative transversal coupling of the second and fifth rectangular waveguide resonators 212 and 222 (conveniently, said second transversal coupling slots 24 being spaced apart from each other (i.e., slots' center-to-center distance), or from the transversal plane PT, by a distance that is half, or a quarter (considering the distance of the slots' centers from said transversal plane PT) of the wavelength in waveguide at the central frequency (also known as "guide wavelength" and typically denoted by Xg) ) ; and
• a third transversal coupling slot 25 for a positive transversal coupling of the third and sixth rectangular waveguide resonators 213 and 223, wherein said third transversal coupling slot 25 lies on the coupling plane Pc at the transversal plane PT (conveniently, is centered with respect to said transversal plane PT) .
The transversal coupling slots 23, 24 and 25 can be conveniently made in the form of apertures on the metallic wall(s) separating, respectively, the first 211 and fourth 221, the second 212 and fifth 222, and the third 213 and sixth 223 rectangular waveguide resonators.
The alternating signs of the transversal coupling
coefficients are due to the geometrical arrangement of the two folded circuits 21 and 22 and to the special symmetry of the overall structure, which is invariant after a rotation of 180 degrees around the symmetry axis As. This structure makes it possible the realization of different coupling signs using only one kind of iris, which can be advantageously selected for the best manufacturability and for the best agreement of its frequency response with the ideal coupling.
In particular, the aspect ratio of the transversal coupling slots 23, 24 and 25 may be advantageously selected for the best manufacturability and for a minimum frequency spreading of the transversal coupling coefficients.
More in general, the values of the transversal coupling coefficients may be conveniently controlled by properly tuning the size of the transversal coupling slots 23, 24 and 25.
Conveniently, the path lengths of the first and second folded circuits 21 and 22 (i.e., curves's radius and straight waveguide lengths) are such that to achieve alignment of resonators fields with respect to the transversal plane PT.
In particular, the longitudinal magnetic field component of the resonant mode TE10N (i.e., the magnetic field component coupled by the transversal coupling slots 23, 24 and 25) is :
• null on the transversal plane PT for the second and fifth rectangular waveguide resonators 212 and 222 which are subject to a negative transversal coupling; and
• maximum on the transversal plane PT for the first 211 and fourth 221, and the third 213 and sixth 223 rectangular waveguide resonators which are subject to positive transversal couplings.
For a better understanding of this feature, Figures 4 and 5 show examples of magnetic field lines in, respectively,
• the second rectangular waveguide resonator 212 and the fifth rectangular waveguide resonator 222 subject to the negative transversal coupling via the second transversal coupling slots 24, and
• the first/third rectangular waveguide resonator 211/213 and the fourth/sixth rectangular waveguide resonator 221/223 subject to the positive transversal coupling via the first/third transversal coupling slot 23/25.
In particular, as shown in Figure 4, for the negative transversal coupling realized by the two second transversal coupling slots 24, that are spaced apart from each other by a distance D equal to half the guide wavelength Ag (i.e., D=Ag/2) and by D/2 (i.e., Xg/4) from the transversal plane PT, the longitudinal magnetic field component is null on said transversal plane PT.
More in detail, the coupled magnetic field component (i.e., the longitudinal one) oscillates with a sinusoidal shape along the longitudinal direction (i.e., along the waveguide path represented by z axis in Figure 4) . For being null at the transversal plane PT, the coupled magnetic field component achieves a couple of maximum absolute values, with opposite signs, at the centers of the two second transversal coupling slots 24, which, as previously said, are spaced apart from the transversal plane PT by a quarter of the guide wavelength (i.e., Ag/4) along the longitudinal direction (i.e., along the z axis) .
Instead, as shown in Figure 5, for the positive transversal couplings realized by the first and third transversal coupling slots 23 and 25 centred on the transversal plane PT, the longitudinal magnetic field component is maximum on said transversal plane PT and, hence, at the centers of said first and third transversal coupling slots 23 and 25.
As previously described, the resonant field associated with each rectangular waveguide resonant cavity
211,212,213,221,222,223 is the TE10N mode. As broadly known to those skilled in the art, the first index of "TE10N" (i.e., 1) is related to a direction (y axis in Figures 3-5) that is orthogonal to the coupling plane Pc and is associated with waveguide width. The second index (i.e., 0) is related to a direction (x axis in Figures 3-5) that is parallel to the coupling plane Pc and is associated with waveguide height. The last index (i.e., N) is related to the longitudinal direction (z axis in Figures 3-5) parallel to (i.e. aligned with) waveguide path (obviously, the three axes x, y and z being orthogonal to each other) and corresponds to the number of magnetic field loops enclosed in each rectangular waveguide resonant cavity. The value of the last index can be selected to adjust the lengths of the rectangular waveguide resonant cavities so that it is possible to meet the geometrical constraints imposed by the waveguide structure. Preferred values for the last index are N=3 or N=4.
As previously explained, the first, second and third transversal coupling slots 23, 24 and 25 located on the coupling plane Pc establish a coupling between the longitudinal magnetic field components of the TE10N modes associated with adjacent resonators. This is true for the single-mode filter 2 in rectangular waveguide technology according to the preferred, non-limiting embodiment of the present invention shown in Figure 3. In fact, in waveguide technology, the longitudinal component is the only magnetic field component that is non-null on the coupling plane Pc and, hence, is the magnetic field component coupled by the transversal coupling slots 23, 24 and 25.
Anyway, as it will be described hereinafter, the present invention might be conveniently carried out also with other technologies based on TEM mode (such as stripline or coaxial technology) . In this case, the transversal coupling slots would couple the transversal magnetic field component, which would be the only non-null
magnetic field component on the coupling plane Pc. Therefore, in this case, the transversal magnetic field component would be:
• maximum on the transversal plane PT for positive transversal couplings; and
• null on the transversal plane PT for negative transversal couplings.
Therefore, in view of the foregoing, according to a more general embodiment of the present invention, the magnetic field component coupled by the transversal coupling slots (e.g., the longitudinal one for TElON-based solutions, or the transversal one for TEM-base solutions) is :
• maximum on the transversal plane PT for positive transversal couplings; and
• null on the transversal plane PT for negative transversal couplings.
The electrical response of the single-mode filter 2 is in a very good agreement with the ideal response over a wide frequency region, as it is demonstrated by a comparison between a simulated frequency response of the single-mode filter 2 shown in Figure 6 and the corresponding ideal one shown in Figure 2.
Thanks to the geometrical orientation of the rectangular waveguides (which are laid out in the respective E planes), the configuration of the single-mode filter 2 is compatible with a clam-shell-like (or, equivalently, sandwich-like) realization (conveniently, by using a manufacturing process based on milling machines), wherein the single mode-filter 2 is split up into three parts at the first and second planes of symmetry Psi and PS2.
For example, as shown in Figure 7, the single mode- filter 2 may be conveniently split up into:
• a top part 201 including a first symmetrical half of the first folded circuit 21;
• a middle part 202 including, on opposite sides (respectively, top and bottom sides thereof) , the second symmetrical half of the first folded circuit 21 and a first symmetrical half of the second folded circuit 22; and
• a bottom part 203 including the second symmetrical half of the second folded circuit 22.
This kind of splitting introduces a negligible degradation of the insertion loss because the electric currents associated with the TE10N resonant mode of a rectangular waveguide are null across the first and second planes of symmetry Psi and Ps2.
Actually, the symmetry of the first and second folded circuits 21 and 22 is only approximate because of the presence of the transversal coupling slots 23, 24 and 25. However, said transversal coupling slots 23, 24 and 25 are quite small in comparison with the waveguide size and, in use, generate a small perturbation of the field distribution in proximity of the first and second planes of symmetry Psi and Ps2. The electric currents across said first and second planes of symmetry Psi and Ps2 are, thence, not exactly null (as it would be in case of a perfect symmetry), but are anyway small.
Each of the three parts 201,202,203 is substantially a planar structure and does not present any discontinuity in the normal (out of plane) direction, except for the transversal coupling slots 23, 24 and 25. This kind of structure can be easily manufactured using a milling machine .
The clam-shell (or, equivalently, sandwich-like) realization as an assembly of three parts and the use of milling machines permits a substantial cost reduction with respect to a canonical dual-mode filter configuration, which must be manufactured as an assembly of a higher number of parts (typically, at least one part for each resonant cavity) . This advantage becomes more evident with the increasing of the filter order and of the number of
resonant cavities.
From the foregoing, it is immediately clear to those skilled in the art that the configuration of the single mode filter 2 of order six may be used for making symmetric folded resonator circuits of an any even order M (e.g., M=4,6,8,. ) by simply reducing/extending the structure of order six shown in Figure 3, with the possibility of allocating any even number of transmission zeros lower than M (i.e., the possible numbers of transmission zeros are MTZ=2, 4, ...,M-2) .
Moreover, from the foregoing it is also immediately clear to those skilled in the art that the first and second folded circuits 21 and 22 might be conveniently based on technologies different than the rectangular waveguide one, by maintaining the same symmetry features and the same geometrical features taught by the present invention about the overall filter structure and the transversal couplings.
For example, the first and second folded circuits 21 and 22 might be conveniently based also on square coaxial technology, microstrip technology, stripline technology, etc .
In fact, according to a more general embodiment of the present invention, the first and second folded circuits may be conveniently based on rectangular waveguide technology, or also on a different technology (such as square coaxial, microstrip or stripline technology) , wherein each resonator is :
• separated and electrically insulated from the respective transversally-coupled resonator by means of a respective wall lying on the coupling plane (conveniently, a metal or metallized wall, such as a metal thick wall in case of rectangular-waveguide-based filter, or a thin metallization for a planar multilayer structure, e.g., based on microstrip or stripline technology) ; and
• transversally coupled to said respective resonator by means of
- a respective positive transversal coupling including a respective single slot made through said respective wall at the transversal plane (conveniently, centered with respect to the transversal plane) , or
- a respective negative transversal coupling including a respective pair of slots made through said respective wall in proximity of the transversal plane (conveniently, symmetrically spaced apart from the transversal plane by a predefined distance (preferably, that is quarter of the wavelength in waveguide at the central frequency, i.e., the guide wavelength Xg) ) .
Additionally, it is important to point out that, according to said more general embodiment of the present invention, the first and second folded circuits might have a non-prefect symmetry with respect to a rotation of 180 degrees around the symmetry axis As, or even not have any symmetry with respect to said axis.
From the foregoing, the technical advantages of the present invention are immediately clear to those skilled in the art .
In particular, it is important to point out that the present invention allows to design microwave filters operating at high frequencies with a narrow pass-band, a high frequency selectivity, the allocation of multiple transmission zeros, and a good agreement of filters' electrical response with respect to the ideal one.
In view of the foregoing, the present invention can be advantageously exploited in satellite multiplexers/demultiplexers and, more in general, for the design of microwave filters characterized by a high frequency selectivity.
In particular, the present invention allows to achieve the aforesaid technical advantages by means of single-mode
microwave filters of the symmetric folded resonator circuit type characterized by a simple, symmetry-driven mechanism for the implementation of transversal-couplings with mixed signs .
With specific reference to the previous attempts to design single-mode filters in rectangular waveguide, the present invention provides a new technique for the realization of the transversal couplings with alternating signs. In fact, according to the present invention, different transversal coupling signs are obtained by a exploiting the symmetry of the structure and a special arrangement of the waveguide layouts. It is then possible to use inductive windows for all the transversal couplings allowing an easier manufacturability than previous designs of single-mode filters in rectangular waveguide.
Moreover, the single-mode filter according to the present invention is well suited for a clam-shell-like (or sandwich-like) realization and can be manufactured by means of milling machines in a smaller number of parts and with a cost saving with respect to the canonical dual-mode filter configuration .
In conclusion, it is clear that numerous modifications and variants can be made to the present invention, all falling within the scope of the invention, as defined in the appended claims.
Claims
1. Microwave filter (2) comprising:
• a first folded circuit (21) including a plurality of first rectangular waveguide resonators (211,212,213) connected in cascade by means of first line couplings (214,215) in rectangular waveguide technology; and
• a second folded circuit (22) including a plurality of second rectangular waveguide resonators (221,222,223) connected in cascade by means of second line couplings (224,225) in rectangular waveguide technology;
characterized in that the first and second folded circuits (21,22) are designed for operating with a TE10N resonant mode and are transversally coupled on a coupling plane (Pc) that is perpendicular to a transversal plane (PT) crossing all the first and second rectangular waveguide resonators (211,212,213,221,222,223);
wherein each first rectangular waveguide resonator (211,212,213) is:
• separated from a respective second rectangular waveguide resonator (221,222,223) by means of a respective metal or metallized wall lying on the coupling plane (Pc) ; and
• transversally coupled to said respective second rectangular waveguide resonator (221,222,223) by means of a respective positive transversal coupling including a respective single slot (23,25) made through said respective metal/metallized wall and centred with respect to the transversal plane (Rt) , or
a respective negative transversal coupling including a respective pair of slots (24) made through said respective metal/metallized wall and symmetrically spaced apart from the transversal plane (PT) by a predefined distance;
wherein , for each positive transversal coupling, each
of the first (211,213) and second (221,223) rectangular waveguide resonators transversally coupled by means of the respective single slot (23,25) is crossed by the transversal plane (PT) at a respective resonator section where magnetic field component coupled by said respective single slot (23,25) is maximum;
and wherein, for each negative transversal coupling, each of the first (212) and second (222) rectangular waveguide resonators transversally coupled by means of the respective pair of slots (24) is crossed by the transversal plane (PT) at a respective resonator section where magnetic field component coupled by said respective pair of slots (24 ) is null .
2. The microwave filter of claim 1, wherein the first and second folded circuits (21,22) are symmetrical with respect to a rotation of 180 degrees around a symmetry axis (As ) defined by an intersection of the transversal and coupling planes (PT,PC) .
3. The microwave filter according to claim 1 or 2, wherein :
• the first folded circuit (21) is symmetrical with respect to a first plane of symmetry ( P si ) parallel to the coupling plane ( P c ) ;
• the second folded circuit (22) is symmetrical with respect to a second plane of symmetry ( P s2 ) parallel to the first plane of symmetry ( P si ) and to the coupling plane ( Pc ) ;
• said coupling plane ( Pc ) is equidistant from said first ( P si ) and second ( P s2 ) planes of symmetry;
• each first rectangular waveguide resonator includes a respective rectangular waveguide resonant cavity (211,212,213) ;
• each second rectangular waveguide resonator includes a respective second rectangular waveguide resonant cavity (221,222,223);
• the first line couplings include first curved
rectangular waveguide lines (214,215) connecting in cascade the first rectangular waveguide resonant cavities (211,212,213) ; and
• the second line couplings include second curved rectangular waveguide lines (224,225) connecting in cascade the second rectangular waveguide resonant cavities
(221.222.223) .
4. The microwave filter of claim 3, wherein the first and second curved rectangular waveguide lines (214,215,224,225) have smaller height than the first and second rectangular waveguide resonant cavities
(211.212.213.221.222.223), whereby said first and second curved rectangular waveguide lines (214,215,224,225) have smaller characteristic impedance than said first and second rectangular waveguide resonant cavities
(211.212.213.221.222.223) .
5. The microwave filter according to claim 3 or 4, split up, at the first and second planes of symmetry (Psi,Ps2) , into three parts that comprise:
• a first part (201) including a first symmetrical half of the first folded circuit (21) ;
• a second part (202) including a second symmetrical half of the first folded circuit (21) and a first symmetrical half of the second folded circuit (22) ; and
• a third part (203) including a second symmetrical half of the second folded circuit (22) ;
wherein said first, second and third parts (201,202,203) are designed to be stacked on one another to form the microwave filter (2) .
6. Apparatus designed for use in satellite transmissions and including the microwave filter (2) as claimed in any preceding claim.
7. Satellite including the microwave filter (2) as claimed in any claim 1-5.
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| EP18826107.7A EP3718165B1 (en) | 2017-11-29 | 2018-11-29 | High frequency selectivity filter for microwave signals |
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| IT102017000137455 | 2017-11-29 |
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| CN115051133A (en) * | 2022-07-19 | 2022-09-13 | 北京星英联微波科技有限责任公司 | Waveguide broadside broadband coupling bridge |
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| EP3718165A1 (en) | 2020-10-07 |
| EP3718165B1 (en) | 2022-01-05 |
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