Abstract:
A new technique is presented that allows for controlling the phase of a propagating signal by selectively switching in and out relatively small perturbations along a transmission line section that provide slightly different physical paths for the currents to follow. By using relatively minor perturbations, the phase of a transmission line section can be controlled without drastically altering the impedance of the section, thereby maintaining good impedance matching properties. Also, by keeping the alternate current paths small, generally fine control of phase shift is possible along with allowing the design to remain relatively simple. Such tunable elements can then be incorporated in designs where resonators (or other elements) are separated by specific phase lengths to construct other signal processing functions, such as filters.

Description:
CROSS-REFERENCED APPLICATIONS 
       [0001]    This application claims priority from U.S. Patent Application No. 60/949,446, entitled METHOD AND APPARATUS FOR ALTERING PHASE SHIFT ALONG A TRANSMISSION LINE SECTION, filed on Jul. 12, 2007. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    The present invention relates generally to transmission lines and, more specifically, to tunable transmission lines. 
         [0004]    2. Description of the Related Art 
         [0005]    Most modern radar and communication systems rely on phase shifting elements as signal processing components, often in systems that combine multiple radiating signals of varying amplitudes and phases to control the directionality of radiated signals (i.e. phased-array systems). As these elements are often one of the last in a signal&#39;s transmit chain, their relative losses contribute significantly to the overall system performance (and limitations). In addition to loss, since any signals reflected due to mismatch will re-enter other components in the signal chain and potentially cause unwanted effects, their ability to stay “matched” while controlling their relative phase is very important. 
         [0006]    There are many well known techniques for developing phase-shifting components for radio frequency (RF) systems; most introductory textbooks in the field include the basic concepts for such structures. The most common techniques can be broken down into three primary categories; reflective, loaded-line and switched-line phase shifters. 
         [0007]    In reflective-type phase shifters (see for example,  FIG. 1 ), the incoming signal is guided to a strongly reflective circuit element, often a variable capacitance, whose relative phase angle is related to the impedance of the element (as normalized to the impedance of the connecting transmission line). If the relative impedance can be controlled, the reflected signal will then have a phase angle directly related to the varied impedance of the circuit element. This element is often a varactor or other variable capacitance. In most cases the incoming and outgoing signals are separated from each other through the use of a 90° hybrid element, although a 3-port circulating device is also sometimes used. Drawbacks of this approach are that multiple components are needed (hybrids, varactors, RF chokes, etc) which all introduce loss along the signal path. 
         [0008]    In “loaded-line” type phase shifters, the distributed capacitance (or inductance) of a section of line is designed to be adjustable. Since the velocity of the signal traveling along the line obeys the relationship 
         [0000]        v= 1/√{square root over ( L′C′ )} 
         [0000]    As the capacitance (or inductance) per unit length is varied, the signal will speed up or slow down accordingly. Such phase shifting elements are referred to as true time-delay phase shifters. Since the impedance (and therefore the reflections) of a transmission line section vary with the same parameters as velocity, it can be difficult to maintain good impedance properties while achieving large phase shifts. 
         [0009]    Now, in switched-line phase shifters, multiple transmission-line paths are arranged in parallel from the input to the output of the circuit. Switching components are then added to the circuit to control which physical path the signal travels along. As such, different paths can be designed to provide whatever fixed phase lengths are desired. While good impedance matching can be achieved, one drawback of this technique is that large circuit areas are required for laying out the multiple transmission paths, which can be prohibitively expensive on many integrated circuit processes. 
       SUMMARY OF THE INVENTION 
       [0010]    A technique is disclosed that can allow for a tunable phase delay that has relatively low loss, is simple to design and control, is compact, and is easy to incorporate with various transmission line topologies. The technique allows for variable phase shifts by switching in and out small perturbations distributed along a transmission line element. By making the perturbations along the signal path generally small, the effective phase shift of a transmission line section can be achieved while reflections due to any impedance mismatch can be kept relatively small. 
         [0011]    In some embodiments, small slots may be placed orthogonally along the transverse direction of a slot-type transmission line section to form a corrugated structure. The currents travel generally along the edges of the conductors and therefore follow the corrugations in the metal. In some embodiments, capacitive switches are placed in shunt with these “corrugations” such that when the switches are turned “on” they short circuit the particular corrugation they are placed across, allowing substantially all of the current to bypass that particular groove, thereby altering the phase delay of the signal. This process can be repeated on a “per unit length” basis so that varying amounts of total phase change can be achieved. 
         [0012]    Small perturbations along a transmission line or waveguide section may provide an effective signal path that can be controlled by selectively switching in and out the perturbations to vary the phase length of that path. In this manner, the input-to-output phase delay can be controlled. The impedance of the transmission line section may also be varied in the same manner, in order to control the magnitude of a signal (from input to output), in addition to or instead of controlling the phase of a signal. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0013]    For a more complete understanding of the present invention and the advantages thereof, reference is now made to the following Detailed Description taken in conjunction with the accompanying drawings, in which: 
           [0014]      FIG. 1  illustrates a schematic representation of a prior-art reflective phase shifter; 
           [0015]      FIG. 2  provides a schematic representation of a prior art slot-type transmission line showing the current flow; 
           [0016]      FIG. 3  provides a schematic representation of a corrugated slot-type transmission line showing how the current flow changes to conform to the slots; 
           [0017]      FIG. 4  provides a schematic representation of a corrugated slot-type transmission line showing how the current flow changes when an individual slot is “shorted out”; 
           [0018]      FIG. 5  provides a graph showing the change in phase shift through a section of transmission line of  FIG. 4  as the states of the corrugations are changed; 
           [0019]      FIG. 6  provides a schematic representation of a transmission line resonator; 
           [0020]      FIG. 7  provides a schematic representation of the present invention as implemented in a folded-waveguide topology; and 
           [0021]      FIG. 8  provides a graph showing the frequency response of the tunable resonator shown in  FIG. 7 . 
       
    
    
     DETAILED DESCRIPTION 
       [0022]    In the following discussion, numerous specific details are set forth to provide a thorough understanding of the present invention. However, those skilled in the art will appreciate that the present invention may be practiced without such specific details. In other instances, well-known elements have been illustrated in schematic or block diagram form in order not to obscure the present invention in unnecessary detail. Additionally, for the most part, details concerning network communications, electromagnetic signaling techniques, and the like, have been omitted inasmuch as such details are not considered necessary to obtain a complete understanding of the present invention, and are considered to be within the understanding of persons of ordinary skill in the relevant art. 
         [0023]      FIG. 1  illustrates a schematic representation of a prior-art reflective phase shifter  100 . Reflective phase shifter  100  may have terminals  102  and  104  coupled to a hybrid power divider  106 . Variable capacitors  108  and  110  may act as tunable elements. 
         [0024]      FIG. 2  shows a schematic representation of a prior art slot-type transmission line  200 , also known as a slot-line. The transmission line  200  consists of two conducting surfaces  202  and  204 , separated by a non-conducting gap comprising the slot  206 . A signal source  208  is illustrated as providing power to a load  210  via the conducting surfaces  202  and  204 . The power is illustrated by currents  212  and  214 , which are induced at the radio frequency (RF) frequency of the signal source  208 . In order to satisfy the boundary conditions described by Maxwell&#39;s equations, the currents  212  and  214  will flow substantially along the edges of the conducting surfaces  202  and  204  respectively, along the slot  206 . As a result, currents  212  and  214  will travel along an electrical path length of the transmission line  200  that is related to the physical length of slot  206 . 
         [0025]      FIG. 3  illustrates a transmission line  300 , comprising conductors  302  and  304 , which may be separated by a slot  306 . The transmission line  300  may further comprise ten perturbations,  308   a  through  308   e  and  310   a  through  310   e,  along the length of the slot  306 , which may be configured to alter an electrical path length traversed by currents  312  and  314  on the transmission line  300 . Thus, the transmission line  300  may comprise a corrugated slot-line. Since the currents  312  and  314  may follow the conducting edges, the effective electrical path length (as seen by the currents  312  and  314 ) of the transmission line  300  may be longer than the electrical path length (as seen by the currents  212  and  214 ) of the transmission line  200 , as shown in  FIG. 2 , even when the physical length (distance from source  208  to load  210 ) is the same. Since the impedance of a transmission line is a function of its cross-sectional geometry, the impedance in the area of the slots will be different than the reference line. However, if perturbations  308   a  through  308   e  and  310   a  through  310   e  are small enough, the electrical path length may be changed while the overall effect on the impedance of transmission line  300  may remain small. 
         [0026]    Referring to  FIG. 3 , the perturbations  308   a  through  308   e  and  310   a  through  310   e  may comprise secondary slots, illustrated as generally orthogonal to a primary slot  306 , which is between the conductors  302  and  304 . In some embodiments, the perturbations  308   a  through  308   e  and  310   a  through  310   e  may be configured to cause a change in the electrical resistance of the transmission line  300 . The perturbations  308   a  through  308   e  in conductor  302  may be disposed generally opposite the slot  306  from a corresponding one of the perturbations  310   a  through  310   e  in conductor  304 , forming pairs. While the illustrated embodiment comprises ten perturbations in two conductors, a greater or lesser quantity of either perturbations or conductors may be used. Further, other arrangements of perturbations could be implemented, in addition to the arrangement illustrated in  FIG. 3 , that allow for a relatively uniform cross section, such that the impedance remains relatively constant in substantially all areas. 
         [0027]    The transmission line  300  further may comprise one or more switching elements  316   a  through  316   e  and  318   a  through  318   e  coupled to the conductors  302  and  304 , respectively, and configured to selectively bypass a corresponding one of the perturbations  308   a  through  308   e  and  310   a  through  310   e  for tuning the transmission line  300 . By placing the switching elements  316   a  through  316   e  and  318   a  through  318   e  across the slots comprising the perturbations  308   a  through  308   e  and  310   a  through  310   e,  individual perturbations  308   a  through  308   e  and  310   a  through  310   e  may be substantially removed (or shorted out) from the signal path traversed by the current  312  and/or the current  314 . The switching elements  316   a  through  316   e  and  318   a  through  318   e  may comprise transistors, diodes and/or microelectromechanical systems (MEMS) switches, and may be actuated either individually or along in pairs for tuning the transmission line  300 . In the illustrated embodiment of  FIG. 3 , the switching elements  316   a  through  316   e  and  318   a  through  318   e  may be arranged in pairs, with switching elements  316   a  through  316   e  disposed generally opposite the primary slot  306  from a corresponding one of the switching elements  318   a  through  318   e.  However, it should be understood that a different arrangement may be used, other than a pairing configuration on opposing sides of a primary slot, to selectively shorten the electrical path length. 
         [0028]    The switching elements  316   a  through  316   e  and  318   a  through  318   e  may be configured to be reactive and/or resistive in order to selectively tune a signal. If a switching mechanism is reactive, the phase shift of the current flowing through it can be further adjusted or varied if desired as there will be a phase delay associated with the reactive element. Resistive switching elements may allow for selectively tuning the signal by selectively adjusting or varying the electrical loss of the signal over the transmission line  300 . 
         [0029]      FIG. 4  illustrates that the current flow may be modified by “switching” out one of the corrugations in the transmission line  300 ; the overall phase shift through a line such as the transmission line  300  (as the corrugations are switched out) is shown in the graph of  FIG. 5 . In  FIG. 4 , switching elements  316   c  and  318   c  may be actuated as a pair, to short out the perturbations  308   c  and  310   c,  respectively, for tuning the transmission line  300 . The electrical path length of transmission line  300  may thus be altered, allowing the currents  312  and  314  to take shorter electrical path routes than going around the slots of the perturbations  308   c  and  310   c.    
         [0030]    As shown in  FIG. 5 , Curves  502 - 510  of graph  500  depict the change in phase shift, as a function of frequency, through a section of an embodiment of transmission line  300  as different numbers of the perturbations  308   a  through  308   e  and  310   a  through  310   e  are bypassed. For example, the curve  502  shows the insertion phase of an embodiment of transmission line  300  if a single pair of switching elements is actuated to bypass a single pair of perturbations, as illustrated in  FIG. 4 . The curves  504 ,  506 ,  508  and  510  illustrate insertion phase when two, three, four and five pairs of switching elements are actuated, respectively. 
         [0031]    Present technology provides for a number of different circuit elements that allow for the slots to be selectively switched in and out for tuning the transmission line. Some examples include transistors, p-n and metal-semiconductor junction diodes, and MEMS switches (both ohmic and capacitive-contact varieties). Each technology offers different advantages and disadvantages, depending on the final design goals and the manufacturing processes available to the designer. The present invention differs from the classic “switched-line” phase shifter where the current flow is designed to be switched drastically from state to state (along alternative transmission line sections), while in the present invention the changes in the current flow may be designed to be small and may not drastically alter the current flow along a single transmission line section. 
         [0032]    Filters are one of the most common RF elements used in radar and communication systems. Bandpass filters in particular are used extensively to eliminate unwanted signals that are spectrally close to the signal of interest. Such filters often consist of one or more resonator elements coupled together in a way to obtain the desired passband characteristic for tuning the transmission line. 
         [0033]    At microwave frequencies, resonator elements are often formed using “distributed” techniques, exploiting the electrical length between one or more circuit elements to obtain the desired electrical response. In the case for bandpass filters, large reflections are spaced 90° apart at the center frequency of the filter, with the constructive interference resulting in a “bandpass” response that has low loss at the center frequency and higher loss at frequencies above and below the center. Once the coupling is designed (to achieve the proper filter shape), the entire filter response can be tuned across frequency by adjusting the lengths of lines that make up the resonator elements. Since the present invention may be designed to provide a simple mechanism to alter the electrical path length of a transmission line, it may be well suited to be incorporated into a filter design to provide tunability. 
         [0034]      FIG. 6  provides an illustrative drawing of a filter device comprising a transmission line resonator  600 , which may comprise two conductors  302  and  304  coupled to two frequency-dependent resonator elements  602  and  604 . The resonator elements  602  and  604  may be coupled on different ends of the transmission line resonator  600 , separated by a distance  606 . Generally, when two or more resonator elements are connected together, various filter shapes can be achieved by adjusting the amount of coupling from resonator to resonator. However, once the overall filter shape is achieved, the response of the filter can be tuned to different center frequencies by adjusting the electrical length (and therefore the resonant frequency) of each resonator using the current invention. 
         [0035]    In the embodiment shown in  FIG. 6 , switching elements  316   a  through  316   e  coupled to the conductor  302  at perturbations  308   a  through  308   e,  respectively, and switching elements  318   a  through  318   e  coupled to the conductor  304  at perturbations  310   a  through  310   e,  respectively, may be selectively switched to bypass one or more of the perturbations  308   a  through  308   e  and  318   a  through  318   e.  The selective bypassing of the one or more perturbations  308   a  through  308   e  and  318   a  through  318   e  may allow for tuning of the electrical signal across the transmission line  300  by altering the electrical path of the signal, thereby increasing or decreasing attenuation of the signal to vary, adjust or tune electrical loss. Further, multiple transmission line resonators, such as transmission line resonator  600 , may be placed end-to-end. 
         [0036]    The current invention may be further applied to other filter structures that rely on transmission line elements for electrical performance.  FIG. 7A  illustrates a transmission line filter structure  700  that may consist of a conductor  702  embedded within a folded “H-Plane” waveguide  710 , yielding a tunable filter based on waveguide technology. Conductor  702  may comprise nine perturbations  704   a  through  704   i  and two shorting posts  706   a  and  706   b.  The section of the transmission line  700  between shorting posts  706   a  and  706   b  may form a distributed resonator  714 . 
         [0037]    As shown in further detail in  FIG. 7B , shorting out perturbation corrugations  704   a  through  704   i  along the transmission line  700  within the resonator  714  may alter the electrical path between shorting posts  706   a  and  706   b.  Switching elements  712   a  through  712   i  may be coupled to the conductor  702  at perturbations  704   a  through  704   i  to allow for selectively switching the elements to bypass one or more of the perturbations  704   a  through  704   i.  If the switching elements  712   a  through  712   i  are also resistive, then the signal may be attenuated by adjusting, varying or tuning the electrical loss of the signal across the transmission line. Therefore, the resonant frequency of structure  700  may be changed by switching in and out the various corrugations  704   a  through  704   i  along the line using switching elements  712   a  through  712   i.    
         [0038]    The frequency response is plotted in curves  802  through  806  of graph  800  in  FIG. 8 . Curves  802 ,  804  and  806  show the change in resonant frequency as zero, one and two of perturbations  704   a  through  704   i  are bypassed, respectively. 
         [0039]    Having thus described the present invention by reference to certain of its preferred embodiments, it is noted that the embodiments disclosed are illustrative rather than limiting in nature and that a wide range of variations, modifications, changes, and substitutions are contemplated in the foregoing disclosure and, in some instances, some features of the present invention may be employed without a corresponding use of the other features. Many such variations and modifications may be considered desirable by those skilled in the art based upon a review of the foregoing description of preferred embodiments. Accordingly, it is appropriate that the appended claims be construed broadly and in a manner consistent with the scope of the invention.