Tunable electromagnetic cavity resonator

Abstract

A cavity resonator (1) for electromagnetic waves comprises: an electrically conductive wall (2) configured to be electrically connected to a predetermined potential (12); a cavity (6) enclosed by the wall (2) and configured to contain standing electromagnetic waves; an electrically conductive plate (7) attached to a dielectric substrate (8) inside the cavity (6); and a switch (11) electrically connected to the plate (7). The switch (11) has a first state and a second state. The plate (7) is arranged inside the cavity (6) such that, when the cavity resonator (1) is in use, the cavity resonator (1) has a lower fundamental frequency when the switch (11) is in the first state than when the switch (11) is in the second state. The cavity resonator (1) finds use in for example frequency-domain multiplexing applications.

Description

[0001] TUNABLE ELECTROMAGNETIC CAVITY RESONATOR

[0002] TECHNICAL FIELD

[0003] The present invention relates to a cavity resonator for electromagnetic waves and to a method for using a such resonator. The present invention further relates to a multiplexer for frequencydomain multiplexing.

[0004] BACKGROUND

[0005] An electromagnetic cavity resonator has an enclosed cavity in which electromagnetic waves can travel back and forth between the boundary of the cavity so that standing waves appear at certain frequencies which depend on for example the dimensions of the cavity. These standing waves and frequencies are often referred to as resonance modes and resonance frequencies, respectively.

[0006] Electromagnetic cavity resonators have a wide variety of applications, including as components in cavity filters, cavity oscillators and diplexers. Some applications require or benefit from tunable resonators, i.e., resonators the fundamental frequency of which can be adjusted, the fundamental frequency being the lowest resonance frequency of the resonator. For such applications, there exist for example resonators that are electrically tunable. The tuning is then often achieved by means of varactor diodes. They allow for fast tuning, but the tuning range is typically limited and, since varactor diodes are non-linear devices, these types of resonators usually suffer from intermodulation problems even for low signal power levels. Moreover, the Q (quality) values of electrically tunable resonators are often low, making them unsuitable for some applications, such as narrow bandpass filters. The higher the Q value, the narrower the range around the fundamental frequency at which the resonator will resonate. A higher Q value may thus be seen as being associated with a more stable output signal from the cavity resonator. Frequency fluctuations around the fundamental frequency are sometimes referred to as phase noise.

[0007] There also exist resonators that are mechanically tunable, allowing tuning by means of for example a movable cavity wall. Mechanically tunable resonators can have high Q values, but the tuning is usually fairly slow because of the need to move mechanical components. Further, the movable components are typically sensitive to temperature changes and acoustical vibrations, something which influences the performance of the resonator negatively. The movable components are also subject to wear and tear and eventually need to be replaced. Moreover, the motors and linear actuators, which are typically used for moving the movable components, increase energy consumption.

[0008] Thus, while existing tunable electromagnetic cavity resonators perform acceptably for many applications, there nevertheless exists a need for continued innovations efforts aimed at overcoming or alleviating at least some of the above-mentioned shortcomings of the existing technology. In particular, there is a need for tunable electromagnetic cavity resonators that offer an attractive trade-off between high Q values and other desirable properties related to performance and cost. SUMMARY

[0009] To better address one or more of the concerns discussed above, the present inventors have provided the invention as claimed herein. The invention is defined by the independent patent claims. Embodiments of the invention are set forth in the dependent patent claims, in the following description and in the drawings.

[0010] According to a first aspect of the invention, there is presented a cavity resonator for electromagnetic waves. The cavity resonator comprises: an electrically conductive wall configured to be electrically connected to a predetermined potential, the wall having a proximal portion and a distal portion arranged opposite to the proximal portion and at a first distance from the proximal portion; a cavity enclosed by the wall and configured to contain standing electromagnetic waves; at least one dielectric substrate arranged inside the cavity and attached to the proximal portion; and at least one electrically conductive plate attached to the substrate and arranged inside the cavity. The plate is arranged opposite to the distal portion and at a second distance from the distal portion, the second distance being smaller than the first distance. The cavity resonator further comprises at least one switch electrically connected to the plate. The switch has: a first state in which the switch is configured to electrically disconnect the plate from the predetermined potential, whereby the plate is left electrically floating; and a second state in which the switch is configured to electrically connect the plate to the predetermined potential. The cavity resonator has a first fundamental frequency when the switch is in the first state and a second fundamental frequency when the switch is in the second state. The first fundamental frequency is defined by the first distance and the second fundamental frequency is defined by the second distance.

[0011] According to a second aspect of the invention, there is presented a multiplexing device for frequency-domain multiplexing of electrical signals. The multiplexing device comprises: a first port configured to receive a first external signal including a first frequency range; a second port configured to receive a second external signal including a second frequency range, the second frequency range being different from the first frequency range; a common port configured to send a multiplexed signal to an external device; at least one control port configured to receive an external control signal; and at least a first and a second cavity resonator according to the first aspect of the invention. The first frequency range comprises at least one of the first and second fundamental frequencies of the first cavity resonator, and the second frequency range comprises at least one of the first and second fundamental frequencies of the second cavity resonator. The switches of the first and second resonators are electrically connected to the control port and changeable between their respective first and second states by the external control signal. The first cavity resonator is electrically connected to receive the first external signal from the first port and to send an output signal to the common port. The second cavity resonator is electrically connected to receive the second external signal from the second port and to send a further output signal to the common port. The multiplexed signal comprises the output signal and the further output signal.

[0012] According to a third aspect of the invention, there is presented a method of using a cavity resonator according to the first aspect of the invention. The method comprises changing the switch from the first state to the second state.

[0013] By the term potential is meant electric potential. When the plate is electrically floating, the plate is not electrically connected to a known potential and the potential of the plate is therefore unknown. The predetermined potential is on the other hand a known reference potential. The fundamental frequency of the resonator is the lowest resonance frequency of the cavity resonator. That is to say, the fundamental frequency is the frequency of the fundamental mode of the cavity resonator. It may be noted that a resonance frequency may alternatively be referred to as a resonant frequency.

[0014] The first external signal, the second external signal and the external control signal are to be received from one or more external devices, i.e., devices that are external to the multiplexing device. These signals may be received from different devices, or two or all of them may be sent from the same external device. Further, the external device to which the multiplexed signal is sent may, but does not have to, be another device than the one or ones sending the control signal, the first external signal and the second external signal. Still further, the first port, the second port and the common port are typically configured to both receive external electrical signals and send electrical signals.

[0015] In general, arranging components inside the cavity of a resonator tends to lower the Q value substantially. The present invention is based on the surprising realization that it is possible to provide a tunable electromagnetic cavity resonator having a comparatively high Q value by arranging an electrically conducting plate inside the cavity and letting the plate be switchable between being electrically floating and having a predetermined potential that is the same as the potential of the cavity wall. Thereby, the cavity resonator has a first fundamental frequency when the switch is in the first state and a second fundamental frequency when the switch is in the second state, the second fundamental frequency being higher than the first fundamental frequency. Differently stated, the switching of the potential of the plate shifts the fundamental frequency of the cavity resonator between two different values in a stepwise manner. Thus, the inventive cavity resonator enables stepwise frequency tuning.

[0016] The cavity resonator can have a large tuning range and a Q value that is sufficiently high for applications which require filters having sharp edges and oscillators having low phase noise. The tuning can be sufficiently precise for manufacturing tolerances to be relaxed and, consequently, for production costs to be reduced. The cavity resonator can also be manufactured from relatively simple, low-cost and energy-efficient components, and does not require movable parts that are highly susceptible to vibrations and mechanical wear.

[0017] Moreover, since the cavity resonator according to the invention allows for its fundamental frequency to be tuned or, differently put, adjusted, the cavity resonator is versatile and capable of meeting a variety of application needs. For example, a diplexer incorporating the cavity resonator may be able to realize several different duplex distances and, consequently, it may be possible to reduce the number of diplexers needed for each frequency band. This helps to reduce the cost and complexity of the diplexer. As another example, the cavity resonator may enable fast and easy digital tuning of cavity oscillators instead of a more complicated type of tuning, such as an analog phase-locked loop together with frequency control using digital signal processing. As yet another example, it is possible to use the tuning made possible by the cavity resonator for temperature compensation in filters.

[0018] The first fundamental frequency may correspond to a first fundamental resonance mode and the second fundamental frequency may correspond to a second fundamental resonance mode. The first fundamental resonance mode may have a node at the proximal portion and a further node at the distal portion, and the second fundamental resonance mode may have a node at the plate and a further node at the distal portion. By a frequency corresponding to a mode is meant the mode has that frequency. By the term "node" is meant a point where a standing wave has a minimum amplitude. By a node being arranged "at" a location is here meant that the node is located close to that location or, differently put, in the vicinity of that location. It may be noted that a resonance mode may alternatively be referred to as a resonant mode.

[0019] The substrate may be attached to a cavity-facing surface of the wall, and an area of the plate may be less than 15% of an area of the cavity-facing surface, alternatively less than 10%, or less than 5%. The "area of the plate" here refers to the area of the plate that faces the cavity. If the cavity resonator comprises several plates, the "area of the plate" is meant to be understood as the combined area of all of the plates. The cavity-facing surface at which the plate is arranged is usually the cavity-facing surface of the wall to which the substrate is attached. Now, typically, the larger the area of the plate, the greater the impact on the fundamental frequency. However, too large a plate may result in a low Q value. The inventors have realized that using a plate that has an area within the aforementioned ranges may provide an especially advantageous compromise between these competing effects.

[0020] The cavity resonator may comprise several plates configured to be independently connected to and disconnected from the predetermined potential. Thereby, it is possible to provide a cavity resonator that is tunable between a desired number of fundamental frequencies. A total of n plates can be put in 2ndifferent configurations the first and second states. Because of symmetry or some other reason, some of these configurations may yield the same fundamental frequency, but different configurations often yield different fundamental frequencies. The frequency step size can be adjusted by for example changing the size of the plates. Using small plates typically results in a small frequency step size. The frequency step size is also affected by the positions of the plates inside the cavity, because the position of the plate affects how big an impact the plate has on the standing waves inside the cavity. Typically, arranging the plate close to where a fundamental resonance mode has a node helps to increase the impact the plate has on that mode.

[0021] The cavity resonator may comprise several plates arranged on the same substrate and / or several plates arranged on different substrates. These two options may provide advantages related to, for instance, technical performance or manufacturing cost. Depending on various factors, such as the intended application of the cavity resonator and the way in which it is manufactured, one option may be preferable over the other. It should be noted that these alternatives may be combined. That is, the cavity resonator may include substrates with one plate and substrates with several plates. If the cavity resonator has several plates, it may, but does not have to, have one switch for each plate. A switch may be configured to control the potential of one plate or the potentials of several plates.

[0022] The switch, when in the second state, may be configured to short circuit the plate and the wall. Short-circuiting the plate and the wall is a way to make the plate and the wall have the same potential that does not require complicated electric circuitry and thereby helps to simplify the manufacturing process and reduce costs.

[0023] The switch may be electrically controllable to change between the first state and the second state. Such a switch may allow for particularly fast and user-friendly tuning.

[0024] The switch can be a simple, reliable and inexpensive electronic component which does not add unnecessary complexity or significant cost to the resonator. For example, the switch may comprise a diode and / or a transistor. More generally, the switch may be a two-state switch, i.e., a switch having only two states which may be referred to as on and off, for instance.

[0025] The first fundamental frequency and the second fundamental frequency may be in the microwave range of the electromagnetic spectrum. Making the cavity suitable for containing standing microwaves includes letting the cavity have suitable physical dimensions. Microwave frequencies range from 1 gigahertz to 300 gigahertz. The first fundamental frequency and the second fundamental frequency may for example be in the range from 1 gigahertz to 100 gigahertz, alternatively from 1 gigahertz to 50 gigahertz, from 1 gigahertz to 30 gigahertz or from 5 gigahertz to 15 gigahertz. It is noted that the invention is especially suitable for microwave applications and that microwave resonators and microwave multiplexing devices are important for many industrial applications. Such resonators and multiplexing devices are for example interesting to various applications related to communication networks, such as cellular communication networks and in particular 3rd Generation Partnership Project (3GPP) communication networks, including 2G, 3G, 4G, 5G and 6G networks.

[0026] The wall may comprise: a first part having a U-shaped cross section; and a flat second part which is attached to the first part and arranged at an open end of the U-shaped cross section. The proximal portion is located on the second part and the distal portion is located on the first part at a closed end of the U-shaped cross section. The open end and the closed end of the U- shaped cross section may alternatively be referred to as the top and the bottom, respectively, of the U-shaped cross section. Such a wall can be simple to manufacture and is compatible with cavities having a variety of shapes, in particular rectangular and cylindrical cavities which may have especially clearly defined resonance modes and which may therefore be particularly suitable for the present invention.

[0027] The wall may comprise a printed circuit board, and the substrate may be attached to the printed circuit board. The flat second part of the wall may for example be formed by a printed circuit board. Letting a printed circuit board form a part of the wall may help to simplify the manufacturing process. For instance, many semiconductor devices and other electronic components can be mounted on printed circuit boards using a standard Surface Mount Assembly process, allowing the cavity resonator to be manufactured in an easy and cost- effective manner.

[0028] The substrate may be a die, and the die may be flip-chip mounted to the printed circuit board. Thereby, the manufacturing process can be even further simplified. Of course, a die is but one example of a suitable substrate. Suitable substrates usually have relatively low dielectric losses since such losses typically lowers the Q value. Dielectric loss tends to be lower in materials with lower dielectric constants.

[0029] The predetermined potential may be an electrical ground potential. This choice of predetermined potential can be particularly convenient to implement from a manufacturing perspective. As an example, the cavity resonator may comprise a common grounding element, such as a grounded metal object, which provides an electrical ground.

[0030] It is noted that embodiments of the invention relate to all possible combinations of features in the claims.

[0031] BRIEF DESCRIPTION OF THE DRAWINGS

[0032] For exemplifying purposes, various embodiments of the present invention will now be described in more detail with reference to the accompanying drawings, in which:

[0033] Figure 1 shows a perspective view of a cavity resonator according to an embodiment of the invention; Figure 2 shows a cross-sectional view along the line A-B in Figure 1;

[0034] Figure 3 is a flowchart related to the resonator in Figures 1 and 2;

[0035] Figures 4 and 5 show perspective views of a cavity resonator according to another embodiment of the invention;

[0036] Figure 6 shows a perspective view of a cavity resonator according to yet another embodiment of the invention;

[0037] Figure 7 is a table showing numerical results from computer simulations; and

[0038] Figure 8 shows a schematic top view of a multiplexing device according to an embodiment of the invention.

[0039] All of the figures are schematic, not to scale and generally only show parts that are necessary in order to elucidate the embodiments, wherein other parts may be omitted. The dimensions of some of the features may have been exaggerated for the sake of clarity.

[0040] DETAILED DESCRIPTION

[0041] The invention will now be described more fully hereinafter with reference to the accompanying drawings, in which embodiments of the invention are shown. The invention may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art. It should be noted that in the following the term exemplary is to be construed as serving as an example, instance or illustration.

[0042] Figures 1 and 2 illustrate an example of a cavity resonator 1 for electromagnetic waves, henceforth referred to as the resonator 1 for brevity. The resonator 1 allows for electrically controlled frequency tuning and may for example be incorporated in cavity oscillators, a voltage-controlled oscillators, filters, such as bandpass filters, or multiplexing devices for frequency-domain multiplexing. The resonator 1 is in this case a microwave resonator, i.e., the resonance waves of the resonator 1 are microwaves. However, in a different example, the resonator 1 may be adapted to support resonance waves that are outside the microwave region of the electromagnetic spectrum.

[0043] The resonator 1 comprises an electrically conductive wall 2, which will be referred to as the wall 2 for brevity. The wall 2 here comprises two distinct parts. Specifically, the wall 2 comprises a first part 3 and a second part 4 which are attached to each other. Both parts are capable of conducting electricity. As is most clearly seen in Figure 2, the first part 3 has a U-shaped cross section. The first part 3 is in this case made of copper, but other metals, such as iron, and electrically conductive plastics may alternatively be used. The second part 4 is flat and arranged at the open part of the U-shaped cross section of the first part 3. The second part 4 closes the U- shaped cross section. In this case, the second part 4 is a printed circuit board and will therefore in the following be referred to as the PCB 4. The PCB 4 comprises a layer 5 which is electrically conductive. The layer 5 is in this case a copper layer but could in a different example be a layer of another type of metal or a layer of an electrically conductive plastic. The layer 5 is arranged on a side of the PCB 4 that faces the closed end of the U-shaped cross section. It is noted that. in a different example, the wall 2 could be formed by three or more pieces or by two pieces that have other shapes than in this example.

[0044] The resonator 1 further comprises a cavity 6 for containing electromagnetic waves. Specifically, the first part 3 and the second part 4 are attached together so that the wall 2 encloses the cavity 6. The layer 5 faces the cavity 6, thus forming a cavity-facing surface of the wall 2. The wall 2 forms a closed, hollow structure, and the cavity 6 is an air-filled space inside that structure. The cavity 6 may alternatively be referred to as a chamber. The cavity 6 can contain standing electromagnetic waves because the surface of the wall 2 that faces the cavity 6 is electrically conducting substantially everywhere. The cavity 6 here has the shape of a rectangular parallelepiped having a long side of length di and two short sides of equal length h. In Figures 1 and 2, the long side is the height of the cavity 6 and the short sides are the sides of its base. Hence, the cross section of the cavity 6 perpendicular to the long side is a square. Since the resonator 1 in this example is a microwave resonator, the lengths di and h are in the millimetre or centimetre range. The length di of the long side is about twice as long as the length h of the short side. It is noted that, in a different example, the lengths di and h may have other values or the cavity 6 may have a different shape, such as a cylindrical shape or a rectangular parallelepiped with all sides having different lengths. In general, the shape of the cavity 6 is usually such that the resonator 1 has a single dominant resonance mode.

[0045] Moving on, the resonator 1 comprises an electrically conductive plate 7, henceforth referred to as the plate 7 for brevity. The plate 7 is in this case a metal plate. The plate 7 is more specifically made of copper but could in a different example be made of another type of metal or an electrically conductive plastic. The plate 7 is here a flat plate, specifically a square flat plate. The plate 7 could have another shape in a different example, such as a rectangular, triangular, polygonal, circular, oval or irregular shape. The plate 7 has side length l2which in this case in the millimetre range. The thickness ti of the plate 7 is here in the micrometre range, such as in the range from 10 micrometres to 80 micrometres or from 20 micrometres to 50 micrometres. The area of the plate 7 may for example be less than 15% of the area of the cavity-facing surface of the PCB 4, alternatively less than 10%, or less than 5%. The cavity-facing surface of the PCB 4 is here formed by the layer 5.

[0046] The plate 7 is here centrally located on the cavity-facing surface of the PCB 4. The plate 7 is indirectly attached to the PCB 4. More precisely, the plate 7 is attached to a dielectric substrate 8 which is attached to the PCB 4 and which, for brevity, will hereinafter be referred to as the substrate 8. The substrate 8 is arranged on the side of the PCB 4 that faces the cavity 6. Thus, the substrate 8 is arranged on the same side of the PCB 4 as the layer 5. The plate 7 is attached to an end of the substrate 8 which is distal to the PCB 4. The plate 7 is thereby arranged completely inside the cavity 6. Stated differently, the entire plate 7 is inside the cavity 6. In this example, the shape of the substrate 8 is approximately that of a rectangular parallelepiped. The substrate 8 has a thickness t2, so the plate 7 is here arranged at a distance t2from the PCB 4. The thickness t2of the substrate 8 is typically in the range from 100 micrometres to 300 micrometres, such as approximately 200 micrometres. The two other sides of the substrate 8 here have equal length, this length being substantially equal to the side length l2of the plate 7. It is noted that the substrate 8 may have a different shape and / or a different size. For instance, the substrate 8 may be larger or smaller than the plate 7.

[0047] As mentioned above, the substrate 8 is a dielectric substrate and, as such, comprises one or more dielectric materials, for example silicon. In this case, the substrate 8 is a die that is flipchip mounted to the PCB 4. It is noted that this is but one example of a suitable substrate 8 and way of attaching the substrate 8 to the PCB 4. It is preferable, but not necessary, to use a low- loss substrate, i.e., a substrate that exhibits low magnetic and dielectric losses, since such losses reduce the Q value of the cavity resonator. Low-loss substrates include certain types of chiplets.

[0048] Focusing again on the plate 7, it is noted that the plate 7 is arranged between two portions of the wall 2, namely a proximal portion 9, which is proximal to the plate 7, and a distal portion 10 which is distal to the plate 7. The plate 7 is arranged opposite to the distal portion 10. The proximal portion 9 is arranged on the PCB 4 and opposite to the distal portion 10. The distal portion 10 is located on the first part 3, specifically at the closed end of the U-shaped cross section. The distal and proximal portions 9, 10 are in this case flat and substantially parallel with each other. The distal and proximal portions 9, 10 are also substantially parallel with the plate 7. The distal and proximal portions 9, 10 are located a first distance di apart, the first distance here being equal to the long side of the cavity 6. That is, the first distance di is in this case the vertical height of the cavity 6 in Figures 1 and 2. There is a second distance d2between the plate 7 and the distal portion 10. The second distance d2is smaller than the first distance di at least partly because the substrate 8 is located between the plate 7 and the proximal portion 9. As a result, the plate 7 is situated a bit closer to the distal portion 10 than the proximal portion 9. The distance between the plate 7 and the proximal portion 9 is in this example substantially equal to the thickness t2of the substrate 8. It is worth noting that, in this case, the distance di between the proximal and distal portions 9, 10 is a perpendicular distance therebetween, the distance d2between the plate 7 and the distal portion 10 is a perpendicular distance therebetween, and the distance t2between the plate 7 and the proximal portion 9 is a perpendicular distance therebetween.

[0049] The resonator 1 also includes a switch 11 to which the plate 7 is electrically connected. The switch 11 is in this case a diode, such as a p-i-n diode. This is however but one example of a suitable switch 11. A transistor is another type of suitable switch. The switch 11 is integrated with the substrate 8 but could be arranged on the PCB 4 or somewhere else in a different example. The switch 11 has a first state and a second state, so the switch 11 is switchable between two different states. Further, the switch 11 is here electrically controllable. That is to say, switching the switch 11 between the first and second states is done by means of electrical signals. As will be further discussed below, the resonator 1 is configured to have a first fundamental frequency when the switch 11 is in the first state and a second fundamental frequency when the switch 11 is in the second state, the first fundamental frequency being defined by the first distance diand the second fundamental frequency being defined by the second distance d2.

[0050] It is possible to control the potential of the plate 7 using the switch 11. Putting the switch 11 in the first state electrically disconnects the plate 7 from a predetermined potential 12 and results in the plate 7 having a floating potential. Putting the switch 11 in the second state electrically connects the plate 7 to the predetermined potential 12. The plate 7 and the wall 2 then have the same potential because the wall 2 is electrically connected to the predetermined potential 12. The PCB 4 is here electrically connected to the predetermined potential 12 and the first part 3 of the wall 2 is in electrical contact with the PCB 4.

[0051] In this case, the plate 7 and the wall 2 are short-circuited when the switch 11 is in the second state. Further, in this case, the predetermined potential 12 is an electrical ground potential. The electrical ground may for example be provided by a grounded metal object, such as a steel frame or steel chassis forming part of the resonator 1 or a device which includes the resonator 1. It should be noted, however, that the predetermined potential does not have to be a ground potential but could be some other, known reference potential.

[0052] Moving on, the resonator 1 here comprises a terminal 13 configured to introduce electromagnetic waves into the cavity 6 and to transfer electromagnetic energy away from the cavity 6. Thus, the terminal 13 allows for the coupling of an electric signal into and out of the cavity 6 and may be referred to as an input / output terminal. The terminal 13 is configured to be electrically connected to a power source (not shown) that provides the energy required to excite electromagnetic waves inside the cavity 6. The terminal 13 is here arranged on the PCB 4 but may alternatively be arranged elsewhere on the wall 2. Also, it should be noted that instead of having a single terminal 13, which has both input and output functionality, the resonator 1 may in a different example have two separate terminals: an output terminal configured to introduce electromagnetic waves into the cavity 6 and an input terminal configured to transfer electromagnetic energy away from the cavity 6.

[0053] In use, the resonator 1 is connected to receive electrical signals. A first electrical signal having a first range of frequencies enters the cavity 6 via the terminal 13, whereby electrical currents in the wall 2 produce standing electromagnetic waves inside the cavity 6. A second electrical signal leaves the resonator 1 via the terminal 13 to a device connected thereto. The second electrical signal has a second range of frequencies which includes the fundamental frequency of the resonator 1. The second range of frequencies is here a range which is centered at or close to the fundamental frequency and which is narrower than the first range of frequencies. Hence, the resonator 1 may be used as a filter that removes frequencies from a signal.

[0054] The value of the fundamental frequency of the resonator 1 depends on the state of the switch 11. This is because the plate 7 is arranged inside the cavity 6 such that, when the cavity resonator 1 is in use, the fundamental frequency of the resonator 1 is lower when the switch 11 is in the first state than when the switch 11 is in the second state. Differently stated, the resonator 1 has a first fundamental frequency when the switch 11 is in the first state and a second fundamental frequency when the switch 11 is in the second state, the second fundamental frequency being higher than the first fundamental frequency. Hence, the fundamental frequency of the resonator 1 is tunable, or differently put adjustable, by switching the switch 11 between the first and second states.

[0055] More specifically, when the switch 11 is in the first state so that the plate 7 is electrically floating, a first fundamental resonance mode 14, which has a node at the proximal portion 9 and a further node at the oppositely-arranged distal portion 10, is formed inside the cavity 6. The first fundamental resonance mode 14 is defined by a first wavelength that is substantially equal to twice the distance between the proximal and distal portions 9, 10 (the line with longer dashes in Figure 2 schematically illustrates half that wavelength). The frequency of the first fundamental resonance mode 14 is the above-mentioned first fundamental frequency. In a manner of speaking, the electrically floating plate 7 behaves basically as the equipotential surface of a perfect conductor and as if it were substantially invisible to the electromagnetic waves in the cavity 6.

[0056] On the other hand, when the switch 11 is in the second state so that the plate 7 and the wall 2 have the same predetermined potential 12, a second fundamental resonance mode 15, which has a node at the plate 7 and a further node at the oppositely-arranged distal portion 10, is formed inside the cavity 6. The second fundamental resonance mode 15 is defined by a second wavelength which is substantially equal to twice the distance between the plate 7 and the distal portion 10 (the line with the shorter dashes in Figure 2 schematically illustrates half of that wavelength). The second wavelength is shorter than the first wavelength because the distal portion 10 is closer to the plate 7 than to the proximal portion 9. The frequency of the second fundamental resonance mode 15 is the above-mentioned second fundamental frequency. In a manner of speaking, putting the switch 11 in the second state has basically the same effect as if the PCB 4 with the layer 5 had been moved upwards a distance substantially equal to the sum of the thicknesses labelled ti and t2in Figure 2.

[0057] Thus, changing the switch 11 from the first state to the second state raises the fundamental frequency of the resonator 1, here in one step from a lower value, which corresponds to the first wavelength, to another value which is higher and corresponds to the second wavelength. Both frequencies may for example be in the microwave frequency range, such as in the range from about 5 gigahertz to about 15 gigahertz. Furthermore, it is noted that the first and second fundamental resonance modes are TE101 modes in this case, but they may be a different type of modes in a different example.

[0058] Figure 3 is a flowchart showing the steps of: providing SI the resonator 1 by, for example, manufacture or purchase; electrically connecting S2 the resonator 1 to receive and send electrical signals, such as electrical signals transmitted in a cellular communications network; and using the resonator 1 by changing S3 the switch 11 between the first and second states in order to alternate between the first and second fundamental frequencies. It is worth noting that a user of the resonator 1 performs the last step S3. One or both of the preceding steps SI and S2 may, but does not have to, be performed by the user of the resonator 1.

[0059] Figures 4 and 5 illustrate an example of a resonator 20 which is similar to the resonator 1 in Figures 1 and 2. However, the resonator 20 in Figures 4 and 5 comprises a plurality of plates 21, here twelve plates 21. All of the plates 21 are flat squares plates and have the same size. The plates 21 are arranged to form a rectangular array on a single substrate 22. In this case, the resonator 20 includes twelve switches (not shown), each of which is electrically connected to one of the plates 21. Thus, each plate 21 is here electrically connected to one switch and each plate 21 may be switched between having a floating potential and the predetermined potential independently from the other plates 21. Thereby, the resonator 20 allow for 212possible combinations of plates 21 having the floating potential or the predetermined potential. Figures 4 and 5 show two different combinations, the plates having the predetermined potential being indicated by a striped pattern. Many of these combinations will affect the electromagnetic field inside the cavity in different ways and, consequently, may result in different fundamental frequencies. So, the resonator 20 allows for multi-step tuning. The ways in which different combinations affect the electromagnetic field inside the cavity can be determined through for example practical experiments or computer simulations, as will be further discussed below in connection with Figure 7.

[0060] It should be noted that, in a different example, all or several of the plates 21 may be connected to a single switch capable of switching the plates connected thereto independently between having a floating potential or the predetermined potential. It should also be noted that all of the plates 21 do not need to be arranged on the same substrate. For instance, each plate may be arranged on its own separate substrate, or there may be several substrates, some of which support several plates and others only one plate. Further, the number of plates may be smaller or greater than twelve, the plates do not need to be square, and all plates do not need to have the same shape. Nor do the plates need to be arranged as a rectangular array. The plates could alternatively be arranged as an array forming a different shape, such as a square, a rotationally symmetric shape, a meandering shape or a shape similar to a triangle, an oval or a circle. As yet another alternative, the plates could be arranged in an irregular pattern.

[0061] Figure 6 illustrates another example of a resonator 30 which is similar to the resonator 1 illustrated in Figures 1 and 2. However, the resonator 30 in Figure 6 has a cubic cavity and comprises a plurality of plates, here four plates 31, 32, 33, 34. Each plate 31, 32, 33, 34 is arranged on a separate substrate, so there are four substrates. All of the plates 31, 32, 33, 34 are here flat square plates and have the same size, and the plates 31 are arranged next to each other so as to form a larger square.

[0062] Figure 7 is a table showing computer simulation results for an exemplary implementation of the resonator 30 in Figure 6. In these computations, the plates are copper plates with two sides of length 4 millimetres and a thickness of 50 micrometres. The substrates have two sides of length 4 millimetres and a thickness of 250 micrometres. The substrates are made of silicon and have a relative permittivity of 11.9. The cavity is an air-filled cube with sides that are 2 centimetres long. For comparison purposes, it may be noted that, without plates inside, the fundamental frequency of the resonator would have been 10,6015 gigahertz.

[0063] Each row of the table shows the result of one of six simulations. The leftmost column shows the number of grounded plates (N). The other plates are electrically floating. In the fourth row from the bottom, the two grounded plates are along a side of the square formed by the four plates (i.e., the plates labelled 31 and 33 or 32 and 34 in Figure 6), whereas in the third row from the bottom the two grounded plates are along the diagonal (i.e., the plates labelled 31 and 34 or 32 and 33 in Figure 6). The second and third columns from the left show the fundamental frequencies in gigahertz (GHz) and the Q values, respectively. Finally, the rightmost column shows the tuning range in parts per million (ppm). The tuning range is computed using the fundamental frequencies. For example, the tuning range in the fifth and fourth rows from the bottom are computed, respectively, as (105177 - 105083) / 105083 = 895 ppm and (105207 - 105083) / 105083 = 1180 ppm. If smaller plates had been used in the simulations, the frequency step size would have been smaller than in the table.

[0064] The table shows surprisingly high Q factors of 2430 and greater. By comparison, the Q value of a varactor-tuned resonator is typically less than about 600. The table also shows a large total tuning range of 2826 ppm. Such a tuning range is about one order of magnitude greater than the tuning range of a typical voltage-controlled crystal (VCXO) oscillator.

[0065] With reference to Figure 8, an example of a multiplexing device 40 for frequency-domain multiplexing of an electrical signal will now be described. In this example the multiplexing device 40 is a diplexer and will therefore be referred to as such in what follows. Triplexers and quadplexers are other examples of multiplexing devices in which the inventive resonator disclosed herein may advantageously be incorporated.

[0066] The diplexer 40 here includes a housing 41 which for example can be made of one or more metals, plastic materials or combinations thereof. The diplexer 40 also includes a first port 42, a second port 43, a common port 44 and a control port 45. The ports 42, 43, 44, 45 are in this case integrated with the housing 41, so the housing 40 comprises the ports 42, 43, 44, 45. Each of the ports 42, 43, 44, 45 is configured to allow electric signals to pass therethrough and to electrically connect the diplexer 40 to external devices, such as antennas, transmitters, receivers, transceivers, etc. Thereby, electrical communication between the diplexer 40 and external devices via the ports 42, 43, 44, 45 is possible. It should be noted that the ports 42, 43, 44, 45 do not have to be integrated with the housing 41 and could be arranged elsewhere on the diplexer 40 in a different example. Further, although the ports 42, 43, 44, 45 are separate ports in the example illustrated in Figure 8, some or all of them may be integrated with each other in another example. The diplexer 40 may also have more than one control port 45.

[0067] The diplexer 40 further includes a first resonator 1 and a second resonator 1', which are located inside the housing 41. The resonators 1, 1' are of the type described in connection with Figures 1 and 2 above, although in a different example the resonators 1, 1' could be of another type. For instance, the resonators 20, 30 described in connection with Figures 3 to 6 are also suitable for being incorporated in diplexers as well as other types of multiplexing devices. The resonators 1, 1' have different operational frequencies, which in this case means that the first and second fundamental frequencies of the first resonator 1 are lower than the first and second fundamental frequencies of the second resonator 1'. In a different example, all of the possible fundamental frequencies of the first resonator 1 do not have to be lower than the lowest fundamental frequency of the second resonator 1'.

[0068] The terminal 13 of the first resonator 1 is electrically connected to the first port 42 and to the common port 44. The terminal 13' of the second resonator 1' is electrically connected to the second port 43 and to the common port 44. The control port 45 is electrically connected to the switches 11, 11' of the resonators 1, 1'. The resonators 1, 1' are electrically connected to the ports 42, 43, 44, 45 via internal connections 46 located inside the housing 41. The internal connections 46 can be of various types, for example wires, cables or electrically conductive tracks. A control device (not shown) configured to electrically control the switches 11, 11' can be electrically connected to the control port 45.

[0069] It is worth noting that the resonators 1, 1' are here directly electrically connected to the ports 42, 43, 44, 45 but may in a different example be indirectly connected to one or more thereof, i.e., there may be other components connected between the resonators 1, 1' and the ports 42, 43, 44, 45. Further, the diplexer 40 may comprise two control ports electrically connected to a respective one of the switches 11, 11'. Stated differently, the diplexer 40 may have one control port for each resonator 1, 1'.

[0070] Moving on, the diplexer 40 is coupled to an antenna 47, which is an example of an external device. The antenna 47 is electrically connected to the common port 44, here via an exterior connection 48 located outside the housing 41 of the diplexer 40. The exterior connection 48 can for example be a wire, a cable or an electrically conductive track. The antenna 47 may be an antenna for a cellular communication network, such as a 2G, 3G, 4G, 5G or 6G network. For instance, the antenna 47 may be a base station antenna configured to connect wirelessly with mobile phones, laptops and other types of user equipment, thereby providing users access to the network. Alternatively, the antenna 47 may be included in the so-called transport infrastructure of a cellular communication network. For instance, the antenna 47 may be integrated in a microwave point-to-point link for a wireless fronthaul network or a wireless backhaul network. Examples of such links are the ones sold under the name "MINI-LINK" by Ericsson AB.

[0071] It should be noted that even though the diplexer 40 and the antenna 47 are illustrated as separate units in Figure 8, they may form a single unit in a different example. That is, the diplexer 40 may be integrated with the antenna 47. Further, the diplexer 40 and the antenna 47 may in a different example be indirectly electrically connected instead of directly electrically connected as in Figure 8. Stated differently, the diplexer 40 may be electrically connected to the antenna 47 via one or more other components.

[0072] An exemplary usage scenario, in which the diplexer 40 multiplexes two electrical signals, is as follows. The diplexer 40 receives a first external signal and a second external signal at the first port 42 and the second port 43, respectively. These signals may be sent from two different external devices or from the same external device. For example, the first port 42 may be connected to a radio transmitter and the second port 43 may be connected to a radio receiver, or vice versa. As another example, the first and second ports 42, 43 may be connected to the same radio transceiver.

[0073] The first external signal has a first frequency range, which may alternatively be referred to as a first frequency band. The second external signal has a second frequency range which is different from, i.e., higher or lower than, the first frequency range. In this case the first frequency range is lower than the second frequency range, so the frequency ranges will henceforth be referred to as the low frequency range and the high frequency range, respectively, in what follows. The low and high frequency ranges are typically completely disjoint. The low frequency range includes the first and second fundamental frequencies of the first resonator 1, and the high frequency range includes the first and second fundamental frequencies of the second resonator 1'. Hence, in this case, the low frequency range includes all of the fundamental frequencies which the first resonator 1 is configurable to have (by changing its switch 11), and the high frequency range includes all of the fundamental frequencies which the second resonator 1' is configurable to have (by changing its switch 11').

[0074] The first external signal enters the diplexer 40 through the first port 42 and continues to the terminal 13 of the first resonator 1. Similarly, the second external signal enters the diplexer 40 through the second port 43 and continues to the terminal 13' of the second resonator 1'. The resonators 1, 1' then process the received signals. An output signal leaves the first resonator 1 through its terminal 13, and a further output signal leaves the second resonator 1' through its terminal 13'. The output signal and the further output signal continue to the common port 44, where they are combined into a multiplexed signal which leaves the diplexer 40 for the antenna 47.

[0075] The multiplexed signal includes a narrow range of frequencies around, depending on the state of its switch 11, either the first or second fundamental frequency of the first resonator 1. The multiplexed signal also includes a narrow range of frequencies around, depending on the state of its switch 11', either the first or the second fundamental frequency of the second resonator 1'. Hence, by switching the switches 11, 11' using a control device coupled to the control port 45, different multiplexed signals can be generated by the diplexer 40.

[0076] Now, the diplexer 40 can also be used for demultiplexing, i.e., the reverse of the multiplexing process described above. An exemplary usage scenario, in which the diplexer 40 is used for demultiplexing electrical signals, is described below.

[0077] The diplexer 40 receives a signal from the antenna 47, the signal including two separate frequency ranges: a low frequency range that includes the first and second fundamental frequencies of the first resonator 1 and a high frequency range that includes the first and second fundamental frequencies of the second resonator 1'. The signal enters the diplexer 40 through the common port 44. The signal is split and continues to both resonators 1, 1' which receive the signals through their respective terminals 13, 13'. The resonators 1, 1' then process the signals. A processed signal leaves the first resonator 1 through its terminal 13, and another processed signal leaves the second resonator 1' through its terminal 13'. The two processed signals then leave the diplexer 40 through the second port 42 and the third port 43, respectively, and continue to the devices connected thereto. The processed signals leaving the diplexer 40 are demultiplexed signals and have different frequency characteristics than the signal received by the diplexer 40 from the antenna 47.

[0078] A brief description of the processing performed by the resonators 1, 1' is as follows. The first resonator 1 and the second resonator 1' remove high-range frequencies and low-range frequencies, respectively, so that the processed signals leaving the resonators 1, 1' include only a narrow range of frequencies around either, depending on the state of the switches 11, 11', the first or the second fundamental frequency of the respective resonator. Hence, the diplexer 40 here processes a multi-frequency input signal by separating, based on frequency, this signal into different output signals. These output signals may be referred to as demultiplexed signals. Since it is possible to tune the resonators 1,1' by electrically controlling the switches 11, 11', the diplexer 40 is capable of processing a variety of multi-frequency signals from the antenna 47 into different demultiplexed output signals.

[0079] The person skilled in the art realizes that the present invention by no means is limited to the embodiments described above. On the contrary, many modifications and variations are possible within the scope of the appended claims. For example, in addition to or instead of silicon, the substrate may include other low-loss dielectric materials.

[0080] In the claims, any reference signs placed between parentheses shall not be construed as limiting to the claim. The word "comprising" does not exclude the presence of other elements or steps than those listed in the claim. The word "a" or "an" preceding an element does not exclude the presence of a plurality of such elements.

Claims

CLAIMS1. A cavity resonator (1; 20; 30) for electromagnetic waves, comprising: an electrically conductive wall (2) configured to be electrically connected to a predetermined potential (12), the wall (2) having a proximal portion (9) and a distal portion (10) arranged opposite to the proximal portion (9) and at a first distance (dj from the proximal portion (9); a cavity (6) enclosed by the wall (2) and configured to contain standing electromagnetic waves; at least one dielectric substrate (8; 22) arranged inside the cavity (6) and attached to the proximal portion (9); at least one electrically conductive plate (7; 21; 31, 32, 33, 34) attached to the substrate (8; 22) and arranged inside the cavity (6), the plate (7; 21; 31, 32, 33, 34) being arranged opposite to the distal portion (10) and at a second distance (d2) from the distal portion (10), the second distance (d2) being smaller than the first distance (di); and at least one switch (11) electrically connected to the plate (7; 21; 31, 32, 33, 34), the switch (11) having: a first state in which the switch (11) is configured to electrically disconnect the plate (7; 21; 31, 32, 33, 34) from the predetermined potential (12), whereby the plate (7; 21; 31, 32, 33, 34) is left electrically floating; and a second state in which the switch (11) is configured to electrically connect the plate (7; 21; 31, 32, 33, 34) to the predetermined potential (12), wherein the cavity resonator (1; 20; 30) has a first fundamental frequency when the switch (11) is in the first state and a second fundamental frequency when the switch (11) is in the second state, and wherein the first fundamental frequency is defined by the first distance (di) and the second fundamental frequency is defined by the second distance (d2).

2. The cavity resonator (1; 20; 30) according to claim 1, wherein the first fundamental frequency corresponds to a first fundamental resonance mode (14) and the second fundamental frequency corresponds to a second fundamental resonance mode (15), wherein the first fundamental resonance mode (14) has a node at the proximal portion (9) and a further node at the distal portion (10), and wherein the second fundamental resonance mode (15) has a node at the plate (7) and a further node at the distal portion (10).

3. The cavity resonator (1; 20; 30) according to claim 1 or 2, wherein the substrate (8; 22) is attached to a cavity-facing surface (5) of the wall (2), and wherein an area of the plate (7) is less than 15% of an area of said cavity-facing surface (5), alternatively less than 10%, or less than 5%.

4. The cavity resonator (20; 30) according to any of the preceding claims, comprising several plates (7; 21; 31, 32, 33, 34) configured to be independently connected to and disconnected from the predetermined potential.

5. The cavity resonator (20) according to any of the preceding claims, comprising several plates (21) arranged on the same substrate (22).

6. The cavity resonator (30) according to any of the preceding claims, comprising several plates (31, 32, 33, 34) arranged on different substrates.

7. The cavity resonator (1; 20; 30) according to any of the preceding claims, wherein the switch (11), when in the second state, is configured to short circuit the plate (7; 21; 31, 32, 33, 34) and the wall (2).

8. The cavity resonator (1; 20; 30) according to any of the preceding claims, wherein the switch (11) is electrically controllable to change between the first state and the second state.

9. The cavity resonator (1; 20; 30) according to any of the preceding claims, wherein the switch (11) comprises at least one of a diode and a transistor.

10. The cavity resonator (1; 20; 30) according any of the preceding claims, wherein the first fundamental frequency and the second fundamental frequency are in a microwave range of an electromagnetic spectrum.

11. The cavity resonator (1; 20; 30) according to claim 10, wherein the first fundamental frequency and the second fundamental frequency are in the range from 1 gigahertz to 100 gigahertz, alternatively from 1 gigahertz to 50 gigahertz, from 1 gigahertz to 30 gigahertz, or from 5 gigahertz to 15 gigahertz.

12. The cavity resonator (1; 20; 30) according to any of the preceding claims, wherein the wall (2) comprises: a first part (3) having a U-shaped cross section; and a flat second part (4) attached to the first part (3) and arranged at an open end of the U-shaped cross section, wherein the proximal portion (9) is located on the second part (4) and the distal portion (10) is located on the first part (3) at a closed end of the U-shaped cross section.

13. The cavity resonator (1; 20; 30) according to any of the preceding claims, wherein the wall (2) comprises a printed circuit board (4), and wherein the substrate (8; 22) is attached to the printed circuit board (4).

14. The cavity resonator (1; 20; 30) according to claim 13, wherein the substrate (8;22) is a die, and wherein the die is flip-chip mounted to the printed circuit board (4).

15. The cavity resonator (1; 20; 30) according to any of the preceding claims, wherein the predetermined potential (12) is an electrical ground potential.

16. A multiplexing device (40) for frequency-domain multiplexing of electrical signals, comprising: a first port (42) configured to receive a first external signal including a first frequency range; a second port (43) configured to receive a second external signal including a second frequency range, the second frequency range being different from the first frequency range; a common port (44) configured to send a multiplexed signal to an external device (47); at least one control port (45) configured to receive an external control signal; and at least a first and a second cavity resonator (1, 1') according to any of the claims 1 to 15, wherein the first frequency range comprises at least one of the first and second fundamental frequencies of the first cavity resonator (1), wherein the second frequency range comprises at least one of the first and second fundamental frequencies of the second cavity resonator (1'), wherein the switches (11, 11') of the first and second resonators (1, 1') are electrically connected to the control port (45) and changeable between their respective first and second states by the external control signal, wherein the first cavity resonator (1) is electrically connected to receive the first external signal from the first port (42) and to send an output signal to the common port (44), wherein the second cavity resonator (1') is electrically connected to receive the second external signal from the second port (43) and to send a further output signal to the common port (44), and wherein the multiplexed signal comprises said output signal and said further output signal.

17. A method of using a cavity resonator (1; 20; 30) according to any of the claims 1 to 15, comprising changing the switch (11) from the first state to the second state.

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