Patent Grant
6930568
1. Statement of the Technical Field
The present invention relates to the field of delay lines, and more particularly to variable RF delay lines.
2. Description of the Related Art
Delay lines are used for a wide variety of signal processing applications. For example, broadband time delay circuits are used in beam-forming applications in phased array antennas. Typical fixed geometry, true time delay circuits used in phased array antennas are comprised of switched lengths of transmission line. Despite the importance of broadband delay lines in such systems, the conventional approach to designing and implementing these components suffer from a number of drawbacks. For example, conventional delay line devices often require a relatively large number of RF switches that can result in signal losses. Also, conventional time delay circuits can be limited with regard to the delay resolution that can be achieved.
RF delay lines are often formed as ordinary transmission lines coupled to a dielectric. Depending upon the structure of the transmission line, the dielectric can be arranged in different ways. For example, microstrip, stripline circuits commonly are formed on a dielectric substrate. Two important characteristics of dielectric materials are permittivity (sometimes called the relative permittivity or ∈r) and permeability (sometimes referred to as relative permeability or μr). 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 effect the electrical length of a transmission line and therefore the amount of delay introduced to signals that traverse the line.
Further, ignoring loss, the characteristic impedance of a transmission line, such as stripline or microstrip, is equal to √{square root over (Ll/Cl)} where Ll is the inductance per unit length and Cl is the capacitance per unit length. The values of Ll and Cl 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.
For a given geometry, an increase in dielectric permittivity or permeability necessary for providing increased time delay will generally cause the characteristic impedance of the line to change. However, this is not a problem where only a fixed delay is needed, since the geometry of the transmission line can be readily designed and fabricated to achieve the proper characteristic impedance. When a variable time delay is needed, however, such techniques have traditionally been viewed as impractical because of the obvious difficulties in dynamically varying the permittivity and/or permeability of a dielectric board substrate material and/or dynamically varying transmission line geometries.
Other types of variable delay lines include implementations of lines that have used mechanical means to vary the electrical length. One such arrangement included a plurality of telescoping tubes to produce a variable length coaxial line. These devices were at one time fairly common in laboratories for tuning circuits, but they suffered from certain drawbacks. For example, they were subject to wear, difficult to control electronically, and not easily scaleable to microwave frequencies. Accordingly, the only practical solution for electronic implementations of variable delay lines has been to use conventional fixed length RF transmission lines with delay variability achieved using a series of electronically controlled switches.
The invention concerns a method and apparatus for producing a variable delay for an RF signal. The method can include the step of propagating the RF signal along an RF transmission line, coupling a fluidic dielectric to the RF transmission line, and dynamically changing a composition of the fluidic dielectric to selectively vary its permittivity and/or its permeability in response to a time delay control signal. The method can also include the step of dynamically changing a composition of the fluidic dielectric to vary its permeability. The permittivity and the permeability can be varied concurrently in response to the time delay control signal. In a preferred embodiment the method can include selectively varying the permeability concurrently with the permittivity to maintain a characteristic impedance of the transmission line approximately constant.
A continuously variable true time delay line apparatus in accordance with the invention includes a fluidic dielectric and a composition processor adapted for changing a composition of the fluidic dielectric to vary its permittivity. The delay line can also include an RF transmission line at least partially coupled to the fluidic dielectric substrate. A controller controls the composition processor for selectively varying the permittivity in response to a time delay control signal. The composition processor is further adapted for changing a composition of the fluidic dielectric to vary the permeability. The controller causes the composition processor to selectively vary the permittivity and the permeability in response to the time delay control signal.
The transmission line can also be coupled to a solid dielectric substrate material. In that case, the permeability of the fluidic dielectric is varied to be approximately equal to μr,sub(∈r/∈r,sub) where μr,sub is the permeability of the solid dielectric substrate, ∈r is the permittivity of the fluidic dielectric and ∈r,sub is the permittivity of the solid dielectric substrate. The velocity of propagation will be lower for higher values of ∈r and μr. According to one aspect of the invention, the solid dielectric substrate can be formed from a ceramic material. For example, the solid dielectric substrate can be formed from a low temperature co-fired ceramic.
The composition processor can be comprised of a plurality of fluid reservoirs containing fluidic dielectric components. These can include a first fluid reservoir for a low permittivity, low permeability component of the fluidic dielectric, a second fluid reservoir for a high permittivity, low permeability component of the fluidic dielectric, and a third fluid reservoir for a high permittivity, high permeability component of the fluidic dielectric. The delay line system can also include one or more proportional valves, mixing pumps, and conduits for selectively mixing and communicating the components of the fluidic dielectric from the fluid reservoirs to a cavity coupled to the RF transmission line. Further, the composition processor can advantageously separate the component parts of the fluidic dielectric so that they can be reused in subsequent fluidic dielectric mixtures.
The fluidic dielectric can be comprised of an industrial solvent that can have a suspension of magnetic particles contained therein. For example, the fluid dielectric can contain between about 50% to 90% magnetic particles by weight. The paramagnetic particles can be formed of a material selected from the group consisting of a ferrite, metallic salts, and organo-metallic particles.
a is a cross-sectional view of the transmission line structure in
b is a cross-sectional view of an alternative embodiment of a transmission line structure of FIG. 1.
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 time delay and transmission line characteristic impedance. 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 time delay 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 delay line device. However, devices with higher insertion loss may be acceptable in some instances so this may not be a critical factor. Many applications also require delay lines with a broadband response. Accordingly, it may be desirable in many instances to select component mixtures that produce a fluidic dielectric that has a relatively constant response over a broad range of frequencies.
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 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 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 permittivty and are therefore suitable for use as the high permittivity component. Alternatively, the high permittivity component of the fluidic dielectric can be produced 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
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 138 can be provided as shown in
The process can begin in step 202 of
In step 206, the controller can determine an updated permeability value required for maintaining a constant characteristic impedance of transmission line 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
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 are also preferably positioned within solid dielectric substrate 102 to measure the permeability and permittivity of the fluidic dielectric passing through input conduit 112 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.
If the fluidic dielectric 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. 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 time delay control signal.
Significantly, when updated fluidic dielectric is required, any existing fluidic dielectric must 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.
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 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
In theory, constant characteristic impedance can be obtained for a transmission line by maintaining a constant ratio of permittivity to permeability in the dielectric to which the line is coupled. Accordingly, in those instances where the transmission line is for all practical purposes coupled exclusively to the fluidic dielectric, then it is merely necessary to maintain a constant ratio of ∈r/μr, where ∈r is the permittivity of the fluidic dielectric, and μr is the permeability of the fluidic dielectric. A cross-sectional view of such a line is illustrated in
a is a cross-sectional view of one embodiment of the transmission line structure in
b is a cross-sectional view showing an alternative transmission line structure 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 substrate 142′. In the case where the transmission line is also partially coupled to a solid dielectric, then the permeability μr necessary to keep the characteristic impedance of the line constant can be expressed as follows:
μr=μr,sub(∈r/∈r,sub)
where μr,sub is the permeability of the solid dielectric substrate 142, ∈r is the permittivity of the fluidic dielectric 108 and ∈r,sub is the permittivity of the solid dielectric substrate 142.
Transmission line impedance is not independent of the transmission line structure. However, it is always proportional to the square root of the ratio of the permeability to the permittivity of the media in which the conducting structures are embedded. Thus, for any transmission line, if both the permeability and permittivity are changed in the same proportion, and no other changes are made, the impedance will remain constant. The equation specified enforces the condition of a constant ratio of μ to ∈, and thus ensures constant impedance for all transmission line structures.
At this point it should be noted that while the embodiment of the invention in
According to one aspect of the invention, the solid dielectric substrate 102, 138, 142 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's proven suitability for manufacturing miniaturized RF circuits, make it a natural choice for use in the present invention.
The United States Government has rights in this invention pursuant to Contract No. NRO000-02-C-0388 between the National Reconnaissance Office and Harris Corporation.
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| Number | Date | Country | |
|---|---|---|---|
| 20040095208 A1 | May 2004 | US |
