Patent Publication Number: US-8120449-B2

Title: Magnetically tunable filter with coplanar lines

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to a magnetically tunable filter. 
     2. Related Technology 
     Magnetically tunable filters are employed, for example, as variable bandpass filters in spectrum analyzers and network analyzers, the desired resonant frequency being adjusted by means of an external variable magnetic field. 
     U.S. Pat. No. 4,888,569 discloses a variable bandpass filter for frequencies within a frequency range of at most one waveguide band, for example 50-75 GHz, with four resonator spheres. The variable bandpass filter comprises an input waveguide, an output waveguide and a transfer waveguide, which are configured for the propagation of a TE 10  wave mode. The end of the input waveguide terminated by a short-circuit wall, the start of the output waveguide which is likewise provided with a short-circuit wall, and the transfer waveguide fitted below the input waveguide and the output waveguide in the direction of the externally applied homogeneous magnetic field, is arranged during operation between two magnet poles which supply a variable magnetic field for adjusting a resonant frequency. In the direction of the wave propagation, the input waveguide and output waveguide have a rectangular profile which has a much smaller cross-sectional area in the coupling region than at the connection flange. The coupling region of the variable bandpass filter comprises the four resonator spheres, fitted close to a short-circuit wall, and respectively the tapered ends of the input waveguide and of the output waveguide, as well as the transfer waveguide with a constant cross-sectional area. 
     A disadvantage of the variable bandpass filter described in U.S. Pat. No. 4,888,569 is that in the resonant case the field distribution of the wave to be extracted is unfavourable in the coupling region, since it is guided in a waveguide whose profile is reduced towards the coupling region perpendicularly to the propagation direction of the wave to be extracted. This causes undesired reflections which are destructively superposed and therefore reduce the amount of energy transported by the incoming wave. This effect also pertains to the outgoing wave in the output waveguide, which now has a defined frequency, so that overall the insertion loss in relation to the entry of the input waveguide and the exit of the output waveguide is increased since the field distributions in the coupling region are perturbed owing to the tapering geometry of the waveguides. 
     Another disadvantage is the limited bandwidth of the waveguide concept. 
     SUMMARY OF THE INVENTION 
     Therefore, the invention provides a magnetically tunable filter for high frequencies, which has an insertion loss being as low as possible in the resonant case and which has very high isolation of the filter input and the filter output in the off-resonance case. 
     The invention provides a magnetically tunable filter having a filter housing and two tunable resonator spheres comprising magnetizable material and arranged next to one another in two filter branches, each filter branch comprising a coplanar line arranged on a substrate layer and extending in the direction of an electrical connection, the two filter branches being connected by a common coupling opening and a resonator sphere respectively being positioned on each side of the coupling opening inside the two filter branches. 
     The magnetically tunable filter according to the invention comprises a filter housing and two tunable resonator spheres made of magnetisable material. These are arranged next to one another in two filter branches, each filter branch comprising a coplanar line formed on a substrate layer and extending in the direction of an electrical connection, i.e. in the direction of the signal input or in the direction of the signal output. The two filter branches are connected to one another by a common coupling opening, and they have a common filter housing. On either side of the coupling opening, the resonator spheres are arranged on each side inside the two filter branches. 
     The advantages achieved by the invention are in particular that the magnetically tunable filter according to the invention comprises two coplanar lines, so as to ensure good guiding of the incoming electromagnetic wave and the outgoing wave. The coplanar lines do not have a bottom cut-off frequency. 
     It is furthermore advantageous that the resonator spheres are positioned in the vicinity of a short circuit, since here, over a large frequency range, a magnetic field maximum occurs which is independent of the frequency of the incoming electromagnetic wave. Owing to the coupling structure and the line type of the coplanar line, the working range of the filter according to the invention is relatively wide in respect of the frequency and is therefore very suitable for a frequency range to be filtered, for example from 40 GHz to 75 GHz. 
     Furthermore, the coplanar lines which are used offer the advantage that they have a defined characteristic impedance so that good coupling of the resonator spheres can be adjusted. The characteristic impedance of the coplanar line in the vicinity of the resonator spheres is also easy to match by using a λ/4 transformer or a taper. 
     Furthermore, the coplanar line is preferably formed on a substrate whose dielectric constant is as low as possible, in order to keep the wavelength as large as possible in comparison with the diameter of the resonator spheres. A long wavelength in comparison with the diameter of the resonator spheres reduces the excitation of perturbing higher order modes, since the magnetic field distribution in the volume of the resonator spheres is more homogeneous with a long wavelength than with a shorter wavelength. 
     It is also advantageous for the two coplanar lines to be fully embedded in metal channels, so that they are substantially surrounded by metal walls. In the resonant case, energy transfer is made possible by connecting these channels, or the filter branches, to one another through a coupling opening, the coupling opening being designed differently according to the various exemplary embodiments or optionally comprising irises with geometrically different or differently positioned iris apertures. 
     A coupling opening partially closed by means of a metal separating wall has the advantage that the resonator spheres do not have any direct line of sight with one another. The height of the separating wall is in this case advantageously selected so that although line of sight between the resonator spheres is prevented, a sufficient coupling factor is nevertheless still ensured. This is a significant difference from all previous concepts. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Both the structure and the functionality of the invention, as well as its further advantages and objects, will however be best understandable with the aid of the following description in conjunction with the associated drawings. In the drawings: 
         FIG. 1  shows a perspective representation of a schematically represented structure of a first exemplary embodiment of the magnetically tunable filter according to the invention; 
         FIG. 2  shows a plan view of a schematically represented structure of a second exemplary embodiment of the magnetically tunable filter according to the invention; 
         FIG. 3  shows a side view of a schematically represented structure of the second exemplary embodiment of the magnetically tunable filter according to the invention; 
         FIG. 4  shows a front view of a schematically represented structure of the second exemplary embodiment of the magnetically tunable filter according to the invention; 
         FIG. 5  shows a perspective representation of a schematically represented structure of a filter branch according to the second exemplary embodiment of the magnetically tunable filter according to the invention; 
         FIG. 6  shows a first embodiment of the end region of the coplanar line of the magnetically tunable filter according to the invention; 
         FIG. 7  shows a second embodiment of the end region of the coplanar line of the magnetically tunable filter according to the invention; 
         FIG. 8  shows the simulated off-resonance isolation profile of the magnetically tunable filter according to the invention; 
         FIG. 9  shows the simulated profile of the coupling as a function of the resonant frequency of the magnetically tunable filter according to the invention as well as the simulated attenuation loss of the H 10  mode of a 2 mm wide and 0.7 mm long coupling waveguide; and 
         FIG. 10  shows a simulated resonance profile of the magnetically tunable filter according to the invention for a resonant frequency of 67.8 GHz. 
     
    
    
     DETAILED DESCRIPTION 
     Throughout the figures, parts which correspond to one another are provided with the same references so that repeated description is superfluous. 
       FIG. 1  shows a perspective representation of a schematically represented structure of a first exemplary embodiment of the magnetically tunable filter  1  according to the invention, having a filter housing  2  and having two tunable resonator spheres  3   a ,  3   b  made of magnetisable material, in particular hexaferrite. The overall filter housing  2  comprises two filter branches  4   a ,  4   b , as well as a signal input  6   a  and a signal output  6   b , the resonator spheres  3   a ,  3   b  being arranged next to one another in the two filter branches  4   a ,  4   b.    
     Each of the two filter branches  4   a ,  4   b  contains a coplanar line  7  formed on a substrate layer  5  and extending in the direction of an electrical connection  6 , the substrate layer  5 , which preferably has a low dielectric constant, being arranged on the metal bottom  10  of the filter branch  4   a ,  4   b . The two adjacent and touching filter branches  4   a ,  4   b  are connected to one another through a common coupling opening  8 , a resonator sphere  3   a ,  3   b  respectively being positioned on each side of the coupling opening  8  above the coplanar line  7  inside the two filter branches  4   a ,  4   b.    
     The coplanar line  7  comprises two outer line strips  27   a ,  27   b  and a central line strip  28 , which lie on the same side of the substrate layer  5 , away from the metal bottom  10 , and have a short-circuit region  31  in the end region  30  of the filter branch  4   a ,  4   b . In the short-circuit region  31 , the two outer line strips  27   a ,  27   b  and the central line strip  28  are connected conductively to one another by a metal layer. Provided in the short-circuit region  31 , there is furthermore a through-contact  35  which conductively connects the metal layer through the substrate layer  5  to the bottom of the filter branch  4   a ,  4   b , or of the filter housing  2 . 
     These waveguide-coupled coplanar lines  7  have the advantage that the fields are concentrated in the vicinity of the central line strip  28  and the nonconductive slots  29   a ,  29   b , the current density in the longitudinal direction having maximum values in the vicinity of the short-circuit region  31 . The effect achieved by the coplanar line  7  embedded in the metal filter housing  2  is therefore good and guiding, defined by the line geometry, of the electromagnetic wave to be transported. 
       FIG. 2  shows a plan view of a schematically represented structure of a second exemplary embodiment of the magnetically tunable filter  1  according to the invention. In the common coupling opening  8 , there is now a first thin separating wall  9  which extends between the respective substrate layers  5  of the filter branches  4   a,    4   b  as far as the metal bottom  10  of the filter housing  2 . On either side of this separating wall  9 , whose thickness  15  is defined by two arrows and is for example between 10 μm-100 μm, preferably about 50 μm, the resonator spheres  3   a ,  3   b  which consist of a ferrimagnetic or ferromagnetic material and have a diameter of for example 100 μm-1000 μm, preferably approximately 300 μm, are bonded on a quartz carrier (not further represented) using epoxy adhesive. The quartz carrier with the resonator spheres  3   a ,  3   b  is placed in the short-circuit region  31  of the coplanar line  7 . 
     The dashed lines which extend parallel to the signal input  6   a  and the signal output  6   b , respectively, each indicate a second thin separating wall  19  which in this second exemplary embodiment of the magnetically tunable filter according to the invention is additional relative to the exemplary embodiment shown in  FIG. 1  and will be described in more detail with the aid of  FIG. 3 . 
       FIG. 3  shows a side view of a schematically represented structure of the second exemplary embodiment of the magnetically tunable filter  1  according to the invention, with the first separating wall  9  fitted centrally with respect to the two filter branches  4   a ,  4   b  and with the second separating wall  19 , which was indicated merely as a dashed line in  FIG. 2 . 
     In this side view, it may be seen that the height  11  of the first separating wall  9  is less than the total height  12  of the filter housing  2 , or of the filter branch  4   a ,  4   b , so that this first separating wall  9  prevents direct line of sight between the two resonator spheres  3   a ,  3   b  which are arranged on either side of the first separating wall  9 . 
     Between a lid  16  of the filter housing  2  and an upper edge  17  of the first separating wall  9 , which extends inside and parallel to the coupling opening  8  and whose length  13  corresponds to the length  14  of the coupling opening  8 , there is therefore a first quadrilateral gap  18 . 
     In an additional embodiment of the magnetically tunable filter according to the invention (not further represented), instead of the first separating wall  9  inside the common coupling opening  8  of the filter branches  4   a ,  4   b , it is also possible to fit an iris which extends from the bottom  10  of the filter housing  2  as far as the lid  16  of the filter housing  2  and has an arbitrarily shaped and positioned iris aperture. The iris aperture may for example be circular, elliptical, rectangular, triangular, or have the shape of a polygon. 
     The second separating wall  19  is provided inside the filter branches  4   a ,  4   b  and respectively stands perpendicularly to the longitudinal direction of the coplanar line  7  and the first separating wall  9 , the length  21  of the second separating wall  19  corresponding to the width  22  of a filter branch  4   a ,  4   b  and being positioned inside one filter branch  4   a  approximately in the vicinity of a short-circuit wall  20   b  of the neighbouring filter branch  4   b , which may be seen clearly in the plan view of  FIG. 2 . 
     It may furthermore be seen from  FIG. 3  that the second separating wall  19  in the exemplary embodiment is fastened to the lid  16  of the filter housing  2 , the height  23  of the second separating wall  19  being less than the distance  24  between the substrate layer  5  and the lid  16  of the filter housing  2 , so that a second gap  26  with an essentially quadrilateral profile is formed between a lower edge  25  of the second separating wall  19  and the substrate layer  5  with the coplanar line  7 . 
       FIG. 4  shows a front view of a schematically represented structure of the second exemplary embodiment of the magnetically tunable filter  1  according to the invention, with the first separating wall  9  and the second separating walls  19 . The two resonator spheres  3   a ,  3   b  are arranged mirror-symmetrically to one another on either side of the coupling opening  8 , or on the near and far sides of the first separating wall  9 . The midpoint of the resonator spheres  3   a ,  3   b  lies approximately above the symmetry line of the central line strip  28  of the coplanar line  7 , so that each resonator sphere  3   a ,  3   b  lies at the maximum of the magnetic field and optimal excitation of the desired resonant frequency can be carried out via the magnetic field of the radiofrequency source, the region selected for positioning the resonator spheres  3   a ,  3   b  being characterized in that the magnetic field maximum occurs in this region independently of the frequency of the incoming or outgoing electromagnetic wave. 
     The coplanar line  7 , which for example has a characteristic impedance of 50 Ω, is formed on a substrate layer  5  which has a preferably low dielectric constant. The sphere diameter of the resonator spheres  3   a ,  3   b , i.e. for example 300 μm, is therefore much less than the wavelength of the incoming and outgoing waves. The excitation of perturbing higher order modes is therefore reduced, since the magnetic field distribution in the sphere volume is more homogeneous with a long wavelength than with a wavelength whose dimension is only a little larger than the sphere diameter of the resonator spheres  3   a ,  3   b . The first separating wall  9  between the two resonator spheres  3   a ,  3   b prevents direct coupling of stray fields in the vicinity of the resonator spheres  3   a ,  3   b , so that high decoupling is obtained away from resonance. 
       FIG. 5  shows a perspective representation of a schematically represented structure of a filter branch  4   a  according to the second exemplary embodiment of the magnetically tunable filter  1  according to the invention, with the two separating walls  9  and  19 . This filter branch  4   a  forms one half of a cavity resonator or connecting resonator  32  for an H 10  wave mode, the walls of the connecting resonator  32  being formed by the bottom  10  of the filter housing, the two second separating walls  19 , the two sidewalls  36   a ,  36   b  and the two short-circuit walls  20   a ,  20   b  of the filter branches  4   a ,  4   b , and the lid  16  of the filter housing  2 . The sidewall  36   a  and the short-circuit wall  20   a  are marked by shading in this representation. 
     In the short-circuit region of the filter branch  4   a , it may now be seen clearly that the through-contact  35  connects the metal layer of the coplanar line  7  to the metal bottom  10  of the filter branch  4   a.    
       FIG. 6  shows a first embodiment of the end region of the coplanar line  7  of the magnetically tunable filter  1  according to the invention. The coplanar line is designed as a λ/4 transformer  34  in this region, in order to match the characteristic impedance of the input coplanar line  7  to the characteristic impedance of the coplanar line in the sphere region with the resonator spheres. 
       FIG. 7  shows a second embodiment of the end region of the coplanar line  7  of the magnetically tunable filter  1  according to the invention. The coplanar line is designed as a taper  33  in this region, in order to match the characteristic impedance of the coplanar line  7  to the characteristic impedance of the connecting resonator  32  with the resonator spheres. 
       FIG. 8  shows the simulated isolation profile of the magnetically tunable filter  1  according to the invention in the off-resonance case (isolation), curve A giving the magnitude of the scattering matrix element S 11  and curve B giving the frequency-dependent magnitude of the scattering matrix element S 12  of the filter according to the invention, treated as a two-port network. The values of curve B lie in a range of from −75 dB to −115 dB, and they confirm that electromagnetic waves whose frequency lies outside the resonant frequency are attenuated very strongly by the filter  1  according to the invention. 
       FIG. 9  shows the simulated profile of the coupling (curve C) as a function of the resonant frequency of the magnetically tunable filter  1  according to the invention as well as the simulated attenuation loss (curve D) of the H 10  mode of a 2 mm wide waveguide with a length of 0.7 mm. Curve C and curve D show that the frequency-dependent change in the attenuation of a filter  1  according to the invention, when the resonant frequency increases by approximately 17 GHz, corresponds essentially to the frequency-dependent change in the attenuation of the H 10  mode in the coupling waveguide with the aforementioned dimensions, which clearly shows that the H 10  wave mode propagates in the connecting cavity  32  in the resonant case. The absolute attenuation value in the resonant case lying between −3 dB and −8.5 dB is orders of magnitude less than the values in the decoupling case (isolation) shown in  FIG. 8 . 
       FIG. 10  shows a simulated resonance profile of the magnetically tunable filter  1  according to the invention for a desired central frequency of 68 GHz. Curve E shows the frequency-dependent profile of the absorption curve with an absorption maximum at 67.8 GHz and a full width at half maximum of 0.2 GHz and a frequency spread (FWHM) of approximately 0.3%. Curve F shows the frequency-dependent profile of the transmission curve with a pronounced maximum likewise at 67.8 GHz. It may be seen clearly that the frequency positions of the absorption maximum and the transmission maximum coincide very well. 
     The invention is not restricted to the exemplary embodiments represented in the drawings, and in particular not to a filter housing without separating walls. All features described above and represented in the drawing may be combined with one another in any desired way.