Abstract:
Disclosed is one method and one apparatus which teach improved techniques in using a shaped bias magnetic field over the active region of a ferrite stripline circulator/isolator circuit. The axial component of the bias field is decreased from the center toward edge, thus it is able to accommodate the accompanying changes in magnetization. This fulfills the requirements that frequencies are scaled with distances thereby warranting broadband operation. Furthermore, the radial component of the bias field is reduced, so as to minimize the generation of non-circulation volume modes. The discontinuity in magnetization distributed over the circulator/isolator active region is reduced, so as to minimize the generation of magnetostatic surface modes. The resultant circulator/isolator performance can thus show a broad bandwidth with improved characteristics in insertion loss and in isolation.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
   (Not Applicable) 
   FEDERALLY SPONSORED RESEARCH 
   (Not Applicable) 
   SEQUENCE LISTING OR PROGRAM 
   (Not Applicable) 
   BACKGROUND OF INVENTION 
   1. Field of Invention 
   This invention is directed to one method and one apparatus to obtain broadband operation of a ferrite stripline edge-mode/standing-mode circulator/isolator. More specifically, this invention teaches to use a varying magnetic bias to broaden the transmission band of a ferrite stripline edge-mode/standing-mode circulator/isolator with improved characteristics. 
   2. Prior Art 
   Although ferrite stripline junction circulators have been described in the literature since the 1950&#39;s, their operation was only vaguely understood until the theoretical work by Bosma in 1964 (H. Bosma, “On stripline Y-circulation at UHF,” IEEE Microwave Theory Tech., vol. MTT-12, pp. 61-73, January 1964), and by Fay and Comstock in 1965 (C. E. Fay and R. L. Comstock, “Operation of the ferrite junction circulator,” IEEE Trans. Microwave Theory Tech., vol. MTT-13, pp. 15-27, January 1965). The operation of an edge-mode ferrite isolator was described by Hines in 1961 (M. E. Hines, “Reciprocal and Nonreciprocal Modes of Propagation in Ferrite Stripline and Microstrip Devices”, IEEE Trans. vol. MTT-19, pp. 442-451, 1961), and an edge-mode ferrite circulator by How in 2005 (H. How, “Magnetic Microwave Devices,” in Encyclopedia of RF and Microwave Engineering, Vol. 3, pp. 2425-2461, 2005). Since then, the prior art has always assumed that a ferrite circulator or isolator is operational under a magnetic bias field established via the use of permanent magnets whose explicit spatial profile is considered immaterial to the circuit performance, at least deemed not critical. The resultant frequency bandwidth is thus restricted to a 2:1 ratio (Y. S. Wu and F. J. Rosenbaum, “Wide-band operation of microstrip circulators,” IEEE Trans. Microwave Theory Tech., vol. MTT-22, pp. 849-856, October 1974), or a 3:1 ratio (M. G. Mathew and T. J. Weisz, “Microwave Transmission Devices Comprising Gyromagnetic Material Having Smoothly Varying Saturation Magnetization,” U.S. Pat. No. 4,390,853, Jun. 28, 1983). 
   There has been rapid development in RF and microwave technologies during the past decade. RF and microwave wireless applications have been and continue to be among the fastest growth areas. Some of the expanding activities in these fields include wireless communications (mobile, cellular, and satellite), wireless sensors, local area networks, remote control and identification, global positioning systems (GPS), and intelligent highway and vehicle systems (IHVS). Circulators and isolators are indispensable building elements in RF and microwave circuits: they are used whenever isolation is intended among circuit modules, separating the signal paths according to their propagation directions thereby allowing the transmitter and the receiver to multiplex. Also, broadband instrumentations are needed by the electronic testing industries so that universal equipments are possible whose operation is independent of frequency. As the market is always hungry for bandwidths, the need for broadband circulators and isolators with improved transmission characteristics is thus clear and evident. 
   3. Objects and Advantages 
   Accordingly, it is an object of the invention to address one or more of the foregoing disadvantages or drawbacks of the prior art, and to provide such an improved method and apparatus to obtain improved broadband circulator/isolator operation by properly shaping the bias magnetic field. The bias magnetic field is thus shaped not only to satisfy the necessary circulation conditions for the circulator or isolator circuit, but also to partially magnetize the ferrite material thereby forming a gradual transition to warrant broadband operation; the radial component is reduced and discontinuity in magnetization is minimized, resulting in improved characteristics of the circulator or isolator performance. 
   Other objects will be apparent to one of ordinary skill, in light of the following disclosure, including the claims. 
   SUMMARY 
   In one aspect, the invention provides a method which allows the bias magnetic field expressed onto the circulator/isolator active region to be properly shaped to result a broad transmission band on one hand and improved performance characteristics on the other hand. The circulator/isolator circuit comprises of a ferrite junction exciting resonant standing modes invoking the frequency tracking condition, or the edge-mode operation is involved exploiting wave overlap at the adjacent ports. The radial component of the bias field is reduced so as to inhibit the excitation of non-circulation volume modes, and the discontinuity in magnetization is minimized at the edge so as to suppress the excitation of magnetostatic surface modes. This implies improved performance in isolation and in insertion loss of the circulator/isolator device. 
   In another aspect, the invention provides an apparatus which endows a mechanism enabling the bias magnetic field expressed onto the active region of a ferrite stripline circulator/isolator circuit to be adequately adjusted or tailored thereby to result broadband operation with improved performance characteristics. The mechanism includes field condenser means which are effective to gradually reduce the axial field intensity from the center to the edge. Or, the mechanism adopts the use of tapered magnets generating weaker fields at the edge than at the center, or both. 

   
     DRAWINGS 
     Figure 
     For a more complete understanding of the nature and objectives of the present invention, reference is to be made to the following detailed description and accompanying drawings, which, though not to scale, illustrate the principles of the invention, and in which: 
       FIG. 1  shows the prior art that a ferrite edge-mode isolator is operating admitting nonreciprocal wave propagation for broadband transmission. 
       FIG. 2  shows the prior art that a ferrite edge-mode circulator is operating admitting nonreciprocal wave propagation for broadband transmission. 
       FIG. 3  shows one example of the preferred embodiment of the invention that a ferrite stripline circulator/isolator circuit is biased by two permanent magnets in conjunction with a pair of flux condenser caps to properly shape the bias field in the active region. 
       FIG. 4  shows another example of the preferred embodiment of the invention that a ferrite stripline circulator/isolator circuit is biased by two permanent magnets in conjunction with 3 pairs of flux condenser disks to properly shape the bias field in the active region. 
       FIG. 5  shows another example of the preferred embodiment of the invention that a ferrite stripline circulator/isolator circuit is biased by two permanent magnets whose shapes show a tapered geometry to generate a bias field with an adequate profile in the active region. 
       FIG. 6  shows another example of the preferred embodiment of the invention that a ferrite stripline circulator/isolator circuit is biased by 3 pairs of permanent magnets with decreasing diameters to jointly generate a bias field with an adequate profile in the active region. 
       FIG. 7  shows one example of the preferred embodiment of the invention that the ferrite stripline circulator/isolator circuit consists of 3 joining ports sandwiched between a ferrite superstrate and a ferrite substrate covered by ground planes at top and bottom; impedance transformers are also shown and the circuit is devised for the edge-mode operation. 
       FIG. 8  shows another example of the preferred embodiment of the invention that the ferrite stripline circulator/isolator circuit consists of 3 joining ports sandwiched between composite ferrite superstrate and substrate assuming the triangular/strip geometry covered by ground planes at top and bottom; impedance transformers are also shown and the circuit is devised for the edge-mode operation. 
       FIG. 9  shows another example of the preferred embodiment of the invention that the ferrite stripline circulator/isolator circuit consists of 3 joining ports sandwiched between composite ferrite superstrate and substrate assuming the disk/ring geometry covered by ground planes at top and bottom; impedance transformers are also shown and the circuit is devised for the edge-mode operation. 
       FIG. 10  shows another example of the preferred embodiment of the invention that the ferrite stripline circulator/isolator circuit consists of 3 joining ports sandwiched between composite dielectric/ferrite superstrate and substrate assuming the disk/ring geometry covered by ground planes at top and bottom; impedance transformers are also shown and the circuit is devised for the edge-mode operation. 
       FIG. 11  shows one example of calculations that the axial and the radial magnetic fields generated by a pair of permanent magnets are plotted as a function of distance along the radial direction and the bias field is subject to no field shaping without employing flux shielding. 
       FIG. 12  shows another example of calculations that the axial and the radial magnetic fields generated by a pair of permanent magnets are plotted as a function of distance along the radial direction and the bias field is subject to field shaping via the use of a pair of condenser caps without employing flux shielding. 
       FIG. 13  shows another example of calculations that the axial and the radial magnetic fields generated by a pair of tapered permanent magnets are plotted as a function of distance along the radial direction and the bias field is subject to flux shielding. 
   

   REFERENCES NUMERALS 
   
     
       
             
             
             
           
         
             
                 
                 
             
           
           
             
                 
               001 
               Central Conductor 
             
             
                 
               002 
               Superstrate 
             
             
                 
               003 
               Substrate 
             
             
                 
               004, 005 
               Ground Plane 
             
             
                 
               011, 012, 013, 014, 015, 016, 017, 018 
               Magnet 
             
             
                 
               021, 022 
               Condenser Cap 
             
             
                 
               023, 024, 025, 026, 027, 028 
               Condenser Disk 
             
             
                 
               090 
               Flux Shield 
             
             
                 
                 
             
           
        
       
     
   
   DETAILED DESCRIPTION 
   Background and Rationale:— FIG. 1 ,  FIG. 2   
   Broadband 2-port isolators using the traveling displacement modes or edge modes were first reported by Hines in 1961. In  FIG. 1  a stripline is fabricated on top of a ferrite substrate and an dissipation pad, such as a thin layer of poor conductor, is deposited at one side of the substrate next to the stripline circuit. The superstrate, which consists of the same ferrite material, stacks above the substrate, and ground planes are attached to the substrate and superstrate at their outer surfaces. Superstrate and ground planes are not shown in  FIG. 1 . In the presence of a vertically applied bias magnetic field wave propagation long the stripline is nonreciprocal, to be highly transmitting along one direction, but highly attenuating along the other direction. That is, the RF-magnetic field pattern shown as dashed curves in  FIG. 1  displaces toward the edge of the stripline in the presence of the bias magnetic field, which is either shifting away from the dissipation pad, top drawing, or onto the dissipation pad, bottom drawing, resulting in little attenuation, or heavy attenuation, respectively. Hynes has shown the operation of an edge-mode isolator provided a 3:1 transmission band, which is about the same bandwidth reported by Mathew and Weisz in 1983 utilizing a circulator junction with varying magnetization. 
   Edge-mode traveling-wave operation can also be realized by the 3-port junction geometry, as suggested by How in 2005. In  FIG. 2 ,  3  joining ports exhibiting a 3-fold symmetry rather than 2 aligning ports are shown depositing on top of a triangularly shaped ferrite substrate. Again, a similar superstrate covers the substrate on top and two ground planes are applied at their respective outer surfaces. Superstrate and ground planes are not shown in  FIG. 2 . To operate a bias magnetic field is applied along the junction-thickness direction launching the displacement modes or the edge modes to travel, in a manner analogous to the Hines&#39; isolator modes shown in  FIG. 1 . As a consequence, edge modes couple strongly from ports  1  to  2 , due to overlap of waves with phase coherency, but decouples strongly from ports  1  to  3  in lack of the required wave overlap. This results in the desired circulator operation that electromagnetic waves entering port  1  can only exit from port  2 , from port  2  to port  3 , and from port  3  to port  1 , but not the other way around. As such,  FIG. 2  does not need a dissipation pad, as in contrast to the isolator circuit shown in  FIG. 1 . In  FIG. 2  the dashed curves depict schematically the RF magnetic field patterns illustrating the coupling and decoupling situations for wave propagation in ports. More circulator ports other than 3 can be equally assumed in  FIG. 2 . 
   In order to widen the transmission band of an edge-mode circulator it is necessary to enforce phase coherency for wave propagation between the input and the output ports across a broad frequency range. That is, phase coherency needs to be maintained over one half the wavelength distance, which is denoted as λ/2 in  FIG. 2 . Therefore, high frequency signals couple mostly strongly near the center of the circuit, and low frequency signals couple most strongly near the edge of the circuit. Since the operation of a ferrite device requires the magnetization to scale with frequency, which is known as gyromagnetic ratio, one expects a broadband edge-mode circulator to result if the circulator circuit shows different magnetizations to be scaled with the propagation wavelengths, to be large at the center, but small at the edge. In addition, the internal magnetic field needs also to scale along distance, so as to follow and track the circulation condition over frequencies. This means that the bias field needs to be reduced in accordance with the magnetization change from the center of the circulator circuit toward edge. 
   The other advantage of reducing the magnetization and the internal field to nearly zero at the edge of a circulator circuit is to suppress magnetostatic surface waves (MSWs). MSWs are excited near the edge of a circulator circuit whenever there exists discontinuities in magnetization. MSWs are manifested as leaky waves whose presence can degrade significantly the isolation and insertion-loss performance of the circuit. Performance degradation can also result if non-circulation volume modes are excited within the active region of the circulator circuit due to the non-vanishing radial component of the bias magnetic field; only the axial component of the bias field is responsible for the circulation operation. Radial field appears mostly at the edge of a circulator circuit, which can be minimized if the bias field is all reduced near the edge of the circuit. Although the above discussion is made with the edge-mode circulator shown in  FIG. 2 , it can also be applied to the resonant modes or the standing modes excited with a ferrite circulator junction incorporating the frequency-tracking condition introduced by Wu and Rosenbaum in 1974. Since an isolator circuit can be derived from a circulator circuit by connecting the irrelevant ports with dummy loads, the following discussions concern only the circulator circuits. 
   Preferred Embodiments of the Present Invention:— FIG. 3 ,  FIG. 4 ,  FIG. 5 ,  FIG. 6   
   To illustrate the present invention explicit examples are given in  FIG. 3 ,  FIG. 4 ,  FIG. 5 ,  FIG. 6 , which are all effective in shaping the bias magnetic field in the active region of a ferrite stripline circulator. In  FIG. 3 ,  FIG. 4 ,  FIG. 5 ,  FIG. 6  a ferrite stripline circulator circuit is defined by Central Conductor  001  sandwiched between Superstrate  002  and Substrate  003  with Ground Plane  004  and  005  attached at respective outer surfaces from top and below. Explicit examples of ferrite stripline circulator circuits are shown in  FIG. 7 ,  FIG. 8 ,  FIG. 9 ,  FIG. 10  which will be discussed in the next section. In  FIG. 3 ,  FIG. 4 ,  FIG. 5 ,  FIG. 6  the bias magnetic field is generated by Magnet  011  and  012  and Flux Shield  090  is enclosed at outside providing the return path for the generated magnetic fluxes. In  FIG. 3  Condenser Cap  021  and  022  are used, inserted between Magnet  011  and  012  below and above the active region of the ferrite stripline circulator circuit. Condenser Cap  021  and  022  are made of soft magnetic materials showing a high magnetic permeability serving as a low magnetic-reluctance path for magnetic fluxes. As such, magnetic fluxes are attracted and condensed near the center of the active region of the ferrite stripline circulator circuit thereby being able to effectively shape the bias magnetic field therein. 
   Condenser Cap  021  and  022  in  FIG. 3  may be sliced into thin disks with shrinking diameters, as shown by Condenser Disk  023 ,  024 ,  025 ,  026 ,  027 ,  028  in  FIG. 4 . Condenser Cap  021  and  022  in  FIG. 3  and Condenser Disk  023 ,  024 ,  025 ,  026 ,  027 ,  028  in  FIG. 4  can be made of a magnetic metal such as iron, nickel, cobalt, or their alloys. Alternatively, magnetic shaping can be realized via the use of shaped magnets. This is shown in  FIG. 5  where Magnet  013  and  014  are shaped into (truncated) circular cones capable of generating more magnetic fluxes at the center than at the edge of the ferrite stripline circulator circuit. Magnet  013  and  014  in  FIG. 5  can be sliced into disks with shrinking diameters, as shown by Magnet  015 ,  016 ,  017 ,  018  in  FIG. 6 . Typical magnetic profiles appearing in the active region of the circulator circuit shown with  FIG. 3 ,  FIG. 4 ,  FIG. 5 ,  FIG. 6  have been calculated, as shown by  FIG. 11 ,  FIG. 12 ,  FIG. 13  to be discussed shortly. Note that Magnet  011 ,  012 ,  013 ,  014 ,  015 ,  016 ,  017 ,  018 , Condenser Cap  021  and  022 , and Condenser Disks  023 ,  024 ,  025 ,  026 ,  027 ,  028  shown in  FIG. 3 ,  FIG. 4 ,  FIG. 5 ,  FIG. 6  have assumed the circular symmetry, and it is not necessary. For example, the 3-fold or 6-fold symmetry can be assumed and Magnet  011 ,  012 ,  013 ,  014 ,  015 ,  016 ,  017 ,  018 , Condenser Cap  021  and  022 , and Condenser Disk  023 ,  024 ,  025 ,  026 ,  027 ,  028  shown in  FIG. 3 ,  FIG. 4 ,  FIG. 5 ,  FIG. 6  can be shaped into (truncated) prismatic or hexagonal cones to effectively shape the magnetic field to achieve the intended operation of the ferrite stripline circulator. 
   Further Illustration of the Present Invention:— FIG. 7 ,  FIG. 8 ,  FIG. 9 ,  FIG. 10   
     FIG. 7 ,  FIG. 8 ,  FIG. 9 ,  FIG. 10  show further illustrations of the preferred embodiments of the present invention disclosed with  FIG. 3 ,  FIG. 4 ,  FIG. 5 ,  FIG. 6 . That is, Central Conductor  001 , Superstrate  002 , Substrate  003 , Ground Plane  004  and  005  shown in  FIG. 3 ,  FIG. 4 ,  FIG. 5 ,  FIG. 6  are expanded to show the explicit the ferrite stripline circulator circuit. In  FIG. 7  Superstrate  002  and Substrate  003  are two pieces of ferrite slabs enclosing Central Conductor  001  from top and below, and Central Conductor  001  is shown as a Y-branch with 3 joining ports. Transformer sections are included with the ports so as to match the impedance differences for broadband operation. In the presence of a non-uniform magnetic bias field the induced magnetization within the ferrite materials needs not to be uniform. That is, when a varying bias magnetic field is impressed with a maximum intensity at center vanishing at edge, Superstrate  002  and Substrate  003  are magnetized accordingly so that maximum magnetization is attained at the center of the circulator circuit, decreasing gradually to zero at the edge. In other words, Superstrate  002  and Substrate  003  need not to be fully magnetized to perform the broadband operation, and the vanishing magnetization at the circulator edge assures minimum generation of MSWs. 
     FIG. 8  shows the other possibility that ferrites of different saturation magnetization are used in conjunction with a varying bias magnetic field. In  FIG. 8  Superstrate  002  and Substrate  003  assume a composite structure consisting of triangularly/trapezoidally shaped ferrite blocks or strips with decreasing saturation magnetization, μ 1 &gt;μ 2 &gt;μ 3 ; Central Conductor  001  is shown as a Y-branch with 3 joining ports and transformer stubs are included with the ports so as to match impedance differences for broadband operation. In comparison to  FIG. 7  the varying saturation magnetization μ 1 , μ 2 , and μ 3  shown with  FIG. 8  have the advantage of generating a more fully magnetized magnetization profile over Superstrate  002  and Substrate  003  than if one ferrite material is used, say, μ 1 , which favors applications toward higher power ratings. However, the discontinuity in saturation magnetization μ 1 , μ 2 , and μ 3  means the likelihood in generating MSWs thereby offsetting this power-rating advantage. 
     FIG. 9  shows the same composite ferrite structure of  FIG. 8  except that the 3-fold symmetry assumed by  FIG. 8  is replaced by the circular symmetry. The other difference between  FIG. 8  and  FIG. 9  is that transformer stubs are used by  FIG. 8  and transformer sections are used by  FIG. 9 , same as those used by  FIG. 7 . Transformer section shown in  FIG. 9  is able to matching a decreasing impedance difference, whereas those shown in  FIG. 7  is to match an increasing impedance difference. In  FIG. 7 ,  FIG. 8 ,  FIG. 9  the circulator operation launches edge modes in the ferrite materials, whereas in  FIG. 10  resonant modes or standing modes are excited, reinforcing the frequency tracking condition thereby to insure the broadband circulation operation of a ferrite junction. In comparison to  FIG. 9  the outermost ferrite ring, say, μ 3 , is replaced by a dielectric sleeve, ε, or a transformer, capable of matching impedance difference occurring therein. Again, in  FIG. 10  μ 1 &gt;μ 2 . 
   Further Illustration of the Present Invention:— FIG. 11 ,  FIG. 12 ,  FIG. 13   
     FIG. 11 ,  FIG. 12 ,  FIG. 13  show the calculated bias magnetic fields within the ferrite materials of a stripline circulator circuit. In  FIG. 11  the bias field arises from 2 pieces of permanent magnets placed above and below the circulator circuit shown with  FIG. 3 ,  FIG. 4 ,  FIG. 5 ,  FIG. 6  and with  FIG. 7 ,  FIG. 8 ,  FIG. 9 ,  FIG. 10 . In  FIG. 11  normalized units are used for which the length is normalized with respect to the radius of the magnets and the magnetic field is in unit of the saturation magnetization of the magnets. In  FIG. 11  the magnets are of radius  1 , thickness 0.25, and the substrate/superstrate is of thickness 0.1. No flux shield is used in  FIG. 11  and the ground planes are assumed of thickness 0. In  FIG. 11  the solid curve shows the axial component of the resultant bias magnetic field, B z , and the dashed curve shows the radial component of the bias field, B ρ , both of which are calculated at the mid-plane positions of the ferrite materials. In  FIG. 11  it is seen that without incorporating magnetic shaping, the resultant bias magnetic field has a profile far from desirable, not only because the axial component shows an increasing magnitude from center toward edge, but also significant radial component appear near the edge of the circulator active region. It is thus advantageous to incorporate magnetic shaping so as to entail the broadband operation, as discussed with  FIG. 12  and  FIG. 13  below. 
     FIG. 12  shows the calculated bias magnetic field when a pair of condenser caps are used. The condenser caps are assumed to have an infinite permeability; they assume the geometry of a truncated circular cone of thickness 0.25 and radii 1 and 0.5. Other parameters are the same as used with calculations of  FIG. 1 . The calculated axial and radial components of the bias field are shown as B z  and B ρ , respectively. In  FIG. 12  it is seen that the axial component B z  has been shaped into a more desirable profile, decreasing gradually from the center of the circulator circuit toward edge. However, the radial component B ρ  still shows a bump at the edge of the circulator active region, which can be eliminated by adopting the other magnetic shaping configuration calculated with  FIG. 13 . In  FIG. 13  partially cone-shaped magnets are used which are composed of two portions: the un-tapered portion is of a thickness 0.25 and the tapered portion is also of a thickness 0.25. In  FIG. 13  flux shield has been employed, and the other parameters are the same as used with  FIG. 11  and  FIG. 12 . In  FIG. 13  it is seen that the axial component of the bias field, B z , shows a desirable linear taping profile, and the radial component, B ρ , has been almost totally eliminated. Preliminary measurement of a ferrite stripline circulator with the magnetic-shaping bias configuration shown with  FIG. 13  has revealed a bandwidth broader than a 5:1 ratio with improved transmission characteristics; it outperforms the prior art significantly. In  FIG. 12  and  FIG. 13  linearly tapered condenser caps and magnets are used, respectively; other tapering geometries can also be equally used. 
   CONCLUSIONS 
   The present invention teaches a method and an apparatus enabling the bias magnetic field over the active region of a ferrite stripline circulator/isolator circuit to be properly shaped, showing a maximum axial component at the circuit center decreasing gradually toward edge. The radial component is also reduced. This allows the circulator/isolator circuit to result a broad bandwidth with improved transmission characteristics.