Abstract:
A circulator ( 100 ) is comprised of a transmission line three port Y junction ( 104 ). At least one substantially cylindrical cavity structure ( 113, 115  or  117 ) having a plurality of chambers is disposed adjacent to the Y junction and contains a ferromagnetic fluid ( 114 ). One or more magnets ( 112 ) are provided for applying a magnetic field ( 118 ) to the ferromagnetic fluid and the Y junction in a direction normal to a plane defined by said Y junction. A composition processor ( 301 ) is provided for changing a volume of ferromagnetic fluid in at least one among the plurality of chambers in response to a control signal to selectively alter the operating regions of the circulator.

Description:
BACKGROUND OF THE INVENTION 
   1. Statement of the Technical Field 
   The present invention relates to the field of circulators and isolators, and more particularly to circulators and isolators that have variable RF properties. 
   2. Description of the Related Art 
   Circulators and isolators are devices that typically have three or more ports arranged in a ring and which provide unique RF transmission paths. An isolator is a three port circulator in which the third one of the ports has been terminated. Accordingly, for convenience, references to circulators herein shall be understood to also include isolators. Each type of device provides one way sequential transmission of power between its ports. For example, power in at port  1  couples only to port  2  with the exclusion of all other ports. More particularly, circulators and isolators are designed to allow RF energy to pass from a first port to a second port in a forward direction with little or no insertion loss, but present a high degree of attenuation for RF energy passing in a reversed direction from the second port to the first port. Similarly, RF energy is allowed to pass from the second port to a third port with low insertion loss, but is highly attenuated in the direction from the third port to the second port. 
   Circulators are often used to allow a receiver and a transmitter to share a common antenna by connecting a transmitter to port  1 , an antenna to port  2  and a receiver to port  3 . This arrangement provides for concurrent transmission and reception of signals. The antenna is always connected to the receiver and the transmitter but the receiver is isolated from the transmitted signals. 
   Most commonly, the fabrication of a circulator generally involves a three port Y junction of either rectangular waveguides or stripline that is loaded with ferrite cylinders or discs that are magnetized in a direction normal to the plane of the junction. Notably, while most circulators use a fixed direction of magnetic field and circulation, it is known in the art that the direction of circulation can be reversed by reversing the direction of the biasing magnetic field. This feature can be used to affect RF switching. 
   The ferrite discs used in circulators and isolators are typically formed from an iron powder that has been treated to produce an oxide layer on the outer surface. This oxide layer effectively insulates each iron particle from the next. The powder is mixed with a (non magnetic) ceramic bonding material and heated to form a rigid ceramic disc. Most common ferrite contains about 50% iron oxide. The remainder is typically either an oxide of manganese (Mn) and zinc (Zn) or nickel and zinc. Other types of ferrites can also be used to form the disc. 
   The operating frequency of circulators and isolators is primarily determined by the ferrimagnetic resonance frequency of the ferrite disk. The frequency of ferrimagnetic resonance can be affected by several factors including the diameter, permeability, and dielectric constant or permittivity of the ferrite disk. Maximum coupling of the energy from the RF signal to the ferrite material will occur at ferrimagnetic resonance. Accordingly, for reasons of efficiency, circulators and isolators are generally designed to operate either below ferrimagnetic resonance or above ferrimagnetic resonance. The operating frequency for below resonance (B/R) circulators are generally limited to the range from about 500 MHz to more than 30 GHz. By comparison, the practical range of operating frequencies for above resonance (A/R) circulators is lower, namely from about 50 MHz to approximately 2.5 GHz. From the foregoing, it may be observed that it can be difficult to design a single circulator capable of operating over a broad range of frequencies substantially below 500 MHz and more than 2.5 GHz. 
   Ferromagnetic materials (e.g. iron, nickel, cobalt, and various alloys) have atomic or molecular or crystalline magnetic dipole moments that exhibit a paramagnetic (i.e. positive feedback) response to magnetic fields. These dipole moments tend to align with the magnetic field but the alignment is disrupted by thermal motion of the atoms or molecules. In ferromagnetic materials, it is energetically favorable for all the dipole moments to be aligned. In at least some ferromagnetic materials, the field produced by the aligned dipoles is sufficient to maintain the alignment below the Curie temperature, thereby resulting in permanent magnets. 
   In ferrimagnetic materials, sometimes called ferrites, it is energetically favorable for neighboring dipole moments to be antiparallel but different types of atoms are present and the dipole moments do not cancel exactly. There can thus be a net positive dipole moment. Ferrimagnetic materials spontaneously subdivide into domains, small regions where all dipoles are parallel. The division into domains is such that total energy in the domain boundaries and fields is minimized. Arrangement of domains can be manipulated by externally applied electrical fields. It also influences the magnetic response of the material. These two properties are extremely useful in certain applications. 
   SUMMARY OF THE INVENTION 
   The invention concerns a circulator in which the operating region or other characteristics can be selectively altered so as to be above or below ferrimagnetic resonance. The circulator is comprised of a transmission line port junction such as a three port Y junction. At least one, and preferably more, substantially cylindrical cavity structures are disposed adjacent to the junction and contain a ferromagnetic fluid. Each substantially cylindrical cavity structure can include a plurality of chambers. One or more magnets are provided for applying a magnetic field to the ferromagnetic fluid and the junction in a direction normal to a plane defined by the junction. A processor is provided for changing a volume of the ferromagnetic fluid from at least one of the plurality of chambers in response to a control signal to alter the characteristics of the circulator. For example, the processor can vary the number of chambers containing the ferromagnetic fluid. 
   The cavity containing the ferromagnetic fluid has a ferrimagnetic resonance, and the change of the volume or shape of the ferromagnetic fluid causes a change in the ferrimagnetic resonance. By changing the ferrimagnetic resonance, an operating region of the circulator can be selected to be either above ferrimagnetic resonance or below ferrimagnetic resonance. More particularly, the change in volume and/or shape of the ferromagnetic fluid causes a change in the operating region. According to one aspect of the invention, a plurality of chambers in the form of a plurality of concentric tubes are filled or emptied responsive to the control signal to form the ferromagnetic fluid within the substantially cylindrical cavity structure or structures. The ferromagnetic fluid can be selected from the group consisting of a low permittivity, low permeability fluid, a high permittivity, low permeability fluid, and a high permittivity, high permeability fluid. 
   According to another aspect, the ferromagnetic fluid can be comprised of an industrial solvent and a suspension of magnetic particles contained therein. The magnetic particles can be formed of a material selected from the group consisting of ferrite, metallic salts, and organo-metallic particles and the ferromagnetic fluid can comprise between about 50% to 90% of the magnetic particles by weight. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a perspective view of a circulator that is useful for understanding the invention. 
       FIG. 2  is a cross-sectional view of the circulator of  FIG. 1 , taken along lines  2 — 2 . 
       FIG. 3  is a schematic representation of a portion of a circulator including a processor for varying the volume of a ferromagnetic fluid in a substantially cylindrical cavity structure formed from a plurality of concentric tubes. 
       FIG. 4  is a flowchart illustrating a process that can be used for using ferromagnetic fluid in altering the operating characteristics of a circulator in accordance with the present invention. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1  is a perspective view of a circulator  100  that is useful for understanding the invention. For convenience, the term circulator as used herein should also be understood to also include isolators, which are really a special case of a circulator. As illustrated in  FIG. 1 , the circulator is comprised of metal case  116  and three transmission line ports  101 ,  102 ,  103  that are terminated in a junction  104 , in particular a Y junction in this instance. Electric ground planes  108 ,  110  are shown above and below the transmission line ports  101 ,  102 , and  103 . 
   Referring now to  FIG. 2  in a cross-sectional view across line  2 — 2 , it can be seen that the circulator includes several components within the metal case  116 . In conventional circulators, ferrite discs are positioned in the area between the transmission line Y junction  104  and the electric ground planes  108 ,  110 . In the present invention, however, the ferrite discs are preferably eliminated in favor of ferromagnetic fluid  114  and  324  that is contained within substantially cylindrical cavity structures  301 ,  302 . More particularly, fluid  114  can be contained within chambers  317  and  319  and fluid  324  can be contained within chambers  313  and  315  of substantially cylindrical cavity structures  301  and  302  respectively. Magnets  112  are preferably provided above and below electric ground planes  108  and  110 , respectively. These can be either permanent magnets or electromagnets. The metal case  116  is preferably formed of steel or aluminum with steel cladding to provide a magnetic return circuit. The volumes of ferromagnetic fluid in each of the substantially cylindrical cavity structures  301 , 302  can be manipulated using at least one processor and/or reservoir. As shown in  FIG. 2 , the volume of ferromagnetic fluid in chambers  317  and  319  is controlled by processor  210  whereas the volume of ferromagnetic fluid in chambers  313  and  315  is controlled by processor  215 . Fluid is pumped in and out of chamber  315  via conduit  220  and in and out of chamber  313  via conduit  221 . Conduits  220  and  221  help recirculate ferromagnetic fluid through the processor  210 . Likewise, fluid is pumped in and out of chamber  317  via conduit  230  and in and out of chamber  319  via conduit  231 . Conduits  230  and  231  help recirculate ferromagnetic fluid back through processor  215 . Valves (not shown) can also be used to provide further control in the communication of fluid between processors and cavities or chambers. A particular volume of a specified ferromagnetic fluid can be used to change the ferrimagnetic resonance of the circulator which enables the selection of an operating region of the circulator to be either above ferrimagnetic resonance or below ferrimagnetic resonance. 
   A fluid suspension of ferromagnetic particles can behave ferrimagnetically, with the suspended particles acting the role of domains. In such cases, it will be energetically favorable for the particles to pair up in antiparallel sets (this can be visualized as particle sized bar magnets in suspension.) The exact response of the ferromagnetic fluid will depend on the shape and size distribution of the particles. For example, disk shaped particles will behave differently as compared to bar magnets. Significantly, however, the ferromagnetic fluid can be selected to have a ferrimagnetic resonance that is similar to the conventional type ferrite disks that are presently used in circulators and isolators. 
   In the absence of a magnetic field, an RF signal applied at a transmission line port  101  (of circulator  100  of  FIG. 1 ) will be transferred equally to ports  102  and  103 , provided that all of the transmission lines are equally spaced from one another. This power transfer is due to a pattern of standing waves that are established relative to the input transmission line port  101 . These standing waves are symmetrical relative to the input transmission line port  101 . However, when an axial magnetic field  118  is applied to the ferromagnetic fluids  114  and  324  in cavity structures  301 ,  302 , the presence of such axial magnetic field alters the symmetrical pattern of standing waves. 
   As is known from conventional circulator design, the desired characteristics of circulation and isolation are obtained by causing the standing wave pattern to rotate approximately 30 degrees. With the magnetic field oriented in a first axial direction, it will produce a null at transmission line port  102 , making it the isolation port. The shift in standing wave pattern also causes transmission line port  103  to be fully coupled to the input port  101 . Those skilled in the art will appreciate that the invention is not limited to one particular direction of circulation. Rather, a direction of circulation, and the coupling or isolation of the ports, will be determined by the axial direction of the magnetic field. Reversing the direction of the magnetic field reverses the direction of circulation. 
   The operational frequency of the circulator will be determined substantially by the ferrimagnetic resonance frequency of the ferromagnetic fluid  114  and  324  contained in cylindrical cavity structures  301  and  302 . The ferrimagnetic resonance frequency can be selected by controlling one or more of several design parameters, including the cavity diameter, permeability, and dielectric constant or permittivity of the “ferrite disk”. In general, for A/R operation the ferromagnetic fluid will need to have a higher effective permeability as compared to the permeability required for B/R operation. According to a preferred embodiment of the invention, the permeability and dielectric constant of the ferromagnetic fluid can be dynamically controlled to select the ferrimagnetic resonance frequency and thereby obtain efficient circulator operation over a range of RF frequencies not otherwise obtainable. Note that although the cavity structure  301  is formed by concentric chambers  317  and  313  and the cavity structure  302  is formed by concentric chambers  319  and  315 , the cavity structures  301  and  302  are not limited to such arrangement. Such cavity structures can have more concentric rings or other concentric shapes or other non-concentric chambers defining the cavity structures without departing from the scope of the present invention. Note also that the composition of the fluids  114  and  324  can be the same or be made to have different permeability, permittivity or other characteristics. 
   For example, in another embodiment, a circulator  300  can include a processor  350  and at least one substantially cylindrical cavity structure  375  having a plurality of concentric chambers  360 . The plurality of concentric chambers  360  can be formed from a plurality of concentric capillary tubes. Ferromagnetic fluid can be fed or withdrawn from each of the concentric chambers  360  via conduit feeds  370  coupled between the processor  350  and respective concentric chambers  360 . The processor  350  can also include a reservoir for storage or removal of ferromagnetic fluid as required. Other portions of the circulator such as the magnetic sources and other chambers discussed in the prior embodiment are not shown for simplicity. 
   It is known that circulators and isolators are generally designed to operate either below ferrimagnetic resonance or above ferrimagnetic resonance. The operating frequency for below resonance (B/R) circulators are generally limited to the range from about 500 MHz to more than 30 GHz. By comparison, the practical range of operating frequencies for above resonance (A/R) circulators is lower, namely from about 50 MHz to approximately 2.5 GHz. At high frequencies, the A/R circulator requires a very high intensity magnetic field to operate efficiently. Therefore, in order to obtain efficient operation of a circulator over a range of frequencies that extend substantially below about 500 MHz and substantially above about 2.5 GHz, it can be advantageous to selectively control the characteristics of the ferromagnetic fluid contained in the cylindrical cavity structures  301 ,  302 . This will allow the ferromagnetic resonance frequency to be dynamically changed. Consequently, the circulator can be configured to operate above ferrimagnetic resonance for lower operating frequencies, and below ferrimagnetic resonance when the device is used for higher operating frequencies. 
   In addition to allowing control over the ferrimagnetic resonance frequency, dynamic control over the permeability and permittivity of the ferromagnetic fluid can also permit the impedance of the ferromagnetic fluid contained in the cylindrical cavity structures to be adjusted for an improved match at different frequencies of operation. This ability to adjust impedance can help to reduce the need for external transformer sections as are commonly required for broad bandwidth circulator applications. 
   Composition of Ferromagnetic Fluid 
   The ferromagnetic fluid as described herein can be comprised of several component parts that can be mixed together to produce a desired permeability and permittivity required for a particular ferromagnetic resonance and Y junction impedance. The mixture preferably has a relatively low loss tangent to minimize the amount of RF energy that is lost. The component parts can be selected to include a first fluid made of a high permittivity solvent completely miscible with a second fluid made of a low permittivity oil. A third fluid component can be comprised a ferrite particle suspension in a low permittivity oil identical to the first fluid such that the first and second fluids do not form azeotropes. 
   A nominal value of relative permittivity (ε 1 ) for fluids is approximately 2.0. However, a mixture of such component parts can be used to produce a wide range of permittivity values. For example, component fluids could be selected with permittivity values of approximately 2.0 and about 58 to produce a ferromagnetic fluid with a permittivity anywhere within that range after mixing. Dielectric particle suspensions can also be used to increase permittivity. 
   According to a preferred embodiment, the component parts of the ferromagnetic fluid can be selected to include a high permeability component. High levels of magnetic permeability are commonly observed in magnetic metals such as Fe and Co. For example, solid alloys of these materials can exhibit levels of μ r  in excess of one thousand. By comparison, the permeability of fluids is nominally about 1.0 and they generally do not exhibit high levels of permeability. However, high permeability can be achieved in a fluid by introducing magnetic particles/elements to the fluid. For example typical magnetic fluids comprise suspensions of iron, ferro-magnetic or ferrite particles in a conventional industrial solvent such as water, toluene, mineral oil, silicone, and so on. Other types of magnetic particles include metallic salts, organo-metallic compounds, and other derivatives, although Fe and Co particles are most common. The size of the magnetic particles found in such systems is known to vary to some extent. However, particles sizes in the range of 1 nm to 20 μm are common. The composition of particles can be varied as necessary to achieve the required range of permeability in the final mixed ferromagnetic fluid. However, magnetic fluid compositions are typically between about 50% to 90% particles by weight. Increasing the number of particles will generally increase the permeability. 
   Processing for Communicating Ferromagnetic Fluid Between Reservoirs, Cavities &amp; Chambers 
   A cooperating set of proportional valves, pumps (as may be included in the processor/reservoirs  210  and  215 ), and connecting conduits can be provided for selectively communicating the ferromagnetic fluids  114  and  324  from the fluid reservoirs to cylindrical cavity structures  301  and  302 . The operation of the processor(s) shall now be described in greater detail with reference to FIG.  2  and the flowchart shown in FIG.  4 . 
   The process can begin in step  402  of  FIG. 4 , with processor  210  and/or  214  checking to see if an updated configuration control signal has been received on a control signal input line  337 . If so, then the processor ( 210  and/or  215 ) continues on to step  403  to determine an updated volume or radius for the new circulator configuration. The updated volume and/or radius necessary for achieving circulator operating parameters is preferably determined using a look-up table but can be calculated directly based on the specific operating configuration indicated by the control signal. 
   In step  410 , the processor causes the ferromagnetic fluid  114  and/or  324  to be circulated into the respective cavities  301  and  302  defined by chambers  317 ,  319 ,  313  and  315 . The ferromagnetic fluid can be communicated to the chambers and excess fluid can be re-circulated to the processor through the conduits. In step  412 , the controller can check one or more sensors to determine if the ferromagnetic fluid being circulated to the cavity structures  313  and  315  has the proper values of volume and/or permeability and permittivity. The sensors can include inductive type sensors capable of measuring permeability, capacitive type sensors capable of measuring permittivity, as well as flowmeters. 
   In step  419 , the processor can compare the measured volume (and or shape) to the desired updated cylinder volume value (or shape) determined at step  403 . If the updated value does not match or meet a particular predefined range of values, then at step  421 , the ferromagnetic fluid can be added or removed as indicated from predetermined chambers. If the volume and/or shape are the proper values and optionally the values for permittivity and permeability passing into and out of the cavities defined by cavity structures  301  and  302  are the proper value, then the system can stop circulating the ferromagnetic fluid and the system returns to step  402  to wait for the next updated control signal. 
   Significantly, when updated ferromagnetic fluid is required, any existing ferromagnetic fluid can be circulated out of the cavity structures  301  and  302 . Any existing ferromagnetic fluid not having the proper permeability and/or permittivity can be deposited in a collection reservoir. The ferromagnetic fluid deposited in the collection reservoir can thereafter be re-used at a later time to provide additional ferromagnetic fluid as needed. 
   An example of a set of component parts that could be used to produce a ferromagnetic fluid as described herein would include oil (low permittivity, low permeability), a solvent (high permittivity, low permeability) and a magnetic fluid, such as combination of an oil and a ferrite (low permittivity and high permeability). A hydrocarbon dielectric oil such as Vacuum Pump Oil MSDS-12602 could be used to realize a low permittivity, low permeability fluid, low electrical loss fluid. A low permittivity, high permeability fluid may be realized by mixing the same hydrocarbon fluid with magnetic particles such as magnetite manufactured by FerroTec Corporation of Nashua, N.H., or iron-nickel metal powders manufactured by Lord Corporation of Cary, N.C. for use in ferrofluids and magnetoresrictive (MR) fluids. Additional ingredients such as surfactants may be included to promote uniform dispersion of the particle. Fluids containing electrically conductive magnetic particles require a mix ratio low enough to ensure that no electrical path can be created in the mixture. 
   Solvents such as formamide inherently posses a relatively high permittivty and therefore can be used as the high permittivity component of the ferromagnetic fluid for the invention. Permittivity of other types of fluid can also be increased by adding high permittivity powders such as barium titanate manufactured by Ferro Corporation of Cleveland, Ohio. For broadband applications, the fluids would not have significant resonances over the frequency band of interest. 
   It should be noted that the present invention is not limited to the embodiments shown in  FIG. 2  or  3 . In particular, the circulator can be configured to have more than two substantially cylindrical cavity structures or more than two chambers in any particular cavity structure as shown in FIG.  3 . The circulator is not limited to a particular number of ports ( 3  and  4  ports are common) or a particular number of processors as evidenced by the embodiments of  FIGS. 2 and 3 . Furthermore, the ferromagnetic fluids  114  and  324  do not necessarily need to have the same composition or characteristics. For example, ferromagnetic fluid in chamber  313  can have a different permeability and permittivity and/or volumes than the ferromagnetic fluid in chamber  315 . 
   Those skilled in the art will also recognize that the specific process used to communicate, mix or to separate the component parts from one another will depend largely upon the properties of materials that are selected and the invention. Accordingly, the invention is not intended to be limited to the particular process or structure outlined above.