Abstract:
A circuit for processing radio frequency signals. The circuit is a transmission line stub ( 110 ) coupled to a fluidic dielectric ( 108 ) and includes a composition processor ( 101 ) for selectively varying a composition of the fluidic dielectric. Varying the fluid dielectric composition allows the electrical characteristics of the transmission line stub ( 110 ) to be dynamically varied in response to a control signal  137 . The electrical characteristics that can be varied with the fluid dielectric include, but are not limited to, an electrical length of the stub, a characteristic impedance of the stub, and a frequency response of the stub. The transmission line stub ( 110 ) can be electrically shorted to ground or electrically open with respect to ground.

Description:
BACKGROUND OF THE INVENTION 
     1. Statement of the Technical Field 
     The inventive arrangements relate generally to transmission line stubs, and more particularly for transmission line stubs that can be dynamically tuned. 
     2. Description of the Related Art 
     Transmission line stubs are commonly used in radio frequency (RF) circuits. For example, a resonant transmission line stub is sometimes said to be resonant at a particular frequency, meaning the line has impedance characteristics similar to a resonant circuit at that frequency, although resonant line characteristics are actually a function of voltage reflections, not circuit resonance. On printed circuit boards or substrates, resonant lines are typically implemented by creating a line with at least one port at the input and either an open-circuit or short-circuit to ground at the termination. The input impedance to an open or shorted resonant line is typically resistive when the length of the resonant line is an even or odd multiple of a quarter-wavelength of the operational frequency. That is, the input to the resonant line is at a position of voltage maxima or minima. When the input to the resonant line is at a position between the voltage maxima and minima points, the input impedance can have reactive components. Consequently, properly chosen transmission line stubs may be used as parallel-resonant, series-resonant, inductive, or capacitive circuits. 
     Transmission lines stubs in RF circuits are typically formed in one of three ways. One configuration known as microstrip, places the signal line on the top of a board surface. A second conductive layer, commonly referred to as a ground plane, is spaced apart from and below the signal line. A second type of configuration known as buried microstrip is similar except that the signal line is covered with a dielectric substrate material. In a third configuration known as stripline, the signal line is sandwiched between two electrically conductive (ground) planes. Other configurations, including waveguide stubs, are also known in the art. 
     Low permittivity printed circuit board materials are ordinarily selected for implementing RF circuit designs, including transmission line stubs. For example, polytetrafluoroethylene (PTFE) based composites such as RT/duroid® 6002 (permittivity of 2.94; loss tangent of 0.009) and RT/duroid® 5880 (permittivity of 2.2; loss tangent of 0.0007), both available from Rogers Microwave Products, Advanced Circuit Materials Division, 100 S. Roosevelt Ave, Chandler, Ariz. 85226, are common board material choices. 
     Two important characteristics of dielectric materials are permittivity (sometimes called the relative permittivity or ∈ r ) and permeability (sometimes referred to as relative permeability or μ r ). The relative permittivity and permeability determine the propagation velocity of a signal, which is approximately inversely proportional to √{square root over (μ∈)}. The propagation velocity directly affects the electrical length of a transmission line and therefore the physical length of a transmission line stub. 
     Further, ignoring loss, the characteristic impedance of a transmission line, such as stripline or microstrip, is equal to √{square root over (L l /C l )} where L l  is the inductance per unit length and C l  is the capacitance per unit length. The values of L l  and C l  are generally determined by the permittivity and the permeability of the dielectric material(s) used to separate the transmission line structures as well as the physical geometry and spacing of the line structures. Accordingly, the overall geometry of a stub will be highly dependent on the permittivity and permeability of the dielectric substrate. 
     The electrical characteristics of transmission line stubs generally cannot be modified once formed on an RF circuit board. This is not a problem where only a fixed frequency response is needed. The geometry of the transmission line can be readily designed and fabricated to achieve the proper characteristic impedance. When a variable frequency response is needed, however, use of a fixed length stub can be a problem. 
     A similar problem is encountered in RF circuit design with regard to optimization of circuit components for operation on different RF frequency bands. Line impedances and lengths that are optimized for a first RF frequency band may provide inferior performance when used for other bands, either due to impedance variations and/or variations in electrical length. Such limitations can limit the effective operational frequency range for a given RF system. 
     SUMMARY OF THE INVENTION 
     The present invention relates to a circuit for processing radio frequency signals. The circuit is a transmission line stub coupled to a fluidic dielectric and includes a composition processor for selectively varying a composition of the fluidic dielectric. Varying the fluid dielectric composition allows the electrical characteristics of the transmission line stub to be dynamically varied in response to a control signal. 
     The composition processor can selectively vary a permittivity and a permeability of the fluidic dielectric. Further, according to one aspect of the invention, the composition processor can vary the permittivity and the permeability concurrently in response to the control signal. 
     The electrical characteristics that can be varied with the fluid dielectric include, but are not limited to, an electrical length of the stub, a characteristic impedance of the stub, and a frequency response of the stub. The transmission line stub itself can be electrically shorted to ground or electrically open with respect to ground. 
     According to another aspect of the the transmission line stub is also coupled to a solid dielectric. For example, the solid dielectric can be a circuit board upon which the stub is formed. A cavity can be disposed within the dielectric circuit board substrate, and the fluidic dielectric can be disposed within the cavity. 
     According to one aspect, the invention can also include a component mixer. The component mixer can be arranged for dynamcially mixing a plurality of component parts of the fluidic dielectric responsive to the control signal to form the fluidic dielectric. The component parts can be selected from a low permittivity, low permeability component, a high permittivity, low permeability component, and a high permittivity, high permeability component. The composition processor further can include one or more proportional valves, one or more mixing pumps, and at least one conduit. The composition processor selectively mixes the plurality of component parts of the fluidic dielectric and transfers the fluidic dielectric to a cavity where the fluidic dielectric is coupled to the transmission line stub. According to another aspect of the invention, the composition processor can also include a component part separator adapted for separating the component parts of the fluidic dielectric for subsequent reuse. 
     According to one aspect, the fluidic dielectric can be comprised of an industrial solvent that has 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. For example, the fluidic dielectric can be between about 50% to 90% magnetic particles by weight. 
     The invention can also include a method for dynamically controlling a frequency response of a transmission line stub. The method can include the steps of coupling the transmission line stub to a fluidic dielectric and, in response to a control signal, selectively varying a composition of the fluidic dielectric. The method permits dynamic changes to be performed relative to an electrical characteristic of the transmission line stub. The composition of the fluidic dielectric can be varied so as to modify a permittivity and a permeability of the fluidic dielectric. According to one aspect of the invention, this step can include varying the permittivity and the permeability concurrently in response to the control signal. The result is a corresponding variation in an electrical length, a characteristic impedance, and a frequency response, of the transmission line stub. 
     The method can also include the step of electrically shorting one end of the transmission line stub to a ground potential or forming an open circuit at one end of the transmission line stub. According to another aspect, the method can include the step of also coupling the transmission line stub to a solid dielectric. For example, this can include selecting the solid dielectric to be a circuit board upon which the stub is formed. In that case, the method can also advantageously include step of disposing the fluid dielectric in a cavity within the dielectric circuit board substrate. 
     According to yet another aspect, the method can include selecting a component part of the fluid dielectric from the group consisting of a low permittivity, low permeability component, a high permittivity, low permeability component, and a high permittivity, high permeability component. These components can be selectively mixed and communicated from respective fluid reservoirs to a cavity where the fluidic dielectric is coupled to the transmission line stub. Notably, the method can also include the step of separating the component parts of the fluidic dielectric for subsequent reuse. 
     Finally, the method can also include the step of selecting the fluidic dielectric to be an industrial solvent, either with or without a suspension of magnetic particles contained therein. If magnetic particles are used, the invention can further include the step of selecting the magnetic particles from the group consisting of ferrite, metallic salts, and organo-metallic particles. This step can include selecting a ratio of the component parts so that the fluid dielectric contains between about 50% to 90% magnetic particles by weight. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a block diagram useful for understanding the transmission line stub of the invention. 
         FIG. 2  is a flow chart that is useful for understanding the process of the invention. 
         FIG. 3   a  is a cross-sectional view of the transmission line structure in  FIG. 1 , taken along line  3 — 3   
         FIG. 3   b  is a cross-sectional view of an alternative embodiment of a transmission line structure of  FIG. 1  taken along line  3 — 3 . 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       FIG. 1  is a conceptual diagram that is useful for understanding the tunable transmission line stub system of the present invention. The tunable transmission line stub system  100  includes a conductive RF transmission line stub  110  at least partially coupled to a fluidic dielectric  108 . In most instances, the stub  110  will be coupled to a larger circuit by associated transmission line circuitry. In  FIG. 1 , such circuitry is illustrated by a first transmission line  107  and a second transmission line  111 . Those skilled in the art will appreciate that these transmission lines are merely shown by way of example and are not intended to limit the scope of the invention. Instead, any suitable input and output circuitry can be provided for communicating signals to and from the stub  110 . 
     The fluidic dielectric  108  is preferably constrained within a cavity region  109  that is generally positioned relative to the RF transmission line stub  110  so as to be electrically and magnetically coupled thereto. It should be understood that while the RF transmission line stub  110  is shown in  FIG. 1  as a conductor suspended within a dielectric layers  102 ,  142  over a ground plane  140 , the invention is not so limited. Other transmission line structures can also be used to form the stub and such structures are within the scope of the invention provided that the stub is coupled to a fluid dielectric as described herein. 
     A composition processor  101  is preferably provided for changing a composition of the fluidic dielectric  108  to vary its permittivity. A controller  136  controls the composition processor for selectively varying the permittivity of the fluidic dielectric  108  in response to a control signal  137  received on control line  138 . The composition processor  101  is also adapted for changing a composition of the fluidic dielectric  108  to vary its permeability. According to a preferred embodiment, the controller  136  can cause the composition processor  101  to selectively vary the permittivity and the permeability of the fluidic dielectric concurrently in response to the control signal. Thus, the controller can vary the frequency response of the stub in accordance with an input control signal  137  by effectively vary the inductance and capacitance per unit length of the stub. According to a preferred embodiment, the composition processor also includes separator units  130 ,  132  for separating out component parts of the fluidic dielectric so that they can be subsequently refused. The composition of the fluidic dielectric, the dynamic mixing process, and the component part separation process shall now be discussed in further detail. 
     Composition of Fluidic Dielectric 
     The fluidic dielectric can be comprised of several component parts that can be mixed together to produce a desired permeability and permittivity required for a particular stub electrical response. In this regard, it will be readily appreciated that fluid miscibility and particle suspension are key considerations to ensure proper mixing. Another key consideration is the relative ease by which the component parts can be subsequently separated from one another. The ability to separate the component parts is important when the stub frequency response requirements change. Specifically, this feature ensures that the component parts can be subsequently re-mixed in a different proportion to form a new fluidic dielectric. 
     The resultant mixture comprising the fluidic dielectric also preferably has a relatively low loss tangent to minimize the amount of RF energy lost in the stub  110 . However, devices with higher insertion loss may be acceptable in some instances so this may not be a critical factor. Also, the components of the fluidic dielectric must be capable of providing the proper permittivity and permeability. Aside from the foregoing constraints, there are relatively few limits on the range of component parts that can be used to form the fluidic dielectric. Accordingly, those skilled in the art will recognize that the examples of component parts, mixing methods and separation methods as shall be disclosed herein are merely by way of example and are not intended to limit in any way the scope of the invention. 
     Also, the component materials are described herein as being mixed in order to produce the fluidic dielectric. However, it should be noted that the invention is not so limited. Instead, it should be recognized that the composition of the fluidic dielectric could be modified in other ways. For example, the component parts could be selected to chemically react with one another in such a way as to produce the fluidic dielectric with the desired values of permittivity and or permeability. All such techniques will be understood to be included to the extent that it is stated that the composition of the fluidic dielectric is changed. 
     A nominal value of permittivity (∈ r ) for fluids is approximately 2.0. However, the component parts for the fluidic dielectric can include fluids with extreme values of permittivity. Consequently, a mixture of such component parts can be used to produce a wide range of intermediate permittivity values. For example, component fluids could be selected with permittivity values of approximately 2.0 and about 58 to produce a fluidic dielectric 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 fluidic dielectric can be selected to include a low permittivity, low permeability component and a high permittivity, high permeability component. These two components can be mixed as needed for increasing permittivity while maintaining a relatively constant ratio of permittivity to permeability. A third component part of the fluidic dielectric can include a high permittivity, low permeability component for allowing adjustment of the permittivity of the fluidic dielectric independent of the permeability. 
     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 metal particles/elements to the fluid. For example typical magnetic fluids comprise suspensions of ferro-magnetic 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 fluidic dielectric after mixing. However, magnetic fluid compositions are typically between about 50% to 90% particles by weight. Increasing the number of particles will generally increase the permeability. 
     An example of a set of component parts that could be used to produce a fluidic dielectric 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 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 permittivity and therefore can be used as the high permittivity component 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. 
     Processing of Fluidic Dielectric for Mixing/Unmixing of Components 
     Referring again to  FIG. 1 , the composition processor  101  can be comprised of a plurality of fluid reservoirs containing component parts of fluidic dielectric  108 . These can include a first fluid reservoir  122  for a low permittivity, low permeability component of the fluidic dielectric, a second fluid reservoir  124  for a high permittivity, low permeability component of the fluidic dielectric, and a third fluid reservoir  126  for a high permittivity, high permeability component of the fluidic dielectric. Those skilled in the art will appreciate that other combinations of component parts may also be suitable and the invention is not intended to be limited to the specific combination of component parts described herein. 
     A cooperating set of proportional valves  134 , mixing pumps  120 ,  121 , and connecting conduits  135  can be provided as shown in  FIG. 1  for selectively mixing and communicating the components of the fluidic dielectric  108  from the fluid reservoirs  122 ,  124 ,  126  to cavity  109 . The composition processor also serves to separate out the component parts of fluidic dielectric  108  so that they can be subsequently re-used to form the fluidic dielectric with different permittivity and/or permeability values. All of the various operating functions of the composition processor can be controlled by controller  136 . The operation of the composition processor shall now be described in greater detail with reference to FIG.  1  and the flowchart shown in FIG.  2 . 
     The process can begin in step  202  of  FIG. 1 , with controller  136  checking to see if an updated control signal  137  has been received on a control signal input line  138 . If so, then the controller  136  continues on to step  204  to determine an updated permittivity value for producing the stub frequency response indicated by the control signal. The updated permittivity value necessary for achieving the indicated stub frequency response can be determined using a look-up table. Alternatively, the updated permittivity value can be calculated directly using equations well known to those skilled in the art for calculating capacitance per unit length. In step  206 , the controller can determine an updated permeability value required for achieving the desired inductance per unit length for achieving the indicated frequency response for transmission line stub  110 . 
     In step  208 , the controller  136  causes the composition processor  101  to begin mixing two or more component parts in a proportion to form fluidic dielectric that has the updated permittivity and permeability values determined earlier. This mixing process can be accomplished by any suitable means. For example, in  FIG. 1  a set of proportional valves  134 , conduits  135 , and mixing pump  120  are used to mix component parts from reservoirs  122 ,  124 ,  126  appropriate to achieve the desired updated permeability and permittivity. 
     In step  210 , the controller causes the newly mixed fluidic dielectric  108  to be circulated into the cavity  109  through a second mixing pump  121 . In step  212 , the controller checks one or more sensors  116 ,  118  to determine if the fluidic dielectric being circulated through the cavity  109  has the proper values of permeability and permittivity. Sensors  116  are preferably inductive type sensors capable of measuring permeability. Sensors  118  are preferably capacitive type sensors capable of measuring permittivity. The sensors can be located as shown, at the input to mixing pump  121 . Sensors  116 ,  118  can also be positioned within solid dielectric substrate  102  to measure the permeability and permittivity of the fluidic dielectric passing through input conduit  113  and output conduit  114 . Note that it is desirable to have a second set of sensors  116 ,  118  at or near the cavity  109  so that the controller can determine when the fluidic dielectric with updated permittivity and permeability values has completely replaced any previously used fluidic dielectric that may have been present in the cavity  109 . 
     In step  214 , the controller  136  compares the measured permeability to the desired updated permeability value determined in step  206 . If the fluidic dielectric does not have the proper updated permeability value, the controller  136  can cause additional amounts of high permeability component part to be added to the mix from reservoir  126  and continues circulating the modified fluidic dielectric  108  to the cavity  109 . 
     If the fluidic dielectric  108  is determined to have the proper level of permeability in step  214 , then the process continues on to step  218  where the measured permittivity value from step  212  is compared to the desired updated permittivity value from step  204 . If the updated permittivity value has not been achieved, then high or low permittivity component parts are added as necessary in step  210  and the modified fluid is circulated to the cavity  109 . If both the permittivity and permeability passing into and out of the cavity  109  are the proper value, the system can stop circulating the fluidic dielectric and the system returns to step  202  to wait for the next updated control signal. 
     Significantly, when updated fluidic dielectric is required, any existing fluidic dielectric can be circulated out of the cavity  109 . Any existing fluidic dielectric not having the proper permeability and/or permittivity can be deposited in a collection reservoir  128 . The fluidic dielectric deposited in the collection reservoir can thereafter be re-used directly as a fourth fluid by mixing with the first, second, and third fluids or separated out into its component parts in separator units  130 ,  132  so that it may be re-used at a later time to produce additional fluidic dielectric. The aforementioned approach includes a method for sensing the properties of the collected fluid mixture to allow the fluid processor to appropriately mix the desired composition, and thereby, allowing a reduced volume of separation processing to be required. 
     According to a preferred embodiment, the component parts of the fluidic dielectric  108  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 that has a significantly different boiling point. 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. Given the foregoing, the following process may be used to separate the component parts. 
     A first stage separation process in separator unit  130  would utilize distillation to selectively remove the first fluid from the mixture by the controlled application of heat thereby evaporating the first fluid, transporting the gas phase to a physically separate condensing surface whose temperature is maintained below the boiling point of the first fluid, and collecting the liquid condensate for transfer to the first fluid reservoir  122 . A second stage process in separator unit  132  would introduce the mixture, free of the first fluid, into a chamber that includes an electromagnet that can be selectively energized to attract and hold the paramagnetic particles while allowing the pure second fluid to pass which is then diverted to the second fluid reservoir  124 . Upon de-energizing the electromagnet, the third fluid would be recovered by allowing the previously trapped magnetic particles to combine with the fluid exiting the first stage which is then diverted to the third fluid reservoir  126 . 
     Those skilled in the art will recognize that the specific process used 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 outlined above. 
     RF Unit Structure, Materials and Fabrication 
       FIG. 3   a  is a cross-sectional view of one embodiment of the transmission line structure in  FIG. 1 , taken along line  3 — 3 , that is useful for understanding the invention. As illustrated therein, cavity  109  can be formed in solid dielectric layer  102  and continued in solid dielectric layer  142  so that the fluidic dielectric is closely coupled to transmission line stub  110  on all sides of conductor. The transmission line stub  110  is suspended within the cavity  109  as shown. A ground plane  140  is disposed below the conductor forming the transmission line stub  110 . The ground plane is located between solid dielectric layer  102  and base substrate  144 . 
       FIG. 3   b  is a cross-sectional view showing an alternative transmission line stub  110 ′ for a delay line in which the cavity structure  109 ′ extends on only one side of the conductor  111 ′ and the conductor  111 ′ is partially coupled to the solid dielectric layer  142 ′. 
     At this point it should be noted that while the embodiment of the invention in  FIG. 1  is shown essentially in the form of a buried microstrip construction, the invention herein is not intended to be so limited. Instead, the invention can be implemented using any type of transmission line by replacing at least a portion of a conventional solid dielectric material that is normally coupled to the transmission line with a fluidic dielectric as described herein. For example, and without limitation, the invention can be implemented in transmission line configurations including conventional waveguides, stripline, microstrip, coaxial lines, and embedded coplanar waveguides. All such structures are intended to be within the scope of the invention. 
     According to one aspect of the invention, the solid dielectric layers  102 ,  142 ,  144  can be formed from a ceramic material. For example, the solid dielectric substrate can be formed from a low temperature co-fired ceramic (LTCC). Processing and fabrication of RF circuits on LTCC is well known to those skilled in the art. LTCC is particularly well suited for the present application because of its compatibility and resistance to attack from a wide range of fluids. The material also has superior properties of wetability and absorption as compared to other types of solid dielectric material. These factors, plus LTCC&#39;s proven suitability for manufacturing miniaturized RF circuits, make it a natural choice for use in the present invention. 
     While the preferred embodiments of the invention have been illustrated and described, it will be clear that the invention is not so limited. Numerous modifications, changes, variations, substitutions and equivalents will occur to those skilled in the art without departing from the spirit and scope of the present invention as described in the claims.