Patent Grant
6965286
1. Statement of the Technical Field
The inventive arrangements relate generally to methods and apparatus for providing increased design flexibility for RF circuits and, more particularly, to resonant cavities using conductive fluids.
2. Description of the Related Art
Resonant cavities are well known radio frequency (RF) devices and are commonly used in a variety of RF circuits, for example, in conjunction with microwave antennas and local oscillators. Resonant cavities are typically completely enclosed by conducting walls that can contain oscillating electromagnetic fields. An aperture is generally provided in one of the resonant cavity walls through which RF energy can be transmitted into, and extracted from, the resonant cavity. Resonant cavities can be constructed with a variety of shapes and can be used for different applications and frequency ranges. Nonetheless, the basic principles of operation are the same for all resonant cavities.
A resonant cavity resonates at frequencies which are determined by the dimensions of the resonant cavity. As the cavity dimensions increase, the resonant frequencies tend to decrease, and vice versa. For example, the lowest resonant frequency of a three dimensional rectangular resonant cavity is given by the equation:
where a and b the two largest dimensions of the cavity (i.e. length and width), ∈r is the relative permittivity of the dielectric within the resonant cavity, μr is the relative permeability of the resonant cavity, and C0 is the speed of light.
Resonant cavities provide many advantages for RF circuits operating in the microwave frequency range. In particular, resonant cavities have a very high quality factor (Q). In fact, cavities with a Q value in excess of 30,000 are not uncommon. The high Q gives resonant cavities an extremely narrow bandpass, which enables very precise operation of microwave devices utilizing the resonant cavities. In consequence to the narrow bandpass, however, resonant cavities are typically limited to operating only at very specific frequencies.
To alter the resonant frequency of a resonant cavity would typically require a mechanical manipulation of the shape and structure of the dimensions of the cavity. With rigid conventional dielectric or conductive materials, such manipulations would likely be costly and limited to certain specific structures and frequencies. Thus, a need exists for tuning a resonant cavity in a flexible and cost effective manner.
The present invention relates to a tunable resonant system, which includes a resonant cavity, and a method for a varying the resonant characteristics of the resonant cavity. The resonant cavity is enclosed by a conductive material and has at least one aperture in the conductive material for coupling the resonant cavity to an RF signal propagating in a circuit device, for example an antenna element or an oscillator. A conductive fluid having a permeability can be at least partially disposed within the resonant cavity or a plurality of subcavities within the resonant cavity. A dielectric barrier can be provided within the aperture to prevent fluid from escaping the resonant cavity.
In one aspect of the present invention, at least one composition processor or a fluidic pump is adapted for dynamically changing a composition or volume of the conductive fluid to vary the resonant frequency of the resonant cavity. In this manner at least one parameter associated with the resonant cavity can be varied or maintained. The parameter can be a center frequency, a bandwidth, a quality factor (Q) or an impedance. A controller also can be provided for controlling the composition processor in response to a control signal such as a resonant system control signal. The controller can cause the composition processor to selectively vary or alter the volume or types of conductive fluid within the resonant cavity or a plurality of discrete cavities or subcavities within the resonant cavity. The composition processor can include at least one conduit or feed line for selectively pumping conductive fluid from respective fluid reservoirs to the resonant cavity.
The fluidic dielectric used in the various discrete cavities or subcavities of a resonant cavity for example can have different characteristics, for example characteristics selected from (a) a low permittivity, low permeability, (b) a high permittivity, low permeability, and (c) a high permittivity, high permeability. Further, the high permittivity, high permeability fluidic dielectric can have a high loss tangent. The fluidic dielectric can include an industrial solvent which has a suspension of magnetic particles contained therein. The magnetic particles can be formed of ferrite, metallic salts, and organo-metallic particles. Further, the component can contain between about 50% to 90% magnetic particles by weight.
In another aspect of the present invention, a method for varying the resonant characteristics of a resonant cavity includes the step of at least partially filling the resonant cavity or one or more subcavities within the resonant cavity with conductive fluids. The method also includes the step of changing a composition or volume of the conductive fluid to selectively vary at least a permeability value of the resonant cavity in response to a control signal such as a resonant system control signal. The method also can include the step of pumping the conductive fluid from respective fluid reservoirs to the resonant cavity (or to the subcavities within the resonant cavity) to vary or maintain constant, a center frequency, a bandwidth, a quality factor (Q) and/or an impedance associated with the resonant cavity.
The present invention relates to a tunable resonant system. The invention provides the circuit designer with an added level of flexibility by permitting a conductive fluid to be used in a tuned resonant cavity (resonant cavity), thereby enabling the operating properties within resonant cavity to be varied. Since group velocity in a medium is inversely proportional to √{square root over (μ∈)}, increasing the permittivity (∈) and/or permeability (μ) in the dielectric decreases group velocity of an electromagnetic field within a resonant cavity, and thus the signal wavelength. Accordingly, the permittivity and permeability of the conductive fluid can be selected to decrease the physical size of a resonant cavity and to tune the operational characteristics of the resonant cavity. For example, the permittivity and/or permeability can be adjusted to tune the center frequency of cavity resonances. Further, the loss tangent of the fluidic dielectric can be adjusted in addition to the permittivity and/or permeability in order to tune additional operational parameters, for instance, the quality factor (Q), bandwidth of resonances within the resonant cavity, and an impedance of the resonant cavity. Accordingly, a resonant cavity of a given size can be used for a broad range of frequencies and applications without altering the physical dimensions of the resonant cavity. Moreover, if the physical dimensions of the resonant cavity change, for example due to thermal expansion or contraction, during operation of the resonant cavity, the permittivity, permeability and/or loss tangent of the fluidic dielectric can be automatically adjusted to keep the resonant cavity tuned for optimum performance. Importantly, the present invention eliminates the need for manual adjustments, such as tuning screws, to keep the resonant cavity properly tuned.
The conductive fluid 108 can be constrained within the resonant cavity 102 generally or within any number of cavities such as multiple capillary tubes as will be further discussed particularly with reference to
The resonant cavity 102 can be used in any circuit that can include any other type of resonant cavity. For example, the resonant cavity 102 can be used in conjunction with an antenna element 160, as shown in FIG. 1. The resonant cavity 102 also can be used with other circuit devices, for example an oscillator or a filter. Moreover, the resonant cavity 102 can be used as a filter element. Still, there are many other applications where the resonant cavity 102 can be used, and such applications are understood to be within the scope of the present invention.
A composition processor 101 is provided for changing a composition of the conductive fluid 108 to vary permeability or resonant frequency of the resonant cavity. In effect, the presence or lack of presence of the conductive fluid within the resonant cavity alters the shape or dimensions of the resonant cavity and hence its resonant frequency. A controller 136 controls the composition processor for selectively varying the permeability and/or other characteristics such as permittivity of the conductive fluid 108 in response to a resonant system control signal 137. By selectively varying the permeability and/or permittivity of the conductive fluid, the controller 136 can control group velocity and phase velocity of an RF signal within the resonant cavity 102, and thus resonances within the resonant cavity 102. The permeability and/or permittivity also can be adjusted to control the impedance of the resonant cavity. By selectively varying the loss tangent of the fluidic dielectric along with the permittivity and/or permeability, the controller 136 can control the Q and bandwidth of the resonant cavity 102.
In particular, the center frequencies at which the resonant cavity 102 resonates are determined by the dimensions of the resonant cavity, for example the distance between opposing walls 150, 151; 152, 153; 154, 155. A change in permeability and/or permittivity, which results in a change in phase velocity and group velocity of a signal within a resonant cavity, effectively changes the relative dimensions of the resonant cavity with respect to signal wavelength. Accordingly, the controller 136 can control the center frequencies of the cavity resonances by adjusting the permeability and/or permittivity of the conductive fluid 108. For instance, the permittivity and/or permeability of the conductive fluid 108 can be increased to result in a lower group velocity, which will cause the center frequencies to decrease. Likewise, a decrease in permittivity and/or permeability can increase the center frequencies. Additionally, the permittivity and/or permeability also can be adjusted to tune the impedance of the resonant cavity, which is beneficial for optimizing the RF coupling between the resonant cavity 102 and a circuit element, such as the antenna element 160.
Moreover, the permeability and/or permittivity can be adjusted to maintain a resonant frequency of the resonant cavity 102 constant. For instance, the permeability and/or permittivity can be adjusted to compensate for thermal expansion and contraction of the resonant cavity, such as when a resonant cavity is exposed to temperature extremes or when a substantial amount of power loss occurs in the resonant cavity. Such power loss can occur in a resonant cavity which is used in high power microwave transmission applications.
Composition of Fluidic Dielectric
The conductive fluid can be comprised of several component parts that can be either mixed together or provided in discrete quantized volumes to produce a desired permeability and permittivity required for a particular group velocity and resonant cavity resonant frequencies. 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 operational frequency, bandwidth or Q changes. Specifically, this feature ensures that the component parts can be subsequently re-mixed in a different proportion to form a new conductive fluid. Alternatively, desired permittivity and permeability can be achieved without necessarily mixing the components, but by providing a specific volume of particular component of conductive fluid. Thus, fluid miscibility, particle suspension, and separability may not be an important consideration in an embodiment that depends on discrete volumes of conductive fluid to alter the resonant cavity characteristics.
Many applications also require resonant cavities to be tunable over a wide frequency range. Accordingly, it may be desirable in many instances to select component mixtures or varied volumes of conductive fluid that produce a resonant cavity that has a relatively constant response over a broad range of frequencies. If the conductive fluid is not relatively constant over a broad range of frequencies, the characteristics of the fluid or their volume at various frequencies can be accounted for when the conductive fluid is mixed in a given cavity or pumped in as separate components into separate cavities. For example, a table of permittivity, permeability and loss tangent values vs. frequency can be stored in the controller 136 for reference during the mixing and/or pumping process.
Aside from the foregoing constraints, there are relatively few limits on the range of component parts that can be used to form the conductive fluid. Accordingly, those skilled in the art will recognize that the examples of component parts, mixing, pumping & extracting 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 or alternatively pumped in discretized volumes in order to produce the conductive fluid of a given characteristic. However, it should be noted that the invention is not so limited. Instead, it should be recognized that the composition or volume of the conductive fluid 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 conductive fluid 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 or volume of the fluidic dielectric within the resonant cavity is changed.
A nominal value of permittivity (∈r) for fluids is approximately 2.0. However, the component parts for the conductive fluid 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 conductive 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 conductive fluid can be selected to include (a) a low permittivity, low permeability, low loss component, (b) a high permittivity, low permeability, low loss component and (c) a high permittivity, high permeability, high loss component. These three components can be mixed as needed for increasing the permittivity while maintaining a relatively constant loss tangent (dielectric or magnetic) and for increasing the loss tangent while maintaining a relatively constant product of permittivity and permeability. Still, a myriad of other component mixtures can be used.
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 μr 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 conductive fluid 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 range of conductive fluids as described herein would include oil (low permittivity, low permeability and low loss), a solvent (high permittivity, low permeability and low loss), and a magnetic fluid, such as combination of an oil and a ferrite (low permittivity, high permeability and high loss). Further, certain ferrofluids also can be used to introduce a high loss tangent into the conductive fluid, for example those commercially available from FerroTec Corporation of Nashua, N.H. 03060. In particular, Ferrotec part numbers EMG0805, EMG0807, and EMG1111 can be used.
A hydrocarbon dielectric oil such as Vacuum Pump Oil MSDS-12602 could be used to realize a low permittivity, low permeability, and low loss tangent fluid. A low permittivity, high permeability fluid may be realized by mixing the hydrocarbon fluid with magnetic particles or metal powders which are designed for use in ferrofluids and magnetoresrictive (MR) fluids. For example magnetite magnetic particles can be used. Magnetite is also commercially available from FerroTec Corporation. An exemplary metal powder that can be used is iron-nickel, which can be provided by Lord Corporation of Cary, N.C. Fluids containing electrically conductive magnetic particles require a mix ratio low enough to ensure that no electrical path can be created in the mixture. Additional ingredients such as surfactants can be included to promote uniform dispersion of the particles. High permittivity can be achieved by incorporating solvents such as formamide, which inherently posses a relatively high permittivity. Fluid Permittivity also can 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 Conductive Fluids for Mixing/Unmixing or for Moving of Components
The composition processor 101 can be comprised of a plurality of fluid reservoirs containing component parts of conductive fluid 108. These can include: a first fluid reservoir 122 for a low permittivity, low permeability component of the conductive fluid; a second fluid reservoir 124 for a high permittivity, low permeability component of the conductive fluid; a third fluid reservoir 126 for a high permittivity, high permeability, high loss component of the conductive fluid. 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. For example, the third fluid reservoir 126 can contain a high permittivity, high permeability, low loss component of the conductive fluid and a fourth fluid reservoir can be provided to contain a component of the conductive fluid having a high loss tangent.
A cooperating set of proportional valves 134, mixing pumps 120, 121, and connecting conduits 135 can be provided as shown in
The process can begin in step 402 of
The controller 136 can cause the composition processor 101 to begin mixing two or more component parts in a proportion to form conductive fluid that has the updated loss tangent and permittivity values determined earlier. In the case that the high loss component part also provides a substantial portion of the permeability in the conductive fluid, the permeability will be a function of the amount of high loss component part that is required to achieve a specific attenuation. However, in the case that a separate high loss tangent fluid is provided as a high loss component part, the loss tangent can be determined independently of the permeability. This mixing process can be accomplished by any suitable means. For example, in
In step 410, the controller causes the newly mixed conductive fluid (or discrete and separate volumes of different conductive fluid-see
In step 414, the controller 136 compares the measured loss tangent to the desired updated loss tangent value determined in step 404. If the conductive fluid does not have the proper updated loss tangent value, the controller 136 can cause additional amounts of high loss tangent component part to be added or removed to the mix (or to or from discrete cavities within the resonant cavity) from reservoir 126, as shown in step 415.
If the conductive fluid is determined to have the proper level of loss in step 414, then the process continues on to step 416 where the measured permittivity and permeability from step 412 is compared to the desired updated permittivity or permeability value(s) determined in step 404. If the updated permittivity or permeability value(s) has not been achieved, then high or low permittivity or permeability component parts are mixed, added or removed as necessary, as shown in step 417. The system can continue circulating the conductive fluid through the resonant cavity 102 until the loss tangent, permeability and/or permittivity passing into and out of the resonant cavity 102 are the proper value, as shown in step 418. Once the loss tangent, permeability, and/or permittivity are the proper value, the process can continue to step 402 to wait for the next updated resonant cavity control signal.
Significantly, when updated conductive fluid is required, any existing conductive fluid must be circulated out of the resonant cavity 102. Any existing conductive fluid not having the proper loss tangent and/or permittivity can be deposited in a collection reservoir 128. The fluidic dielectric deposited in the collection reservoir 128 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 so that it may be re-used at a later time to produce additional conductive fluid. 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. For example, 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 that has a significantly different boiling point. A third fluid component can be comprised of 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 would utilize distillation system 130 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. A second stage process would introduce the mixture, free of the first fluid, into a chamber 132 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. 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. 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.
Referring to
The different types of conductive fluid can be constrained within the subcavities 250, 252 within the resonant cavity 102 which may be any number of capillary tubes or other cavities or chambers.
The resonant cavity 202 can be used in any circuit that can include any other type of resonant cavity. For example, the resonant cavity 202 can be used in conjunction with an antenna element 260. The resonant cavity 202 also can be used with other circuit devices, for example an oscillator or a filter. Moreover, the resonant cavity 202 can be used as a filter element. Still, there are many other applications where the resonant cavity 102 can be used, and such applications are understood to be within the scope of the present invention.
A composition processor 201 is provided for changing a composition of the conductive fluid to vary the overall permeability and/or permittivity within the resonant cavity 202. A controller 236 controls the composition processor for selectively varying the volume of various conductive fluid in response to a resonant system control signal 237. Volume control enables control of overall permittivity and/or permeability of the resonant cavity as well as control of group velocity and phase velocity of an RF signal within the resonant cavity 202, and thus resonances within the resonant cavity 202. The permittivity and/or permeability also can be adjusted to control the impedance of the resonant cavity. Volume control may also enable the ability to selectively vary the loss tangent of the fluidic dielectric along with the permittivity and/or permeability, to enable the controller 236 to control the Q and bandwidth of the resonant cavity 202.
In particular, the center frequencies at which the resonant cavity 202 resonates are determined by the dimensions of the resonant cavity, for example the distance between opposing walls. A change in permittivity and/or permeability, which results in a change in phase velocity and group velocity of a signal within a resonant cavity, effectively changes the relative dimensions of the resonant cavity with respect to signal wavelength. Accordingly, the controller 236 can control the center frequencies of the cavity resonances by adjusting the volumes of specific fluidic dielectric.
The composition processor 201 can be comprised of a plurality of fluid reservoirs containing component parts of conductive fluid. These can include one or more fluid reservoirs such as reservoirs 228 and 229 that can contain separate conductive fluid. For example one reservoir can have a low permittivity, low permeability component of the conductive fluid and another reservoir can have a high permittivity, high permeability, high loss component of the fluidic dielectric. Those skilled in the art will appreciate that other combinations of component parts may also be suitable based on a particular application and the invention is not intended to be limited to the specific combination of component parts described herein.
A cooperating set of valves and pumps 221, 223, and connecting conduits can be provided as shown in
The operation of the composition processor 201 operates similar to the composition processor 101 of FIG. 1 and can generally follow the process previously described in connection with the flowchart of FIG. 4. The process can begin in step 402 of
In step 410, the controller causes the discrete and separate volumes of different conductive fluids residing in the subcavities 250 and 252 to be circulated into the resonant cavity 202 through pumps 221 and 223. In step 414, the controller 236 can compare a measured loss tangent, permeability or permittivity to desired value(s) determined in step 404. If the conductive fluid does not have the proper updated value(s), the controller 236 can cause additional amounts of a given conductive fluid to be added or removed to or from the discrete cavities or subcavities (250 and 252) within the resonant cavity and to and from reservoirs 228 and 229, as shown in step 415. A simple embodiment may just require a full or empty cavity, but the present invention certainly contemplates partially filled cavities or subcavities to accomplished the desired results. An embodiment with many small cavities as shown in
If the conductive fluid is determined to have the proper level of loss in step 414, then the process continues on to step 416 where the measured permittivity and permeability from step 412 is compared to the desired updated permittivity or permeability value(s) determined in step 404. If the updated permittivity or permeability value(s) has not been achieved, then high or low permittivity or permeability component parts are added or removed as necessary, as shown in step 417. The system can continue circulating the conductive fluid(s) through the resonant cavity 202 until at least one among the loss tangent, permeability and/or permittivity passing into and out of the resonant cavity 202 are the proper value, as shown in step 418. Once the loss tangent, permeability, and/or permittivity are the proper value, the process can continue to step 402 to wait for the next updated resonant cavity control signal.
Referring to
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.
| Number | Name | Date | Kind |
|---|---|---|---|
| 4152567 | Mayfield | May 1979 | A |
| 5792236 | Taylor et al. | Aug 1998 | A |
| Number | Date | Country | |
|---|---|---|---|
| 20040207495 A1 | Oct 2004 | US |
