Patent Publication Number: US-6992550-B2

Title: Adjustable transmission line stub including a conductive fluid

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. 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. 
   SUMMARY OF THE INVENTION 
   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. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a conceptual diagram useful for understanding the variable transmission line stub of the invention. 
       FIG. 2A  is a cross-sectional view of the transmission line stub structure in  FIG. 1 , taken along line section  2 — 2 . 
       FIG. 2B  is a cross-sectional view of an alternative embodiment of a transmission line stub. 
       FIG. 2C  is a cross-sectional view of another alternate embodiment of a transmission line stub. 
       FIG. 3A  is a graphical representation of electrical characteristics of a transmission line stub in an open circuit configuration. 
       FIG. 3B  is a graphical representation of electrical characteristics of a transmission line stub in a short circuit configuration. 
       FIG. 4A  is a top view of yet another embodiment of a transmission line stub. 
       FIG. 4B  is a cross-sectional view of the transmission line stub structure in  FIG. 4A , taken along section line  4 — 4 . 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   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. 
     FIG. 1  is a conceptual diagram that is useful for understanding the variable transmission line stub of the present invention. In  FIG. 1 , a transmission line tuning apparatus  100  is presented which includes a radio frequency circuit  102 . The radio frequency circuit  102  includes a transmission line stub  104  and a signal return conductor  124 , each of which can be at least partially coupled to a dielectric substrate  110 . For example, the signal return conductor  124  can be a ground plane which is coupled to first side  130  of the dielectric substrate  110  opposing the transmission line stub  104  which is coupled to a second side  132 . A cross-sectional view of the transmission line stub structure in  FIG. 1 , taken along line section  2 — 2 , is shown in FIG.  2 A. At this point it should be noted that the present invention is not limited to any particular dielectric substrate or insulator. For example, and without limitation, the transmission line stub  104  can be insulated from the signal return line  124  by vacuum, a gas (e.g. air), rubber, plastic, substrate materials (e.g. ceramic, fiberglass, silicon, etc.), or any other dielectric. 
   While the embodiment of the invention in  FIG. 1  is shown essentially in the form of a microstrip construction, the invention herein is not intended to be so limited. Instead, the invention can be implemented using any type of transmission line. For instance, the invention can be implemented in transmission line configurations including conventional waveguides, stripline, buried microstrip, coaxial lines, and embedded coplanar waveguides. All such structures are intended to be within the scope of the invention. An example of a buried microstrip arrangement is shown in FIG.  2 B. In this configuration the transmission line stub can be sandwiched between the dielectric substrate  110  and a second dielectric substrate  202 . In a stripline configuration, a second ground plane (not shown) can be coupled to a side of the second dielectric substrate  202  opposing the transmission line stub  104 . In a coaxial arrangement, which is shown in  FIG. 2C , the transmission line stub  104  can be a conductor of a coaxial cable  210  which includes a dielectric insulator  212 . In this example, the transmission line stub  104  is presented as the center conductor of the coaxial cable  210  and the outer conductor is shown as the signal return conductor  124 . Nonetheless, one skilled in the art will appreciate that this configuration can be reversed so that the transmission line stub is the outer conductor and the inner conductor is the signal return line. 
   Referring again to  FIG. 1 , the transmission line stub  104  can be configured to have an input port  106  located where the transmission line stub  104  connects to a transmission line  108 , or some other portion of the circuit. The transmission line stub  104  as shown is generally rectangular in shape, but the transmission line stub can be any one of a variety of transmission line stub shapes. For example the transmission line stub  104  can be cylindrical, tapered, or have a complex shape with a variety of different widths, lengths, and/or thicknesses. Moreover, the transmission line stub  104  can be implemented using a printed circuit board, wires, cables and/or other transmission line conductors. 
   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  FIG. 3A , the voltage minima for an open circuit transmission line stub is typically located at one-quarter of a wavelength from the termination  112  and the voltage maxima is typically located at one-half of a wavelength from the termination  112 . Accordingly, the impedance at the point  302  which is three-eighths of a wavelength from the termination  112  will have inductive characteristics since the impedance at that point will increase as the frequency increases and the impedance will decrease as the frequency decreases. It should be noted that the change in impedance is caused by the change in relative positions of voltage maxima and voltage minima resulting from changes in signal wavelength as the frequency is varied. The positions of voltage maxima and minima will move closer to the termination when the frequency increases and further from the termination as the frequency decreases. 
   If the same transmission line stub is short circuited, as shown in the graphical representation  310  of  FIG. 3B , the voltage maxima typically will be located at one-quarter wavelength from the termination  112  and the voltage minima typically will be located at one-half wavelength. Accordingly, the impedance at point  302 ′ which is three-eighths of a wavelength from the termination  112  will have capacitive characteristics since the impedance at this point  302 ′ will decrease with an increase in frequency and increase with a decrease in frequency. Hence, it becomes apparent that by changing the termination of a transmission line stub from open circuit to short circuit, or from short circuit to open circuit, the impedance characteristics of transmission line stub having reactive impedance components also can be changed. For example, a transmission line stub which presents a capacitive input impedance can be changed to have an inductive input impedance, and vice versa. 
   Fluid Control System 
   Referring once again to  FIG. 1 , it can be seen that the invention preferably includes a fluid control system  150  for selectively controlling the presence and/or removal of the conductive fluid  126  from the fluid conduit  114 . The fluid control system can comprise any suitable arrangement of pumps, valves and/or conduits that are operable for effectively injecting and/or removing conductive fluid  126 . A wide variety of such fluid control systems may be implemented by those skilled in the art. For example, in one embodiment, the fluid control system can include a reservoir  152  for conductive fluid  126  and a pump  154  for injecting the conductive fluid into the fluid conduit  114 . 
   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  FIG. 4A , an exemplary radio frequency circuit  400  comprising a plurality of fluid conduits  412 ,  414 ,  416  is shown. A cross-sectional view of  FIG. 4A  taken along section lines  4 — 4  is shown in FIG.  4 B. Notably, three fluid conduits are shown for exemplary purposes, but any number of fluid conduits can be provided. The fluid conduits  412 ,  414 ,  416  can be disposed to provide a short circuit termination of the transmission line stub  404  at various points along the length of the transmission line stub  404 . Accordingly, the distance between the input port  406  and the termination of the transmission line stub can be varied. In consequence, a particular input impedance to the transmission line stub  404  can be selected by injecting a conductive fluid  444  into the appropriate fluid conduit  412 ,  414 ,  416 . 
   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.