Patent Application
20050035834
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. A 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. Accordingly, transmission line stubs are often referred to as tuned lines or resonant lines. It should be noted, however, that transmission line stub impedance characteristics are actually a function of voltage reflections, not circuit resonance.
On printed circuit boards or substrates, transmission line stubs 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. On an open circuited transmission line stub, each point at an even number of quarter-wavelengths from the termination is at a position of voltage maxima and has a high impedance, while each point at an odd number or quarter wavelengths from the termination is at a position of voltage minimum and has a low impedance. Notably, the relative positions of voltage maxima and minima on a shorted-circuited transmission line stub are reversed in comparison to the positions of voltage maxima and minima on an open circuited transmission line stub.
The input impedance to an open or shorted transmission line stub is typically resistive when the length of the transmission line stub is an even or odd multiple of a quarter-wavelength of the operational frequency. That is, the input to the transmission line stub is at a position of voltage maxima or minima. When the input to the transmission line stub 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 to provide complex impedance characteristics.
Transmission line 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.
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 transmission line 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.
The present invention relates to a circuit for processing radio frequency signals that includes an adjustable transmission line stub. The adjustable transmission line stub has an input at one end, an electrical length and a termination. The circuit also includes a signal return conductor and at least one fluid conduit extending from the transmission line stub to the signal return conductor. A fluid control system, which can be responsive to a control signal, is provided for selectively moving a conductive fluid from a first position to a second position. The fluid control system can include a pump for moving the conductive fluid between the first and second positions.
In the first position, the conductive fluid can be disposed in a fluid conduit to provide an electrically conductive path between the transmission line stub and the return conductor to produce a first tuned circuit response. According to one aspect of the invention, the conductive fluid used in the invention can be a liquid metal, a liquid metal alloy and/or a solvent electrolyte mixture.
The fluid conduit can be a bore, a via, a channel and/or a tube. In the second position, the conductive fluid is moved to a second position where the conductive fluid does not provide an electrically conductive path between the transmission line and the return conductor, thereby producing a second tuned circuit response distinct from the first tuned circuit response. A third tuned circuit response, which is different from the first and second tuned circuit responses, can be produced by forming at least a second conductive path with the conductive fluid between the transmission line stub and the signal return conductor.
At least one electrical characteristic of the transmission line stub is changed when the conductive fluid is moved from the first position to the second position. The electrical characteristic can be a position of a voltage maxima or minima on the transmission line stub, and/or an input impedance of the transmission line stub. The transmission line stub can have an electrical length equal to some integer multiple of about one quarter wavelength at a design operating frequency.
The present invention relates to an adjustable transmission line stub. The electrical characteristics of the transmission line stub can be adjusted by changing the termination of the transmission line stub between an open circuit configuration and a short circuit configuration. A conductive fluid is provided to short the transmission line stub to a return conductor in the short circuit configuration. The conductive fluid can be removed to return the transmission line to the open circuit configuration.
While the embodiment of the invention in
Referring again to
A fluid conduit 114 can extend from the transmission line stub 104 to the signal return conductor 124. The fluid conduit 114 can be any conduit that can contain a conductive fluid 126 so that electrical continuity can be provided between the transmission line stub 104 and the signal return conductor 124 when the conductive fluid 126 is present. In particular, the fluid conduit 114 can be a bore, via, channel, tube or any other type of conduit which extends at least from the transmission line stub 104 to the signal return conductor 124. In one arrangement, the fluid conduit 114 can be a bore that extends from the transmission line stub 104, through the dielectric substrate 110 and to the signal return conductor 124. In another arrangement, the bore can extend through the transmission line stub 104 and the signal return conductor 124 as well. Accordingly, the conductive fluid 126 can be injected into the fluid conduit 114 to electrically short the transmission line stub 104 to the signal return conductor 124 in a first operational state.
In a second operational state, the conductive fluid 126 can be purged from the fluid conduit 114 so that the transmission line stub is open circuited with respect to the signal return conductor 124. For example, a vacuum or positive pressure can be used to purge the conductive fluid 126 from the fluid conduit 114. In one arrangement, the conductive fluid can be replaced with a fluid dielectric 162 or a gas. Typical fluid dielectrics can include oil, such as Vacuum Pump Oil MSDS-12602, and/or solvents, such as formamide. Typical gases can include air, nitrogen, helium, and so on. Importantly, the invention is not limited to any particular fluid dielectric 162 or gas. Those skilled in the art will recognize that the examples of fluid dielectric or gas as disclosed herein are merely by way of example and are not intended to limit in any way the scope of the invention.
As noted, the input impedance at the input port 106 of an open circuited transmission line stub is high if the input port 106 is positioned at a voltage maximum, that is an even number of quarter-wavelengths from the transmission line stub termination 112. The input impedance at the input port 106 of an open circuited transmission line stub 104 is low if the input port 106 is positioned at a voltage minimum, which is an odd number of quarter-wavelengths from the termination 112. However, also as noted, the relative positions of voltage maxima and voltage minima can be reversed by changing the transmission line stub termination from an open circuit to a short circuit. Accordingly, an open circuited transmission line stub which has a low input impedance can be short circuited at the termination 112 to change the input impedance to high. Further, the input impedance to an open circuited transmission line stub can be changed from high to low. Likewise, the input impedance to a transmission line stub having a short circuited termination can be changed from high to low, or from low to high, by removing the short circuit condition.
If the input port of an open circuited transmission line stub is at a position between voltage maxima and voltage minima, the input impedance will have reactive components. In particular, as shown in graphical representation 300 of
If the same transmission line stub is short circuited, as shown in the graphical representation 310 of
Fluid Control System
Referring once again to
The conductive fluid 126 can be injected into the fluid conduit 114 by means of a suitable fluid transfer conduit 116. A second fluid transfer conduit 118 can also be provided for permitting the conductive fluid 126 to be purged from the fluid conduit 114 so that the conductive fluid 126 does not provide electrical continuity between the transmission line stub 104 and the signal return conductor 124. Further, fluid valves 120, 122 can be provided between the fluid transfer conduits 116, 118 and the fluid conduit 114. The fluid valves 120, 122 can be mini-electromechanical or micro-electromechanical systems (MEMS) valves, which are known to the skilled artisan. The fluid valves 120, 122 can be closed to contain the conductive fluid 126 within the fluid conduit 114 during the first operational state when the transmission line stub is short circuited, and opened when the conductive fluid 126 is purged from the fluid conduit 114.
When it is desired to purge the conductive fluid from the fluid conduit 114, a pump 156 can be used to draw the conductive fluid 126 from the fluid conduit 114 into reservoir 170. Alternatively, in order to ensure a more complete removal of all conductive fluid from the fluid conduit 114, one or more pumps 158 can be used to inject a dielectric solvent 162 into the fluid conduit 114. The dielectric solvent 162 can be stored in a second reservoir 164 and can be useful for ensuring that the conductive fluid 126 is completely and efficiently flushed from the fluid conduit 114. A control valve 166 can be used to selectively control the flow of conductive fluid 126 and dielectric solvent 162 into the fluid conduit 114. A mixture of the conductive fluid 126 and any excess dielectric solvent 162 that has been purged from the fluid conduit 114 can be collected in a recovery reservoir 170. For convenience, additional fluid processing, not shown, can also be provided for separating dielectric solvent from the conductive fluid contained in the recovery reservoir for subsequent reuse. However, the additional fluid processing is a matter of convenience and not essential to the operation of the invention.
A control circuit 172 can be configured for controlling the operation of the fluid control system 150 in response to an analog or digital fluid control signal 174. For example, the control circuit 172 can control the operation of the various valves 120, 122, 166, and pumps 154, 156, 158 necessary to selectively control the presence and removal of the fluid dielectric and the dielectric solvent from the fluid conduit 114. It should be understood that the fluid control system 150 is merely one possible implementation among many that could be used to inject and purge conductive fluid from the fluid conduit 114 and the invention is not intended to be limited to any particular type of fluid control system. All that is required of the fluid control system is the ability to effectively control the presence and removal of the conductive fluid 126 from the fluid conduit 114.
Composition of Conductive Fluid
According to one aspect of the invention, the conductive fluid used in the invention can be selected from the group consisting of a metal or metal alloy that is liquid at room temperature. The most common example of such a metal would be mercury. However, other electrically-conductive, liquid metal alloy alternatives to mercury are commercially available, including alloys based on gallium and indium alloyed with tin, copper, and zinc or bismuth. These alloys, which are electrically conductive and non-toxic, are described in greater detail in U.S. Pat. No. 5,792,236 to Taylor et al, the disclosure of which is incorporated herein by reference. Other conductive fluids include a variety of solvent-electrolyte mixtures that are well known in the art. As for conductivity, using a non-perfect conductor, some energy will pass through and some will be dissipated as heat in the conductive material. Conductivities greater than 20 would be desirable, although effective systems could be employed utilizing conductivities as low as 1 or 2.
Multiple Fluid Conduits
In the most basic form, the invention can be implemented using a single fluid conduit. However, multiple fluid conduits can be used to adjust the transmission line stub. Referring to
For example, fluid conduit 412 can be filled with conductive fluid 444 to short the transmission line stub 404 at, or near, the end 410 of the transmission line stub 404. Accordingly, the input impedance of the transmission line stub 404 can be changed with respect to the open circuit input impedance, as previously noted. While fluid conduit 412 remains filled with conductive fluid and fluid conduits 414, 416 are unfilled, or filled with a dielectric fluid or gas, the effective length of the transmission line stub will be determined by location of the fluid conduit 412 which is located at the end 410 of the transmission line stub 404.
Fluid conduit 414 can be located at a distance from the end 410 of the transmission line stub 404, for instance one-eighth of a wavelength. The fluid conduit 414 can be filled with conductive fluid 444 if it is desired to short circuit the transmission line stub to the signal return conductor 454 at the location of the fluid conduit 414. Accordingly, the electrical length of the transmission line stub 404 can be effectively reduced by one-eighth of a wavelength, resulting in a corresponding change to the input impedance of the transmission line stub 404. Likewise, fluid conduit 416 can be filled with conductive fluid 444 to further shorten the effective length of the transmission line stub.
As noted, the fluid control system can comprise any suitable arrangement of pumps, valves, conduits and controllers that are operable for effectively injecting and removing conductive fluid 444, or any other fluid or gas, from the fluid conduits 412, 414, 416. For example, the fluid control system can include reservoirs 442, 446, control valves 432, 434, 436, 438, 440 and pumps 450, 452 to inject the conductive fluid 444 or fluid dielectric 448 in the appropriate fluid conduit. The fluid control system also can include fluid transfer conduits 420, 422, 424 to couple the fluid control system to the fluid conduits 412, 414, 416. Further, fluid transfer conduits 426, 428, 438 and an appropriate pump (not shown) can be provided to remove the conductive fluid 444 or fluid dielectric 448 from the fluid conduits 412, 414, 416.
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.
