Patent Publication Number: US-6667672-B2

Title: Compact high power analog electrically controlled phase shifter

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims priority of U.S. Provisional Patent Application No. 60/298,277 filed Jun. 14, 2001 entitled COMPACT HIGH POWER ANALOG ELECTRICALLY CONTROLLED PHASE SHIFTER. 
    
    
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT N/A 
     BACKGROUND OF THE INVENTION 
     The present invention relates generally to analog phase shifters, and more specifically to high power ferrite microwave phase shifters. 
     Ferrite phase shifters are known that employ an applied magnetic field to vary the permeability of ferrite, thereby controlling the velocity and thus the phase shift of signals propagating through the phase shifter device. A conventional ferrite phase shifter comprises a rectangular waveguide structure, a ferrite slab loading and at least partially filling the waveguide, and a coil of wire wrapped around the waveguide. The wire coil is configured to carry a variable control current for generating a magnetic field, which is transversely applied to the ferrite slab to shift the phase of signals propagating in the rectangular waveguide structure. 
     One shortcoming of the conventional ferrite phase shifter is that the phase shifter device can become rather large and bulky when configured to carry lower frequency microwave signals. Such large bulky ferrite microwave phase shifters can also be costly to manufacture and thus not amenable to high volume production processes. 
     It would therefore be desirable to have a more compact ferrite phase shifter for handling microwave signals. Such a ferrite microwave phase shifter would be low cost and suitable for manufacturing in high volume production processes. It would also be desirable to have a compact ferrite microwave phase shifter that can be used in high power applications. 
     BRIEF SUMMARY OF THE INVENTION 
     In accordance with the present invention, a high power ferrite microwave phase shifter is provided that is both compact and low cost. Benefits of the presently disclosed invention are achieved by providing a waveguide structure that not only reduces the size of the phase shifter device, but also enhances the effectiveness of applied Radio Frequency (RF) magnetic fields. 
     In one embodiment, the high power ferrite microwave phase shifter comprises a waveguide structure including a first substantially cylindrical element and a second substantially cylindrical element, in which the radius of the second cylinder is less than the radius of the first cylinder. The second cylindrical element is disposed within the first cylindrical element such that the first and second cylinders have a common axis of symmetry. The waveguide structure further includes a first septum formed as a disk and disposed within the second cylinder. The disk has a pie-shaped aperture formed therethrough that extends through the circumference of the disk and tapers to the disk center. The disk is centrally disposed within the second cylindrical element such that the first cylinder, the second cylinder, and the disk share the same axis of symmetry. The second cylinder has an opening formed therethrough that extends the full length of the second cylinder. The inner wall of the second cylinder is coupled to the circumferential edge of the disk such that the opening in the second cylinder is aligned with the pie-shaped aperture in the disk. The second cylinder is thus coupled to the disk without obstructing the pie-shaped aperture. The waveguide structure further includes a second planar septum that extends from the inner wall of the first cylinder to the disk center while bisecting the pie-shaped disk aperture. The second septum is coupled to the inner wall of the first cylinder and the disk at the disk center such that the second septum is approximately perpendicular to the plane of the disk. 
     In a preferred embodiment, the ferrite microwave phase shifter is loaded and totally filled with ferrite. The ferrite microwave phase shifter includes a coil of wire wrapped around the circumference of the first cylinder and configured to carry a variable control current for generating an RF magnetic field, which is transversely applied to the ferrite for controllably shifting the phase of signals propagating in the compact waveguide structure. 
     Other features, functions, and aspects of the invention will be evident from the Detailed Description of the Invention that follows. 
    
    
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING 
     The invention will be more fully understood with reference to the following Detailed Description of the Invention in conjunction with the drawings of which: 
     FIGS. 1 a - 1   c  are end and cross-sectional views of a rectangular waveguide structure illustrating the evolution of the present invention; 
     FIGS. 2 a - 2   b  are end views of a folded rectangular waveguide structure further illustrating the evolution of the present invention; 
     FIGS. 3 a - 3   b  are end views of ridge waveguide structures further illustrating the evolution of the present invention; 
     FIGS. 4 a - 4   e  are plan, cross-sectional, and perspective views of a high power ferrite microwave phase shifter including a waveguide structure according to the present invention; and 
     FIG. 5 is a flow diagram illustrating a method of fabricating the high power ferrite microwave phase shifter of FIGS. 4 a - 4   e.   
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     U.S. Provisional Patent Application No. 60/298,277 filed Jun. 14, 2001 is incorporated herein by reference. 
     A high power ferrite microwave phase shifter is disclosed that has both a reduced size and a reduced cost of manufacture. The presently disclosed ferrite microwave phase shifter incorporates a waveguide structure that reduces the size of the phase shifter device while enhancing the effectiveness of applied Radio Frequency (RF) magnetic fields. 
     FIGS. 1 a - 1   c ,  2   a - 2   b , and  3   a - 3   b  depict the evolution of the presently disclosed ferrite microwave phase shifter. Specifically, FIG. 1 a  depicts an illustrative embodiment of a rectangular waveguide  100  having a rectangular cross-section in the x-y plane. It should be understood that the rectangular waveguide  100  extends longitudinally along the z-axis, which defines the propagation direction of RF energy in the guide. The rectangular waveguide  100  also has a longer transverse dimension along the x-axis defining its width “a”, and a shorter transverse dimension along the y-axis defining its height “b”. 
     Those of ordinary skill in this art will appreciate that a rectangular waveguide such as the rectangular waveguide  100  normally has an aspect ratio of 2:1. Further, the rectangular waveguide  100  with a 2:1 aspect ratio has an associated cutoff wavelength λc equal to twice the width of the guide, i.e., λc=2a. 
     FIG. 1 b  depicts an RF propagation mode  104  for the rectangular waveguide  100 , which is configured to conduct RF energy. In the illustrated embodiment, the RF propagation mode  104  is the TE 10  or dominant mode of the rectangular guide  100 . According to the RF propagation mode  104 , both an electric (E) field and a magnetic (H) field exist inside the rectangular waveguide  100 . The E field has force lines directed along the y-axis, and the H field has force lines orthogonal to the force lines of the E field. Further, the amplitude of the E field is maximum at the center of the rectangular waveguide  100  and decreases upon approaching the short sides of the guide. 
     FIG. 1 c  is a cross-sectional view of the rectangular waveguide  100  along a line  1   c — 1   c  further depicting the RF propagation mode  104  for the guide. Specifically, FIG.1 c  depicts the circular polarization of the H field inside the rectangular waveguide  100 . 
     FIG. 2 a  depicts an illustrative embodiment of a folded rectangular waveguide  200 . For example, the folded rectangular waveguide  200  may be formed by conceptually folding the longer transverse dimension of the rectangular waveguide  100  (see FIG. 1 a ) back on itself. In the illustrated embodiment, the folded rectangular waveguide  200  has a rectangular cross-section in the x-y plane, a longer transverse dimension measuring a/2 along the y-axis, and a shorter transverse dimension measuring 2b along the x-axis. Further, the rectangular waveguide  200  has a septum  202  coupled to one of the short sides of the guide and extending in the center of the guide along the y-axis. Like the rectangular waveguide  100  (see FIG. 1 a ), the folded rectangular waveguide  200  including the septum  202  extends longitudinally along the z-axis, which defines the propagation direction of RF energy in the guide. Moreover, the folded rectangular waveguide  200  has an associated cutoff wavelength λc equal to 2a, which is four times the longer transverse dimension a/2 of the guide. It is noted that by conceptually folding the rectangular waveguide  100  (see FIG. 1 a ) to form the folded waveguide structure  200 , as described above, at least one of the dimensions of the rectangular waveguide  100  decreases in size by about 50%. 
     FIG. 2 b  is an end view of the folded rectangular waveguide  200  depicting an RF propagation mode  204  for the guide, which is configured to conduct RF energy. As shown in FIG. 2 b , the RF propagation mode  204  is folded about the septum  202 . According to this RF propagation mode  204 , both an E field and an H field exist inside the guide  200 . The E field has force lines emanating from the septum  202 , and the H field has force lines orthogonal to the force lines of the E field. Further, the amplitude of the E field is maximum at the center of the guide parallel to the y-axis and decreases upon approaching the short side of the guide at the base of the septum  202 . It should be understood that the H field inside the folded rectangular waveguide  200  has a circular polarization like the H field inside the rectangular waveguide  100  (see FIG. 1 c ). 
     FIG. 3 a  depicts an illustrative embodiment of another folded rectangular waveguide  300 . It is noted that the folded rectangular waveguide structure  300  is like the folded rectangular waveguide structure  200  (see FIG. 2 a ) except that the folded rectangular waveguide  300  includes a crosspiece  306  perpendicularly coupled to a septum  302  to form a “T”. Both the septum  302  and the crosspiece  306  extend coextensively along the z-axis. The crosspiece  306  is configured to increase the current carrying area of the rectangular waveguide  300  and thus reduce losses. Including the crosspiece  306  in the folded rectangular waveguide  300  also increases the capacitance at the center of the guide and decreases the inductance at side sections of the guide, thereby reducing the effective impedance of the guide. As a result, the impedance of the folded rectangular waveguide  300  can be brought closer to 50Ω to facilitate impedance matching between the guide and a standard coaxial connector. 
     Moreover, including the crosspiece  306  in the folded rectangular waveguide  300  causes the performance of the guide to be similar to the performance of a ridge waveguide. For example, the rectangular waveguide structure  300  can be modified to approximate a ridge waveguide by conceptually inserting hinges  308  at opposing ends of the crosspiece  306 , and conceptually inserting hinges  310  at respective corners of the guide near the hinges  308 . Next, the rectangular waveguide  300  can be conceptually unfolded at the hinges  308  and  310  to achieve a single-ridge waveguide structure, as depicted in FIG. 3 b . It is noted that the cutoff wavelength λc associated with the single-ridge waveguide structure can be increased and the effective impedance of the ridge waveguide can be reduced by decreasing a gap width g (see FIG. 3 b ) of the ridge waveguide. It follows that a corresponding cutoff wavelength λc and a corresponding effective impedance of the folded rectangular waveguide  300  can be similarly adjusted by decreasing the gap width g (see FIG. 3 a ) between the crosspiece  306  and the adjacent short side of the guide. It should be understood that the RF propagation mode (not shown) inside the folded rectangular waveguide  300  is like the RF propagation mode  204  (see FIG. 2 b ) inside the folded rectangular waveguide  200 . 
     FIG. 4 a  depicts an illustrative embodiment of a ferrite microwave phase shifter  400 , in accordance with the present invention. FIGS. 4 b - 4   c  depict cross-sectional views of the ferrite microwave phase shifter  400  along lines  4   b — 4   b  and  4   c — 4   c , respectively, and FIGS. 4 d - 4   e  depict perspective views of the ferrite microwave phase shifter  400 . In the illustrated embodiment, the ferrite microwave phase shifter  400  includes a waveguide  401  that may be formed by conceptually bending the folded rectangular waveguide  300  (see FIG. 3 a ) along the longitudinal dimension until opposing ends of the waveguide structure  300  meet. 
     As shown in FIGS. 4 a - 4   e , the waveguide structure  401  includes a first substantially cylindrical element  420 , a second substantially cylindrical element  422 , a first septum  424 , and a second septum  430 . Specifically, the radius r 2  of the second cylinder  422  is less than the radius r 1  of the first cylinder  420 . It is noted that the difference between the radii r 1  and r 2  generally corresponds to the gap width g of the folded rectangular waveguide  300  (see FIG. 3 a ). The second cylinder  422  is disposed within the first cylinder  420  such that the first and second cylinders  420  and  422  have a common axis of symmetry. The first septum  424  is formed as a disk and centrally disposed within the second cylinder  422  such that the first cylinder  420 , the second cylinder  422 , and the disk  424  share the same axis of symmetry. The disk  424  has a pie-shaped aperture  426  formed therethrough, which extends through the circumference of the disk  424  to the disk center. The second cylinder  422  also has an opening  428  (see FIG. 4 d ) formed therethrough that extends the full length of the cylinder. The inner wall of the second cylinder  422  is coupled to the circumferential edge of the disk  424  such that the opening  428  in the second cylinder  422  is aligned with the pie-shaped aperture  426  in the disk  424 . The second cylinder  422  is thus coupled to the disk  424  so as not to obstruct the pie-shaped aperture  426 . The second septum  430  of the waveguide structure  401  extends from the inner wall of the first cylinder  420  to the disk center while bisecting the pie-shaped disk aperture  426 . The second septum  430  is coupled to both the inner wall of the first cylinder  420  and the disk  424  at the disk center, and is oriented to be approximately perpendicular to the plane of the disk  424 . The second septum  430  is configured to separate an input of the waveguide  401  from an output of the guide. 
     It should be appreciated that the waveguide  401  is loaded and at least partially filled with ferrite. For example, the ferrite loading the waveguide structure  401  may comprise lithium ferrite or any other suitable ferrite material. In the preferred embodiment, the waveguide structure  401  is totally filled with ferrite  440 , as shown in FIG. 4 e . Further, the waveguide  401  includes cover portions  432  and  434  (see FIGS. 4 b - 4   c ) configured to enclose the ferrite  440  within the guide and thus complete the overall structure of the guide. It is noted that by totally filling the waveguide structure  401  with the ferrite  440 , the size of the guide can be reduced by an amount proportional to the square root of the dielectric constant ∈ r  of the ferrite material. For example, in the event the dielectric constant ∈ r  of the ferrite  440  is equal to 14, the size of the guide  401  can be reduced by a factor of (14) 1/2  or about 3.75:1. Moreover, by totally filling the waveguide  401  with the ferrite  440 , the maximum phase shift of signals propagating through the guide can be achieved. 
     It should also be appreciated that a magnetic field can be generated and applied to the ferrite  440  loading the waveguide  401  to vary the permeability of the ferrite  440 , thereby controlling the velocity and thus the phase shift of signals propagating through the ferrite microwave phase shifter  400 . In the presently disclosed embodiment, the ferrite microwave phase shifter  400  includes a coil of wire (not shown) wrapped around the circumference of the first cylinder  420 . The wire coil is configured to carry a variable control current for generating the magnetic field, which is transversely applied to the ferrite  440 . Specifically, the RF magnetic field is applied in line with the axis of symmetry of the first cylinder  420 , the second cylinder  422 , and the disk  424 . It should be understood that the coil of wire is described above for purposes of illustration, and that alternative structures for electromagnetically generating the applied magnetic field may be employed. Further, in alternative embodiments, the magnetic field may be applied by one or more permanent magnets. 
     According to the RF propagation mode  104  for the rectangular waveguide  100  (see FIG. 1 a ), the H field inside the guide  100  has a circular polarization (see FIG. 1 c ). As shown in FIG. 1 c , the circularly polarized H fields inside the guide  100  are in a “side-by-side” orientation. According to the RF propagation mode for the presently disclosed waveguide  401 , the H field inside the guide  401  also has a circular polarization. However, because the RF propagation mode for the waveguide  401  is folded about the disk-shaped septum  424  much like the RF propagation mode  204  for the folded rectangular waveguide  200  (see FIG. 2 b ), circularly polarized H fields on opposite sides of the disk-shaped septum  424  inside the guide  401  are in a “back-to-back” orientation instead of the above-described side-by-side orientation. Because these back-to-back H fields have the same sense of circular polarization, the effectiveness of the RF magnetic field applied to the ferrite  440  for varying the ferrite permeability is enhanced. 
     The operation of the ferrite microwave phase shifter  400  will be better understood with reference to the following discussion. Ferrite material is characterized as having variable permeability. When in the presence of a biasing magnetic field, the iron content of the ferrite material is “stressed”. Specifically, the spin of the iron atoms in the ferrite material is precessed by the biasing magnetic field. Further, an RF magnetic field applied to the ferrite material works either with or against this precession, thereby causing the permeability or inductive quality of the ferrite material to either increase or decrease. 
     Circularly polarized magnetic fields can be used to exploit this variable permeability characteristic of ferrite. For example, circularly polarized biasing magnetic fields can be generated to cause a circular precession that allows the maximum interaction between the spin of the iron atoms precessed by the biasing magnetic field and the atomic spin precessed by the applied RF magnetic field. The circularly polarized permeability of ferrite may be expressed as 
     
       
         μ+=1+ γMo/ ( γHα−ω )  (1) 
       
     
     
       
         μ−=1+ γMo/ ( γHα+ω ),  (2) 
       
     
     in which “γ” is the efficiency characteristic of the ferrite, “Mo” is the saturation characteristic of the ferrite, and “Hα” is the magnetic line width, which may be regarded as a magnetic Quality factor (Q) value. The respective results of equations (1) and (2) above may be multiplied by the fill factor of the waveguide containing the ferrite to obtain a final permeability value. It is noted that in this discussion, the fill factor of the guide may be regarded as being approximately equal to unity. 
     Those of ordinary skill in the art will appreciate that the single-ridge waveguide structure may be employed to widen the bandwidth for any outside dimension of the guide. The lower impedance at the center of the ridge waveguide and the higher impedance at the outside edges of the guide act as a transformer that increases the cutoff wavelength λc while widening the guide bandwidth. As described above with reference to the folded rectangular waveguide  200  (see FIGS. 2 a - 2   b ) and the single-ridge waveguide  300  (see FIGS. 3 a - 3   b ), the RF propagation mode for the guides  200  and  300  is folded about the septa  202  and  302 , respectively. 
     As also described above, the cutoff wavelength λc associated with the rectangular waveguide  100  may be expressed as 
     
       
           λc= 2 a,   (3) 
       
     
     in which “a” is the width dimension on the inside of the guide. When the rectangular waveguide  100  is folded to form the folded rectangular waveguide structures  200  and  300 , the RF propagation mode curves around the region of the fold. The RF field curvature thus follows “π” conventions instead of following a straight path, as in the rectangular waveguide  100 . 
     Accordingly, in the region of the fold of the folded rectangular waveguide, the height dimension “b” on the inside of the guide is replaced by “πb/2”. The cutoff wavelength λc associated with the folded rectangular waveguide may therefore be expressed as 
     
       
           λc= 2(i a−b+πb/2), 
       
     
     or 
     
       
           λc= 2( a+b (π/2−1)).  (4) 
       
     
     It is noted that the relatively thin septum  202  of the folded rectangular waveguide  200  (see FIG. 2 a ) is a high current carrying area, which can cause increased losses due to its reduced cross-section. By providing the crosspiece  306  to form a widened T-top on the septum  302  of the folded rectangular waveguide  300  (see FIG.  3   a ), the T configuration of the septum  302  and the crosspiece  306  can carry an increased amount of current with reduced loss. This T configuration can also lower the impedance of the folded rectangular waveguide structure. 
     As shown in FIG. 1 c , clockwise and counter-clockwise alternating loops of magnetic field pass down the rectangular waveguide  100 , in which the plane of the alternating loops is parallel to the broad side of the guide. On one side of the waveguide  100 , the loops of magnetic field are oriented in a clockwise direction, while on the other side of the guide the magnetic field loops are oriented in a counter-clockwise direction. The rectangular waveguide  100  relies on these clockwise and counter-clockwise alternating magnetic field loops for providing differential phase shift. It is noted that in order to make use of both sides of the rectangular waveguide  100 , two opposite biasing magnetic fields, one on each side of the guide, are typically required. 
     By folding the rectangular waveguide  100  (see FIG. 1 a ) along the propagation direction of RF energy in the guide to form the folded rectangular waveguide  200  (see FIG. 2 a ) and the folded rectangular waveguide  300  (see FIG. 3 a ), the clockwise and counter-clockwise alternating magnetic field loops come into alignment and the perceived sense of circular polarization, when viewed from the broad side of the guide, is the same. The magnetic biasing required for the guides  200  and  300  can thus be achieved using a single magnetic field passing through both channels of the guides disposed on opposite sides of the septa  202  and  302 , respectively. Moreover, by bending the folded rectangular waveguide  300  (see FIG. 3 a ) along the longitudinal dimension of the guide to form the compact waveguide structure  401  (see FIGS. 4 a - 4   e ), the maximum electrical length can be achieved in the compact waveguide  401  while maintaining the magnetic field properties of the folded rectangular waveguide  300 . 
     It is noted that both sides of the RF magnetic field propagating in the waveguide structure  401  extend toward the center of the disk  424  (see FIG. 4 a ). Both the biasing magnetic field and the applied RF magnetic field are thus localized to the center region of the guide. Moreover, by totally filling the waveguide  401  with the ferrite  440 , the size of the guide is minimized and the fill factor is maximized, which in turn maximizes the variability of the ferrite permeability for enhanced control of the phase shift of signals propagating through the ferrite microwave phase shifter  400 . 
     A method of fabricating the ferrite microwave phase shifter  400  including the waveguide structure  401  (see FIGS. 4 a - 4   e ) is illustrated by reference to FIG.  5 . As depicted in step  502 , first and second cylindrical elements are provided, in which the radius of the second cylinder is less than the radius of the first cylinder. Next, an opening is formed, as depicted in step  504 , through the second cylinder extending the full length of the cylinder. The second cylinder is then disposed, as depicted in step  506 , within the first cylinder such that the first and second cylinders have a common axis of symmetry. Next, a first disk-shaped septum is provided, as depicted in step  508 . A pie-shaped aperture is then formed, as depicted in step  510 , through the disk extending through the circumference of the disk and tapering to the disk center. Next, the disk is centrally disposed, as depicted in step  512 , within the second cylinder such that the first cylinder, the second cylinder, and the disk share the same axis of symmetry. The inner wall of the second cylinder is then coupled, as depicted in step  514 , to the circumferential edge of the disk such that the opening in the second cylinder is aligned with the pie-shaped aperture in the disk. Next, a second planar septum is provided, as depicted in step  516 . The second septum is then coupled, as depicted in step  518 , to the inner wall of the first cylinder and the disk at the disk center such that the second septum bisects the pie-shaped aperture and is approximately perpendicular to the plane of the disk. Finally, the ferrite microwave phase shifter is totally filled, as depicted in step  520 , with ferrite. An RF magnetic field may then be transversely applied to the ferrite for controllably shifting the phase of signals propagating through the phase shifter device. 
     It will further be appreciated by those of ordinary skill in the art that modifications to and variations of the above-described compact high power analog electrically controlled phase shifter may be made without departing from the inventive concepts disclosed herein. Accordingly, the invention should not be viewed as limited except as by the scope and spirit of the appended claims.