Patent Publication Number: US-2023163439-A1

Title: A tunable waveguide resonator

Description:
TECHNICAL FIELD 
     The present invention relates to a tunable waveguide resonator and a method of frequency tuning for the tunable waveguide resonator, wherein the waveguide resonator comprises a tuning element arranged therein. 
     BACKGROUND 
     In wireless communication networks there are various radio equipment that comprise a least some form of a resonator for example used in filters, oscillators such as Voltage Controlled Oscillators (VCOs), or short haul diplexers and similar. 
     One of the more recent trends calling for special requirements on resonator design, is the millimeter-wave (mmW) domain which is becoming notably popular thus raising the bar for demands on low phase noise for the frequency generation. The phase noise limitations in oscillators are often the bottleneck for more complex modulation in a communication system and for the resolution and range in radar systems. 
     Tunability is also another important factor being considered in design of resonators for mmW applications, with its practical implementation depending on availability of the tunable resonators with a high Q-factor, which means low losses and low phase noise. It is also important that a tunable resonator is reliable and inexpensive to produce. 
     Based on the intended application, a resonator can be built from discrete LC components, dielectric resonators, waveguide cavities or variants of these. One common tuning approach is electrical tuning of the cavities. The tuning element can be a varactor diode, ferroelectric material or some other variable reactance structure. The total Q of a resonator structure depends on the combined resistive losses of the respective components. 
     However, in all existing solutions, the common problem is that as soon as a tuning element is coupled to the waveguide cavity resonator, the losses of the tuning element will lower the Q factor and thereby the phase noise increases. The tighter the coupling between the tuning element and the resonator, the wider bandwidths may be obtained, alongside more losses, which in turn leads to increase in the phase noise. 
     Several other solutions use mechanical tuning approach for tuning waveguide cavities where e.g. one side is moved and typically is connected to the cavity wall by sliding contacts. Such a design results in relatively high insertion losses, meaning that a high Q factor cannot be achieved. 
     In a mechanical tuning approach disclosed in WO 2016/058642, the cavity comprises a tuning device comprising an electrically conducting wall part which is mechanically movable, thus making it possible to adjust a distance within the cavity. A support wall by means of a sliding adjustment arrangement is pushed against the movable wall part and this changes the distance inside the cavity which results in change of frequency. However, in this approach a manual knob is used for mechanical adjustment of the distance which may not result in accurate adjustments. Alternatively, moving the sliding adjustment arrangement in a controlled manner, requires using an electrical motor which may lead to increased production complexity, malfunctioning and higher costs. 
     There is thus a need for a tunable waveguide resonator and an improved tuning of frequencies that delivers a high Q-factor, wide spurious free band and is also compact. 
     SUMMARY 
     It is an object of the present invention to set forth an apparatus and a method for providing improved and more reliable tunable high Q-factor waveguide cavity resonators. This and other objects of the present invention are defined in the appended set of claims. The dependent claims define several embodiments of the present invention. 
     The term exemplary in the present disclosure is to be construed as an example, instance or illustration. 
     According to a first aspect of the present invention there is provided a tunable waveguide resonator comprising a waveguide part having a plurality of walls. One of the plurality of walls at least partly comprises a tuning element, wherein the tuning element has a first main surface, facing toward a first main surface of an inner wall of one other wall of the plurality of walls. The tuning element is caused to, in response to a change in a temperature of the tuning element, be reversibly displaced with respect to a reference plane of the first main surface of the tuning element along an extension perpendicular to the first main surface of the one other inner wall. Whereby, a dimension of a cavity of the tunable waveguide resonator is changed. 
     According to one exemplary embodiment of the present invention, the tuning element may be configured to be displaced when the temperature of the tuning element is increased. Such that a portion of the tuning element may be caused to bend out of the references plane along the extension perpendicular to the first main surface of the one other inner wall. 
     In some embodiments, the tunable waveguide resonator may be configured such that a resonance frequency of the tunable waveguide resonator can be tuned corresponding to a distance by which the dimension of the cavity of the tunable waveguide resonator may be changed upon the tuning element being displaced in response to the change in the temperature of the tuning element. 
     In yet another exemplary embodiment according to the present invention, one of the plurality of the walls may at least partly comprise an opening. Such that the tuning element when mounted on the wall of the waveguide part, may extend along the entire length of the opening whereby sealing the opening. 
     In some embodiments, the tuning element may be mounted on the waveguide part by means of attachment means. In some embodiments, the attachment means may comprise any one of a screw, a glue portion, or a solder pad. In other embodiments, the attachment means may comprise any combination of screws, glue portions, or solder pads or any other attachment and tightening means. 
     In yet another embodiment according to the present invention, the tuning element may comprise a membrane comprising a first sheet of a first metal and a first sheet of a second metal. The first sheet of the first metal may be arranged on a surface of the first sheet of the second metal, wherein the first metal may be different from the second metal. According to another exemplary embodiment of the present invention, the membrane may comprise a bi-metallic membrane, wherein the first sheet of the first metal may have a thermal expansion coefficient which is greater than the thermal expansion coefficient of the first sheet of the second metal. According to one exemplary embodiment, the bi-metallic membrane may be a bi-metallic strip. Where, the first metal in the bi-metallic strip may be brass and the second metal in the bi-metallic strip may be steel. 
     Accordingly, it has been realized by the inventors that it is advantageous to provide the cavity of the tunable waveguide resonator with a tuning element which is in the form of a bi-metallic membrane configured to be displaced and change shape i.e. bend out of its initial shape and position in response to a change in the temperature of the bi-metallic membrane. This way it is possible to tune the frequency of the waveguide resonator in a simple, controllable, accurate and cost-effective manner while maintaining a high Q-factor of the cavity. Furthermore, low phase noise values can also be achieved by such a resonator. 
     According to an embodiment of the present invention, the tuning element may be electrically conducting. The tuning element may be configured such that when an electric current passes through the tuning element, the temperature of the tuning element may be caused to change. 
     In some other exemplary embodiments, a thermo-element may be arranged at a predetermined distance (D) from the reference plane of the tuning element, wherein in response to a change in a temperature of the thermo-element, the temperature of the tuning element may be caused to change. 
     According to some other embodiments of the present invention, the waveguide resonator may further comprise processing circuitry for determining a deviation in a selected working frequency of the waveguide resonator. Where the processing circuitry may be further configured to change the temperature of the tuning element by means of a temperature adjusting means based on the determining and compensate for the deviation by tuning the selected working frequency of the waveguide resonator. 
     According to a second aspect of the present invention, there is provided a method for tuning a frequency of a tunable waveguide resonator comprising a waveguide part having a plurality of walls. One of the plurality of walls at least partly comprises a tuning element. Wherein the tuning element has a first main surface, facing toward a first main surface of an inner wall of one other wall of the plurality of walls. Wherein the method comprises:
         Changing a temperature of the tuning element;   Causing the tuning element to be reversibly displaced along an extension perpendicular to the first main surface of the one other inner wall in response to the change in the temperature of the tuning element;   Causing a dimension of a cavity of the tunable waveguide resonator to change in response to the tuning element being reversibly displaced;   Tuning a frequency of the tunable waveguide resonator by the change in the dimension of the cavity.       

     According to one exemplary embodiment, the method may further comprise:
         Providing a temperature adjusting means for changing the temperature of the tuning element;   Changing the temperature of the tuning element by the temperature adjusting means.       

     According to yet another exemplary embodiment of the present invention, the method may further comprise:
         Determining, by means of a processing circuitry a deviation in a selected working frequency of the waveguide resonator;   Changing the temperature of the tuning element by means of the temperature adjusting means based on said determining;   Compensating for the deviation by tuning the selected working frequency of the waveguide resonator corresponding to the change in the dimension of the cavity.       

     In some embodiments, the tunable element may be electrically conducting and wherein the method may further comprise:
         Tuning the frequency of the tunable waveguide resonator by electrically connecting the tunable element to an electric current source such that an electric current passes through the tuning element, and causing the tuning element to be reversibly displaced in response to the change in the temperature of the tuning element.       

     In some other exemplary embodiments of the present invention, a thermo-element may be arranged at a predetermined distance from the reference plane of the tuning element, wherein the method may further comprise:
         Changing a temperature of the thermo-element;   Causing a change in the temperature of the tuning element in response to the change in the temperature of the thermo-element;   Tuning the frequency of the tunable waveguide resonator by causing the tuning element to be reversibly displaced in response to the change in the temperature of the tuning element.       

    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1    shows a schematic perspective view of a tunable waveguide resonator comprising a waveguide part and a tuning element in accordance with an embodiment of the present invention. 
         FIGS.  2 A-C  Illustrate schematic side view of a cross section A-A of the waveguide part of  FIG.  1    in accordance with some embodiments of the present invention. 
         FIG.  2 D  shows a schematic side view of a cross-sectional cut-out part of the tuning element in accordance with an embodiment of the present invention. 
         FIGS.  3 A-B  show schematic side view of the cross-section A-A of the waveguide part of the tunable waveguide resonator in accordance with some other embodiments of the present invention. 
         FIG.  4    shows a simplified block diagram of a circuit layout comprising the tunable waveguide resonator in accordance with an embodiment of the present invention. 
         FIG.  5    shows a flowchart of some of the methods in accordance with some embodiments of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     Aspects and various embodiments of the present disclosure will now be described more fully hereinafter with reference to the accompanying drawings. The different devices, systems, computer programs and methods disclosed herein can, however, be realized in many different forms and should not be construed as being limited to the aspects and embodiments set forth herein. Like numbers in the drawings refer to like elements throughout. 
     The terminology used herein is for describing aspects of the disclosure only and is not intended to limit the invention. As used herein, the singular forms “a”, “an” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. 
       FIG.  1    shows a schematic perspective view of a waveguide part  100  of a tunable waveguide resonator  10  according to one embodiment of the present invention. The waveguide resonator  10  comprises the waveguide part  100 . The waveguide part  100  of  FIG.  1    has a rectangular shape, with a longitudinal extension L. The rectangular cross-section A-A has a first length d 1  and a second length d 2 . The skilled person however, would readily understand that the waveguide part  100  may have any other appropriate shape or geometry, for example in some embodiments the waveguide part  100  may be cylindrical (not shown). The waveguide part  100  comprises a plurality of walls e.g. a first  101   a , a second  101   b , a third  101   c  and a fourth  101   d  wall , each wall comprising an inner wall e.g. a first  101   a ′, a second  101   b ′, a third  101   c ′, and a fourth  101   d ′ inner wall, also shown in the cross-section A-A in  FIGS.  2 A-C . Each wall also comprises an outer wall e.g. a first  101   a ″, a second  101   b ″, a third  101   c ″, and a fourth  101   d ″ outer wall corresponding to the inner walls  101   a ′,  101   b ′,  101   c ′,  101   d ′. The waveguide resonator  10  further comprises a waveguide cavity  107 , which is the opening formed by arranging the walls the waveguide part  100 . The inner walls  101   a ′,  101   b ′,  101   c ′,  101   d ′ of the waveguide part  100  are electrically conductive. The waveguide resonator  10  may have other ports and openings (not shown) for coupling to other electrical and/or mechanical components in a circuit, such as active circuits such as an MMIC (Monolithic Microwave Integrated Circuit), or amplifiers such as reflection amplifiers, etc. 
     Each inner wall  101   a ′,  101   b ′,  101   c ′,  101   d ′ has a first main surface  104  which faces toward a first main surface  104  of one other inner wall. As an example, inner wall  101   b ′ and  101   d ′ face each other i.e. each of the two inner walls  101   b ′ and  101   d ′ arranged to be substantially parallel to each other, has a first main surface  104  which faces toward the first main surface  104  of the other inner wall. 
     The waveguide resonator  10  further comprises a tuning element  102 . The tuning element  102  in this embodiment is comprised in the waveguide part  100  of the tunable waveguide resonator  10 . In the embodiment of  FIG.  1   , one of the walls, wall  101   a , of the waveguide part  100  at least partly comprises the tuning element  102  mounted thereto. Thus, the tuning element  102  at least partly forms a part/portion of the wall  101   a . The tuning element  102  has a first main surface  103   a , also referred to as the top surface  103   a . The first main surface  103   a  forms a portion of the main surface  104  of the inner wall  101   a ′ which in some embodiments covers the entire main surface  104  of the inner wall  101   a ′. In some embodiments, the portion covers only a part of the first main surface  104  of the inner wall  101   a ′. The area of the first main surface  103   a  thus corresponds to the area of the portion of the main surface  104 . In some embodiments, other walls  101   b ,  101   c ,  101   d  may comprise a tuning element  102  and consequently the first main surface  103   a  forms a portion of the first main surface  104  of the inner walls  101   b ′,  101   c ′ and  101   d′.    
     The first main surface  103   a  of the tuning element  102  comprised in wall  101   a ′ in this embodiment is arranged to face toward the first main surface  104  of one other inner wall e.g. the third inner wall  101   c′.    
     The tuning element  102  comprises a bi-metallic membrane. the bi-metallic membrane  102  is for example a strip of metal made of at least two sheets of different metals. As shown as a matter of example in  FIG.  2 D , in a side cross-sectional view of a cut-out part of the membrane  102 , the bi-metallic membrane  102  is made of a first sheet  102 ′ of a first metal arranged on a surface  102 ″a of a first sheet  102 ″ of a second metal. The two metals have different expansion rates when exposed to temperature changes. The first metal has a higher thermal expansion coefficient compared to the second metal. This way, when heated up from its initial temperature, the bi-metallic membrane  102  will bend in a first direction compared to its initial flat position e.g. a direction perpendicular to a plane of the membrane in its flat position. If the bi-metallic membrane  102  is cooled down from its initial temperature, it will bend in an opposite direction to the first direction. A displacement of Δd with respect to the reference plane  106  occurs as a response of the membrane  102  to the increase in temperature. The first metal in this embodiment is brass and the second metal is steel. The skilled person however would consider other combinations of metals suitable for achieving the desired tuning in the tunable waveguide resonator for intended temperatures and applications. Other examples of metals without inadvertently limiting the present invention may include copper and steel, or brass and iron or any other standard bi-metal material or alloy. 
     The tuning element  102  can in other embodiments be a metallic foil which is suitable for reversibly changing its shape when exposed to temperature changes and thus result in a change in a dimension of the cavity of the resonator. In other embodiments the tuning element  102  may comprise a plurality of stacks of a bi-metallic membranes, e.g. a second or a third sheet of the first and second metals arranged in stacks. 
     In the following the tuning element  102  may also frequently be referred to as the bi-metallic membrane  102 . 
     The tuning element  102  is, in response to a change in a temperature of the tuning element  102 , caused to be reversibly displaced with respect to a reference plane  106  of the first main surface  103   a  of the tuning element  102  such that a portion  102   a  (see  FIGS.  3 A and  3 B ) of the tuning element  102  is caused to be displaced along an extension  105  perpendicular to the first main surface  104  of the one other inner wall  101   c ′, whereby changing a dimension d 2  of the tunable waveguide cavity  107 . 
     The second length d 2  of the waveguide part  100  is to be understood as the distance between the two inner walls, the first  101   a ′ and the third  101   c ′ inner wall. In other words, the dimension d 2  of the cavity  107  which is changed when the tuning element is caused to be displaced is the same as changing the second length d 2  i.e. the distance between the two parallel inner walls  101   a ′ and  101   c′.    
     When in use, by changing temperature of the tuning element  102  using a temperature adjusting means, the portion  102   a  of the tuning element  102  is moved towards the first main surface  104  of the opposite inner wall  101   c ′ by projecting out of the reference plane  106  of the first main surface  103   a  of the tuning element  102 . In some embodiments the portion  102   a  forms only a part of the tuning element  102 . In other embodiments the portion  102   a  extends along and forms the entire length of the tuning element  102 . 
     The area and volumetric thermal expansion of the bi-metallic membrane  102  can be isotropic in some embodiments. In other embodiments the thermal expansion may be anisotropic. 
     The membrane may be manufactured by any customary production technologies in the field such as 3D printing. 
     By reversibly here it is meant to be understood that when the temperature of the tuning element is increased with the amount ΔT from an initial temperature T e.g. ambient temperature to T+ΔT, the tuning element  102  is accordingly displaced as described above. However, when the temperature of the tuning element  102  returns to T, the tuning element  102  is moved in the opposite direction and returns to its initial position. 
     As shown in  FIG.  2 A , the tunable element  102  may be comprised only partly in one of the walls  101   a  of the waveguide part  100  forming a part of the wall  101   a . This way, the first main surface  103   a  of the tuning element  102  only partly forms a portion of the inner wall  101   a′.    
     Alternatively or additionally, the wall  101   a  of the waveguide part  100  completely comprises the tuning element  102  as shown in  FIGS.  2 B and  2 C . In other words, the tuning element  102  completely forms one of the walls  101   a  of the waveguide part  100  and thus the first main surface  103   a  of the tuning element  102  forms a portion of the inner wall  101   a ′ extending entirely along the length of the inner wall  101   a′.    
     In some embodiments, the bi-metallic membrane  102  is attached to the end portions  108  of the walls as shown in  FIG.  2 A , e.g. where the bi-metallic membrane  102  is only partly comprised in one of the walls  101   a  of the waveguide part  100 . The end portions  108  here are to be construed as the end portions of the wall  101   a  of the waveguide part  100  leading to an opening  109  in the wall  101   a . In  FIG.  2 A , the bi-metallic membrane  102  comprised in the wall  101   a  is shown to have fully covered the length of the opening  109  and the bi-metallic membrane  102  has thus sealed the opening  109 . The top surface  103   a  of the tuning element  102  forms the portion of the main surface  104  of the inner wall  101   a ′ which covers the entire length of the opening  109 . The opening  109  may extend along a part of the wall  101   a  or the entire length of the wall  101   a , i.e. when the wall  101   a  is removed and replaced by the tuning element  102  as shown in  FIGS.  2 B and  2 C . 
     The bi-metallic membrane  102  is attached to the waveguide part  100  at its end portions  110  by means of attachment means  111 . As shown in  FIG.  2 A , the attachment means  111  are arranged between the end portions  108  of the wall  101   a  and end portions  110  of the bi-metallic membrane  102 , thus attaching the bi-metallic membrane  102  to the wall  101   a  of the waveguide part  100 . 
     In some embodiment the bi-metallic membrane  102  is attached to a portion of the inner walls adjacent the wall comprising the bi-metallic membrane  102 . For example, as shown in  FIG.  2 B  when the bi-metallic membrane  102  is comprised in wall  101   a , the bi-metallic membrane  102  is attached to a portion e.g. an end portion  112  of the inner walls  101   b ′ and  101   d ′ by means of attachment means  111 . The bi-metallic membrane  102  is preferably attached to the end portions  112  of the inner walls  101   b ′,  101   d ′ over the entire length of the inner walls i.e. over the entire longitudinal extension L of the inner walls  101   b ′,  101   d ′ as shown in  FIG.  1   . However, it is conceivable that the bi-metallic membrane  102  is attached to the inner walls only over some points (not shown) along the longitudinal extension of the inner walls  101   b ′,  101   d′.    
     Moving on, the bi-metallic membrane  102  in some embodiments is attached to the bottom part of waveguide part  100  i.e. to the bottom portion of the walls of the waveguide part  100 . For example, as shown in  FIG.  2 C , the bi-metallic membrane  102  is attached to the bottom portions  113  of two of the walls  101   b  and  101   d . The bi-metallic membrane  102  is preferably attached to the bottom portions  113  of the walls  101   b ,  101   d  over the entire length of the walls i.e. over the entire longitudinal extension L of the walls  101 b,  101   d  as shown in  FIG.  1   . However, it is conceivable that the bi-metallic membrane  102  is attached to the walls only over some points along the longitudinal extension of the walls  101   b ,  101   d . In this embodiment an end portion  114  of the top surface  103   a  of the bi-metallic membrane  102  is attached to the bottom portions  113  by means of attachment means  111 . 
     The end portions  114  of the other sides of the bi-metallic membrane  102  are attached in the same way to the bottom portions of the other remaining walls of the waveguide part  100  (not shown). This means that the waveguide part  100  is physically as well as electrically sealed by the bi-metallic membrane  102 . 
     The attachment means  111  in the above discussed embodiments may be screws, glue portions/pads, solder pads/bumps or some other tightening or attachment means. 
     In some embodiments, the tunable element  102  may be partly or fully comprised in multiple walls e.g. in two or in three or in four walls of the waveguide part  100 . (not shown) 
       FIGS.  3 A and  3 B  illustrate the waveguide part  100  in use, wherein the wall  101   a  is entirely formed of the tuning element  102 . The bi-metallic membrane  102  has a second main surface  103   b  (bottom surface  103   b ) which in this embodiment forms the outer wall  101   a ″ of the wall  101   a.    
     In the embodiment of  FIG.  3 A , the temperature adjusting means is a thermo-element  115  arranged at a predetermined distance “D” from the reference plane  106 . It can also be said that the thermo-element  115  is arranged at a predetermined distance from the second main surface  103   b  of the tuning element  102 , i.e. arranged under the bottom surface  103   b  of the bi-metallic membrane  102 . Where in response to a change in the temperature of the thermo-element  115 , the temperature of the tuning element  102  is caused to change such that the bi-metallic membrane  102  is displaced from its initial flat position to a tuning or bent position whereby changing the dimension d 2  of the cavity  107  of the tunable waveguide resonator  10 . 
     In some embodiments the distance “D” may be varied during operation e.g. by being mounted on an adjustable stage or platform controlled by a user or processing circuitry  116 . This provides for several advantages such as calibration of the thermo-element, maintenance, test measurements, or adjustment of the distance during a tuning session based on the frequency readout. 
     When the bi-metallic membrane  102  is in its initial position, the first main surface  103   a  and the second main surface  103   b  are substantially parallel with the reference plane  106 . In the initial position, the dimension d 2  of the cavity  107  which is changed when the tuning element is caused to be displaced from the initial position to the tuning position is the same as the second length d 2  of the waveguide part  100  i.e. the distance between the two parallel inner walls  101   a ′ and  101   c′.    
     By using the thermo-element  115 , the temperature of the bi-metallic membrane  102  is changed indirectly e.g. the membrane  102  is heated up or cooled down indirectly. The thermo-element can for example be a Peltier element. 
     When the temperature of the thermo-element changes e.g. when a temperature increase from T to T+ΔT is applied to the thermo-element, the bi-metallic membrane  102  is caused to be displaced corresponding to this increase. This means that the bi-metallic membrane  102  moves along the extension  105  perpendicular to the first main surface  104  of the inner wall  101   c ′. In this embodiment the temperature increase of ΔT causes the bi-metallic membrane  102  to move towards the inner wall  101   c ′. More specifically, when saying the bi-metallic membrane  102  is caused to be displaced, it is meant that the first main surface  103   a  of the bi-metallic membrane  102  moves towards the first main surface  104  of the inner wall  101   c ′. For example, the portion  102   a  of the bi-metallic membrane  102  is caused to be displaced towards the first main surface  104  of the inner wall  101   c ′ such that the highest point  102   b  of the portion  102   a  of the bi-metallic membrane  102 , when forming an arc shape, is displaced a corresponding distance of Δd, with respect to the reference plane  106 , along the extension  105 . Highest point of the arc shape is to be construed with respect to a chord of a circle comprising the arc, wherein the chord connects the two endpoints of the arc. 
     This movement of the bi-metallic membrane  102  cause the dimension d 2  of the cavity  107  to decrease to d 2 -Δd at the highest point  102   b  of the portion  102   a.    
     If the temperature of the thermo-element  115  is then decreased from T+ΔT to T, the tuning element  102  and more specifically the highest point  102   b  of the portion  102   a  of the tuning element  102  is moved in the opposite direction along the extension  105  away from the first main surface  104  and towards its initial position. This causes the dimension d 2 -Δd of the cavity  107  to increase and ultimately return to the initial value of d 2 . 
     It must be clear to the skilled person that the other portions of the bi-metallic membrane  102  other than the portion  102   a  as well as other points than the highest point  102   b  of the portion  102   a  will experience a slightly different thermal expansion and distance alteration than Δd and thus the dimension change over the entire length of the bi-metallic membrane  102  will graduate between d 2  and d 2 -Δd. Stating differently, the bi-metallic membrane  102  forms the arc shape between the two attachment points. 
     By employing the above mechanism, the inventors have found that the dimension or volume of the cavity  107  can be accurately adjusted which results in a change in frequency of the waveguide resonator  10 . For example, when the bi-metallic membrane  102  is heated up, the volume of the cavity will be reduced as discussed above in detail and this will lead to an increase in the frequency of the waveguide resonator, thus a convenient frequency tuning is achieved. This way, the variations of the ambient or working temperature of the tunable waveguide resonator  10  is advantageously compensated for. The present invention advantageously makes possible to tune the resonance frequency of the cavity  107  of the waveguide resonator  10  without sacrificing the high Q-factor of the cavity  107 . Further, the present invention eliminates the need for installing a varactor diode inside the waveguide cavity  107  which when installed in the cavity  107 , negatively affects the high Q-factor of the cavity  107  of the waveguide resonator  10 . The waveguide resonator  10  according to the present invention can also achieve considerably low phase noise values compared to standard available solutions. For instance, a standard VCO available on the market today can deliver a −114 dBc phase noise at a central frequency of 10 GHz. As an example, in comparison, the VCO comprising a waveguide cavity resonator  10  according to the present invention can deliver an improvement of at least 19 dB at the same working frequency over the above standard VCO. 
     In some embodiments the dimensions of the cavity  107  may e.g. be d 1 =21 mm×d 2 =18 mm for a central frequency of 10 GHZ. Other arrangements and dimension are clearly conceivable to the skilled person based on the working frequency of the waveguide resonator  10 . 
     In some exemplary embodiments, the displacement (Δd) of the bi-metallic membrane  102  is in the range of 10 μm to 20 μm for a central frequency of 10 GHz. It is however conceivable that for several other working frequencies , waveguide cavities and corresponding bi-metallic membranes could be designed for achieving desired frequency tuning ranges without departing from the scope of the appended claims. 
     The thermo-element  115  is arranged to be accurately controllable by means of control and processing circuitry  116 . This way the temperature of the thermo-element  115  can be adjusted with high precision. In some embodiments the control circuitry  116  may execute an algorithm to regulate the temperature of the thermo-element  115  such that a certain tuning position of the membrane  102  i.e. a certain frequency tuning target is constantly maintained and fluctuation in the ambient temperature, and/or working temperature of the waveguide resonator  10  are compensated for. 
     In another embodiment according to the present invention illustrated in  FIG.  3 B , the bi-metallic membrane  102  is connected to a current source  117  as the temperature adjusting means, which injects electric current through the bi-metallic membrane  102  and causes a temperature increase in the bi-metallic membrane  102  by means of direct heating compared to the indirect heating of the embodiment of  FIG.  3 A . The electric current source  117  may be a designated electric current source, or it may be an electric current from an output port of another component (not shown), such as a filter unit, of the electric circuitry. The working principles and advantages achieved by this embodiment of the invention is similar to that of the previous embodiments. 
     Furthermore, in some other embodiments, the bi-metallic membrane  102  is configured to operate in the ambient temperature and compensate only for temperature variations in the working environment of the waveguide resonator  10 . In such embodiments no direct and/or indirect temperature regulating means are installed. Instead, it is the fluctuations of the ambient temperature which control the displacement of the bi-metallic membrane  102  and in such way control the volume of the cavity  107  and the changes in the frequency of the waveguide resonator  10 . It is however required that a suitable combination of metals or alloys be used to construct the bi-metallic membrane  102  when it is controlled by the ambient temperature. 
       FIG.  4    shows a block diagram  200  of a phase locked loop (PLL) circuit, wherein the tunable waveguide resonator  10  according to the present invention is implemented by means of example. The PLL circuit  200  comprises, a reflection amplifier  201  connected to the waveguide resonator  10 , a low pass filter (LPF)  202 , and processing and control circuitry including a microprocessor  203  and a comparator  204 . In this example the PLL circuit includes the waveguide resonator  10  and a thermo-element  115  arranged for temperature adjustment of the bi-metallic membrane  120 . The PLL circuit  200  further includes additional means for tuning the frequency of the tunable waveguide resonator  10 . For example, the PLL circuit  200  comprises an electric motor  205  and a tuning screw  206  mounted onto the waveguide part  100  of the resonator  10  via e.g. an aperture (not shown) in the waveguide part  100 . The tuning screw  206  may be coupled to a tuning device (not shown) located inside the waveguide part e.g. between any of the two inners wall of the waveguide part  100 . The frequency of the cavity  107  can be adjusted by the motor  205  rotating the screw  206  which controls a metallic or dielectric puck inside the cavity  107 . This way a broad and rather crude adjustments of the frequency of the cavity  107  is achievable. The PLL circuits additionally comprises a varactor diode  207  which is placed outside the cavity  107  of the waveguide resonator  10 . Such a varactor diode  207  can be used to control small variations in frequency of the cavity  107 . The motor  205 , varactor diode  206  and the temperature-controlled bi-metallic membrane  102  individually and/or in combination provide the user with a great degree of control over tuning the frequency of the waveguide resonator  10  which is very advantageous. 
       FIG.  5    shows a flow chart of a method according to another aspect of the present into for tuning a frequency of a tunable waveguide resonator  10 . The waveguide resonators  10  comprises a waveguide part  100 . The waveguide part  100  comprises a plurality of walls  101   a ,  101   b ,  101   c ,  101   d  and a tuning element  102 . One of the plurality of walls e.g. wall  101   a  at least partly comprises the tuning element  102 , wherein the tuning element has a first main surface  103   a , facing toward a first main surface  104  of an inner wall  101   a ′,  101   b ′,  101   c ′,  101   d ′ of one other wall e.g. inner wall  101   c ′ of wall  101   c  of the plurality of walls, wherein the method comprises changing  51  the temperature of the tuning element  102 , causing S 2  the tuning element to be reversibly displaced along an extension  105  perpendicular to the first main surface  104  of the one other inner wall  101   c ′ in response to the change in the temperature of the tuning element. The method further comprises causing S 3  a dimension d 2  of a cavity  107  of the tunable waveguide resonator  10  to change in response to the tuning element being reversibly displaced and tuning S 4  a frequency of the tunable waveguide resonator by the change in the dimension d 2  of the cavity  107 . 
     In some embodiments the method further comprises providing S 11  a temperature adjusting means  115 ,  117  for changing a temperature of the tuning element  102 , and changing S 12  the temperature of the tuning element  102  by the temperature adjusting means. 
     In other embodiments the bi-metallic membrane  102  may be configured to operate in the ambient temperature and compensate only for temperature variations in the working environment of the waveguide resonator  10 . In such embodiments, temperature adjusting means are not required. Instead, it is the fluctuations of the ambient temperature which control the displacement of the bi-metallic membrane  102  and in such way control the volume of the cavity  107  and the cause the tuning of the frequency of the waveguide resonator  10 . It is however noted that a suitable combination of metals or alloys is to be used to construct the bi-metallic membrane  102  when it is controlled by the ambient temperature. 
     The method can be carried out in any desired order, or parts of the method may be performed repeatedly or sequentially in different applications as desired. 
     In other embodiments, the method may further comprise determining S 5 , by means of a processing circuitry  116 ,  203 ,  204  a deviation in a selected working frequency of the waveguide resonator, and changing S 6  the temperature of the tuning element by means of the temperature adjusting means  115 ,  117  based on the determining. The method may further comprise compensating S 7  for the deviation by tuning the selected working frequency of the waveguide resonator corresponding to the change in the dimension d 2  of the cavity  107 . The deviation may for example be any temperature fluctuations in the working environment leading to a deviation of the frequency of the resonator. The deviation may also be caused due to mechanical vibrations or any other conceivable environmental disturbances such as wind, irradiation, and the like.