Abstract:
A resonator system contains one or more resonator, and has a plurality of degenerate resonant modes. Switching circuitry can be operated for controlling a degree of coupling between the resonant modes, such that resonant properties of the resonator system can be controlled. Where the resonator system includes one resonator, the size of a notch in a conductive patch can be controlled to vary the coupling between the resonant modes within that resonator. Where the resonator system includes multiple resonators, the coupling between the resonant modes in the resonators can be varied. A single device can be tuned as required, in order to provide the desired frequency response properties.

Description:
FIELD 
     This invention relates to the tuning of resonator systems, and to resonator systems whose coupling can be adjusted. 
     BACKGROUND 
     Resonators are widely used in situations where electromagnetic signals or fields are being generated or detected. A body made of a conductive material with a dielectric substrate will resonate at a particular frequency, with that resonant frequency corresponding to a particular wavelength that is related to the dimensions of the body and the material properties of the body. Where the body is effectively one-dimensional, this relationship is relatively straightforward. However, where the body is two- or three-dimensional, or where there is coupling between two or more such bodies, the relationship becomes more complex. 
     In one well-known form of resonator, a generally square, thin (that is, effectively two-dimensional) layer, or patch, of conductive material is provided. An effectively two-dimensional body of this type has two resonant modes, relating to oscillations along the width and the length of the patch respectively. In each case, the resonant frequency of the mode corresponds to a wavelength which is approximately double the respective dimension of the patch. 
     By removing a part of the conductive material, it is possible to establish a degree of coupling between these two resonant modes. For example, a generally square area of conductive material can be removed from a corner of the patch. The degree of coupling affects the overall frequency response of the patch resonator. If the degree of coupling is at a critical level, the frequency response includes a resonance at a particular frequency. If the degree of coupling is below this critical level, the resonator becomes less efficient. If the degree of coupling is above the critical level, the resonant peak is effectively split into two peaks and spread over a wider range of frequencies, that is, the resonator has a larger bandwidth. 
     SUMMARY OF THE INVENTION 
     According to a first aspect of the present invention, there is provided a resonator system, comprising at least one resonator, having a plurality of degenerate resonant modes, and further comprising switching circuitry for controlling a degree of coupling between said resonant modes, such that resonant properties of the resonator system can be controlled. 
     This has the advantage that a single device can be tuned as required, in order to provide the desired frequency response properties. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       For a better understanding of the present invention, and to show how it may be put into effect, reference will now be made, by way of example, to the accompanying drawings, in which: 
         FIG. 1  is a schematic diagram of a first resonator system in accordance with the present invention. 
         FIGS. 2 and 3  show in more detail switches used in the resonator system of  FIG. 1 . 
         FIG. 4  is a schematic perspective view of a second resonator system in accordance with the present invention, in use. 
         FIG. 5  is a schematic plan view of the resonator system of  FIG. 4 , in use. 
         FIGS. 6 and 7  show in more detail switches used in the resonator system of  FIGS. 4 and 5 . 
         FIG. 8  is a schematic diagram of a third resonator system in accordance with the present invention. 
         FIG. 9  is a schematic perspective view of a further resonator system in accordance with the present invention. 
         FIG. 10  is a schematic side view of the resonator system of  FIG. 9 . 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 1  shows a resonator system with a single symmetric resonator  10 . The resonator  10  is formed from a patch of a conductive material  12 , such as a metal, sandwiched between two ground planes (not shown in  FIG. 1 ). The patch  12  is generally square, and planar (extending in the x-y plane in the coordinate system shown in  FIG. 1 ). The patch  12  is mounted on a substrate (not shown in  FIG. 1 ), formed of a dielectric material. The patch has two resonant modes, with a first mode extending in the x-direction and a second mode extending in the y-direction. The dimensions of the patch and the dielectric material properties of the substrate determine the frequencies of these resonant modes. More specifically, the length of the patch in each of these directions determines the wavelength λ, of the respective resonant mode, by the relationship length≈λ/2. 
     By suitable choice of dimensions, resonators in accordance with the present invention, such as the resonator  10 , can be used at any desired frequency, but a typical application is in radio frequency, microwave and millimeter communications, where the required frequency of operation means that the dimensions of the patch  12  in the x- and y-directions will be of the order of the wavelength, usually a few millimeters or a few centimeters. The patch  12  is generally thin, in the sense that the dimension in the direction, perpendicular to the x-y plane, will be considerably smaller than the dimensions in the x- and y-directions. It will be appreciated that the same technique can be used, with suitable modification, in the case of a three-dimensional resonator. 
     A first feed line  14  is connected to supply energy to and/or from the patch  12  at a point which, in this illustrated embodiment, is half way along a first side  16  of the patch, while a second feed line  18  is connected to supply energy to and/or from the patch  12  at a point which, in this illustrated embodiment, is half way along a second side  20  of the patch, the second side  20  being adjacent to the first side  16 . Thus, the first feed line  14  is connected to the first resonant mode of the patch, and the second feed line  18  is connected to the second resonant mode of the patch. 
     In this case, the shape of the patch  12  is modified, in order to achieve a degree of coupling between the first and second resonant modes. Specifically, a notch  22  is formed in the patch  12 , at the corner between the third side  24  and the fourth side  26  of the patch  12 , where the third side  24  is opposite the first side  16  and the fourth side  26  is opposite the second side  20 , although in other embodiments the notch could be of any shape, and could be at any corner of the patch, or could be formed elsewhere in the patch. 
     The size of the notch  22  has an effect on the resonant properties of the patch  12 , because the degree of coupling between the resonant modes affects the overall frequency response of the patch resonator. If there is no notch  22 , the first and second resonant modes are effectively uncoupled from each other, and the degree of coupling between them increases as the size of the notch  22  increases. If the degree of coupling is at a critical level, the frequency response includes a resonance at a particular frequency. If the degree of coupling is below this critical level, the resonator becomes less efficient. As the degree of coupling increases above the critical level, the resonant peak splits into two separate peaks, spread over a wider range of frequencies. Provided that these two peaks are not too widely spread, the resonator has a larger operating bandwidth. 
       FIG. 1  shows an arrangement, whereby the degree of coupling between the resonant modes can be altered, as required, in order to achieve desired resonant properties. 
     Specifically, a track  28  of conductive material surrounds the notch  22 . Then, material is removed from a track  30  surrounding the track  28  of conductive material, while a further track  32  of conductive material surrounds the track  30  where material is removed. Switches  34 ,  36  are then provided, bridging the track  30  where material has been removed, between the tracks  28 ,  32  of conductive material. 
     Similarly, material is removed from a track  38  surrounding the track  32  of conductive material, while a further track  40  of conductive material surrounds the track  38  where material is removed. Switches  42 ,  44 ,  46 ,  48  are then provided, bridging the track  38  where material has been removed, between the tracks  32 ,  40  of conductive material. 
     Further, material is removed from a track  50  surrounding the track  40  of conductive material, while a further track  52  of conductive material surrounds the track  50  where material is removed. Switches  54 ,  56 ,  58 ,  60 ,  62 ,  64  are then provided, bridging the track  50  where material has been removed, between the tracks  40 ,  52  of conductive material. 
     Further, material is removed from a track  66  surrounding the track  52  of conductive material, while a further track  68  of conductive material surrounds the track  66  where material is removed. Switches  70 ,  72 ,  74 ,  76 ,  78 ,  80 ,  82 ,  84  are then provided, bridging the track  66  where material has been removed, between the tracks  52 ,  68  of conductive material. 
     Further, material is removed from a track  86  surrounding the track  68  of conductive material, while the remainder of the patch  12  of conductive material surrounds the track  86  where material is removed. Switches  88 ,  90 ,  92 ,  94 ,  95 ,  96 ,  98 ,  100 ,  102 ,  104  are then provided, bridging the track  86  where material has been removed, between the track  68  of conductive material and the remainder of the patch  12 . 
     In this example, all of the tracks are right angled, but are effectively concentric, centered around the corner of the patch  12  between the third side  24  and the fourth side  26 , although other arrangements are equally possible. 
       FIG. 2  shows a representative one of the switches  36  in an open position, while  FIG. 3  shows the switch  36  in a closed position. 
       FIG. 2  shows the switch  36  bridging a gap between the conductive tracks  28 ,  32 , across the track  30  where the conductive material has been removed. The switch  36  includes a first end portion  36   a  connected to the first conductive track  28 , with a second end portion  36   b  connected to the second conductive track  32 . A central conductive element  36   c  is separated from the first end portion  36   a  by a first gap  36   d , and is separated from the second end portion  36   b  by a second gap  36   e.    
     As shown in  FIG. 2 , the switch  36  is controlled such that the gaps  36   d ,  36   e  remain open, and so there is no continuous conductive path between the conductive tracks  28 ,  32 . The result, assuming that the switch  34  is similarly held open, is that the area covered by the conductive track  28 , and by the track  30  where the conductive material has been removed, forms an effective part of the notch  22 , and so the degree of coupling between the resonant modes of the system is determined accordingly. 
     By contrast,  FIG. 3  shows the situation where the switch  36  is controlled so that the gaps  36   d ,  36   e  present in  FIG. 2  are closed, and there is a continuous conductive path  36   f  between the conductive tracks  28 ,  32 . The result, assuming that the switch  36  is similarly held closed, is that the area covered by the conductive track  28 , and by the track  30  where the conductive material has been removed, forms an effective part of the conductive patch  12 , and so the degree of coupling between the resonant modes of the system is different from when the switch  36  is open. 
     The switches bridging the areas where material has been removed may be MEMS switches, and may be of a type suitable for use at radio or microwave frequencies, where the resonator  10  is intended for use at such frequencies. Each of the switches is controlled by a controller  106 , although for clarity  FIG. 1  only shows the connections from the controller  106  to the switches  102 ,  104 . The controller can have separate connections to each of the switches shown in  FIG. 1 , allowing each of the switches to be controlled individually, or it can have connections which allow all of the switches bridging one track where material has been removed to be opened and closed together. 
       FIG. 4  is a perspective view, showing an alternative resonator  110  in the x, y and z dimensions in more detail, and in use, while  FIG. 5  is a plan view, showing the resonator  110  in the same use configuration. The resonator shown in  FIGS. 4 and 5  is very similar to that shown in  FIG. 1 , and so the common features will not be described further in detail. Thus,  FIGS. 4 and 5  show the patch  12 , and the feed lines  16 ,  18 , mounted between two blocks  120 ,  122  ( FIG. 4 ) of dielectric material, with at least one ground plane (not shown in  FIGS. 4 and 5 ), in a way that will be well known to the person skilled in the art. 
     As in the resonator shown in  FIG. 5 , a track  28  of conductive material surrounds the notch  22 . Then, material is removed from a track  30  surrounding the track  28  of conductive material, while a further track  32  of conductive material surrounds the track  30  where material is removed. Similarly, material is removed from a track  38  surrounding the track  32  of conductive material, while a further track  40  of conductive material surrounds the track  38  where material is removed. Further, material is removed from a track  50  surrounding the track  40  of conductive material, while a further track  52  of conductive material surrounds the track  50  where material is removed. Further, material is removed from a track  66  surrounding the track  52  of conductive material, while a further track  68  of conductive material surrounds the track  66  where material is removed. Finally, material is removed from a track  86  surrounding the track  68  of conductive material, while the remainder of the patch  12  of conductive material surrounds the track  86  where material is removed. 
     As before, switches are provided, bridging each of the tracks  30 ,  38 ,  50 ,  66 ,  86  where the conductive material has been removed. The switches must be placed sufficiently close to each other that, when two adjacent switches are closed, the area between them from which the conductive material has been removed can effectively form part of the patch  12 . For example, the switches can be positioned as shown in  FIG. 1 . 
       FIG. 5  shows a representative one of the switches  60  in an open position, with another representative one the switches  96  in a closed position. 
       FIG. 6  shows the switch  60  in more detail. Specifically,  FIG. 6  shows the switch  60  bridging a gap between the conductive tracks  40 ,  52 , across the track  50  where the conductive material has been removed. As shown in  FIG. 6 , the switch  60  is controlled such that there remains a gap  60   a , and so there is no conductive path between the conductive tracks  40 ,  52 . The result is that the area covered by the conductive track  40 , and by the track  50  where the conductive material has been removed, forms an effective part of the notch  22 , and so the degree of coupling between the resonant modes of the system is determined accordingly. 
       FIG. 7  shows the switch  96  in more detail and, by contrast,  FIG. 7  shows the situation where the switch  96  is controlled so that there is a continuous conductive path  96   a  between the conductive track  68  and the remainder of the patch  12 . The result is that the area covered by the conductive track  68 , and by the track  86  where the conductive material has been removed, forms an effective part of the conductive patch  12 , and so the degree of coupling between the resonant modes of the system is different from when the switch is open. 
     As before, the switches bridging the areas where material has been removed may be MEMS switches, and may be of a type suitable for use at radio or microwave frequencies, where the resonator  10  is intended for use at such frequencies. Again, each of the switches is controlled by a controller. The controller can have separate connections to each of the switches, allowing each of the switches to be controlled independently, or it can have connections which allow all of the switches bridging one track where material has been removed to be opened and closed together. 
     As mentioned above,  FIGS. 4 and 5  show the resonator  110 , with the switches controlled so that the resonator has desired properties. Specifically, as shown in  FIG. 5 , the switches in the track  30  where conductive material has been removed, are open, meaning that there is no conductive path bridging the gap between the tracks  28 ,  32 . Further, the switches in the track  38  where conductive material has been removed, are open, meaning that there is no conductive path bridging the gap between the tracks  32 ,  40 , and the switches (including the switch  60 ) in the track  50  where conductive material has been removed, are open, meaning that there is no conductive path bridging the gap between the tracks  40 ,  52 . 
     By contrast, the switches in the track  66  where conductive material has been removed, are closed, meaning that there are conductive paths bridging the gap between the tracks  52 ,  68 , and the switches (including the switch  96 ) in the track  86  where conductive material has been removed, are closed, meaning that there are conductive paths bridging the gap between the track  68  and the remainder of the patch  12 . The presence of these conductive paths means that, provided the dimensions of the tracks and the gaps between them are chosen to be significantly smaller than the wavelengths of the signals in the conductive material, all of the material in the conductive tracks that are connected to the remainder of the patch  12  can be considered to be an active part of the patch. However, the tracks of conductive material that do not have any conductive path to the remainder of the patch  12  can be considered in effect to be part of the notch  22 . 
     Thus, by determining which of the switches should be opened and closed, the effective size of the notch  22  can be varied, and hence the coupling between the resonant modes of the resonator can be varied, with the result that the bandwidth of the resonator system can be controlled. 
     The resonator systems shown and described above have included a single square patch resonator. However, the invention is equally applicable to patch resonators of other shapes, for example to square patch resonators with cut out portions in their centres, or in their sides. 
       FIG. 8  shows one such alternative embodiment of the invention, where the resonator system includes an annular circular resonator. 
     In this embodiment of the invention, the resonator system  130  includes a ring  132  of conductive material. The ring  132  extends through almost the full 360° of a circle, but there is a notch  134  at one point. The conductive material then has straight arms  136 ,  138  extending inwardly from the circumference of the circle at this notch. At their inward ends, the arms  136 ,  138  are joined by a connecting piece  140 . In addition, switches  142 ,  144 ,  146  bridge the notch  134  at different points along its length. The switches  142 ,  144 ,  146  are controlled by a controller  148 . 
     By selectively opening and closing the switches  142 ,  144 ,  146 , as described above with reference to the other embodiments of the invention, the degree of coupling between the resonant modes of the system  130  can be varied, and hence the resonant properties of the system  130  can also be varied. 
       FIGS. 9 and 10  show a resonator system in accordance with a further embodiment of the invention. Specifically,  FIG. 9  is a perspective view in the x and z dimensions, and  FIG. 10  is a side view, showing a resonator system in the form of a filter  200  formed from two stacked dual-mode resonators  202 ,  204 . 
     The first dual mode resonator  202  is formed from a generally square patch  206  of conductive material, formed between two layers  208 ,  210  of dielectric material. A feed line  212  ( FIG. 9 ) is in the plane of the patch  206  to connect the resonator  200  to a larger circuit. A generally square notch  214  ( FIG. 9 ) is formed in one corner of the patch  206 , in order to introduce a degree of coupling between the resonant modes in the patch  206 . Although not shown in  FIG. 9  or  10 , switches may be provided as described above with reference to  FIGS. 2 and 3 , or  FIGS. 6 and 7 , in order to control the effective size of the notch  214 , in order to control this degree of coupling, and therefore control the resonant properties of the first resonator  202 . 
     The second dual mode resonator  204  is formed from a generally square patch  216  of conductive material, formed between two layers  218 ,  220  of dielectric material. Feed lines  222 ,  224  ( FIG. 9 ) in the plane of the patch  216  connect the resonator  200  to the larger circuit. A generally square notch  223  ( FIG. 9 ) is formed in one corner of the patch  216 , in order to introduce a degree of coupling between the resonant modes in the patch  216 . Although not shown in  FIG. 9  or  10 , switches may be provided as described above with reference to  FIGS. 2 and 3 , or  FIGS. 6 and 7 , in order to control the effective size of the notch  223 , in order to control this degree of coupling, and therefore control the resonant properties of the second resonator  204 . 
     There is thus described a system with three feed lines  212 ,  222 ,  224 , although the invention is equally applicable to systems with two, four, or any other suitable number of feed lines. 
     The first and second resonators  202 ,  204  are coupled together by means of a conductive layer  225  formed between them, and having an aperture or iris  226  formed within it. In this case, the iris  226  ( FIG. 9 ) is of a thin rectangular shape, but it can be any shape. 
     Switches  228 ,  230 ,  232 ,  234  bridge across the iris  226  at different points along its length as shown in  FIG. 9 . The switches  228 ,  230 ,  232 ,  234  can be opened and closed under the control of a controller (not shown in  FIGS. 5 and 6 ). By suitable opening and closing of the switches  228 ,  230 ,  232 ,  234 , the degree of coupling between the first and second resonators  202 ,  204  can be controlled, and hence the resonant properties of the resonator  200  can be controlled. 
     Although the invention is described herein with reference to resonator systems that include two-dimensional patch resonators, it will be appreciated that the invention is equally applicable to three-dimensional resonators. 
     In addition, although the invention is described with reference to resonator systems containing two stacked resonators, it will be appreciated that the invention is equally applicable to larger stacked resonators, or to other methods of controlling the degree of coupling between the resonant modes in separate resonators. 
     Resonator systems as described herein can be useful in tunable radio frequency or microwave antennas, matching networks, phase shifters, duplexers, and other radio frequency or microwave circuitry, where coupling tuning is required.