Abstract:
Various exemplary embodiments include a technique for tuning a filter to have two stop bands. This technique may involve combination of signals from a plurality of high-band notch resonators and low-band notch resonators. Loop wires may couple both high-band and low-band notch resonators to a central conductor, thereby enabling the central conductor to transmit a signal having dual stop bands.

Description:
TECHNICAL FIELD 
     Various exemplary embodiments relate generally to a tunable band stop filter and, more particularly, to a filter having two notches in its frequency response. 
     BACKGROUND 
     Many systems use filters to selectively attenuate certain signal frequencies. Band stop filters greatly reduce signal strength within a particular band of frequencies, but otherwise permit the signal to pass through the filter without attenuation. In some cases, a filter may need to have two stop bands instead of one, selectively removing these dual bands without impacting other frequencies. 
     Band stop filters are also known as notch filters. Other names for such filters include band limit, T-notch, band-elimination, and band-reject. Regardless of the assigned name, all of these filters block transmission of a relatively narrow band of frequencies, where the highest blocked frequency is usually no more than one hundred times the lowest blocked frequency. 
     Existing techniques can couple band stop filters together, but such techniques have certain drawbacks. For example, a cross-slot iris may couple two resonating cavities, transferring a magnetic field from a first cavity to a second cavity. In conventional systems, such magnetic field transfer may involve an elongated string of cavities, where the first cavity is aligned along the same axis as the second cavity. 
     However, it may be difficult to provide tuning when collinear cavities are coupled by an iris. Because the iris may be disposed along the central line, it may not be possible to move the cavities once they are linked together. Moreover, it may not be easy for a user to access the iris if a large number of cavities are coupled together in a string. Such a structure may be cumbersome and difficult to store. 
     In addition, a known technique for combining notch filters to produce a double stop bands may produce a stretched, unwieldy structure. Cascading a first notch filter into a second notch filter, according to this conventional approach, would require an elongated transmission line, stretched out along the length of both the first notch filter and the second notch filter. 
     Moreover, cascading notch filters together may result in a degraded signal. While the initial notch filter would theoretically only subtract a stop band from a signal, it may also produce significant distortion and noise. This is particularly true if the initial notch filter consisted of a plurality of cavity resonators, wherein each resonator might contribute a small amount of distortion or noise. Therefore, the output of the cascaded notch filters would not produce a clean signal with two stop bands but a spectrum with significant noise and distortion. 
     For the foregoing reasons and for further reasons that will be apparent to those of skill in the art upon reading and understanding this specification, there is a need for an improved way of tuning a filter with two stop bands. There is also a need to produce a dual stop band characteristic on a transmission line that uses a more compact configuration. Furthermore, there is a need to produce dual stop bands without using cascaded filters. 
     SUMMARY 
     In light of the present need for an improved technique for tuning a filter with two stop bands, a brief summary of various exemplary embodiments is presented. Some simplifications and omissions may be made in the following summary, which is intended to highlight and introduce some aspects of the various exemplary embodiments, but not to limit the scope of the invention. Detailed descriptions of a preferred exemplary embodiment adequate to allow those of ordinary skill in the art to make and use the inventive concepts will follow in later sections. 
     In various exemplary embodiments, a tunable filter that provides dual stop bands may comprise a central conductor disposed along a first axis; and a plurality of filter elements that encompass the central conductor, each of the filter elements aligned along a respective axis substantially orthogonal to the first axis, each of the filter elements further comprising: a high-band notch resonator disposed on a first side of the central conductor; a low-band notch resonator disposed on a second side of the central conductor, the second side being substantially opposite to the first side; and a coupling element disposed between the high-band notch resonator and the central conductor, disposed between the low-band notch resonator and the central conductor, and soldered so that at least a portion of the coupling element is substantially orthogonal to the central conductor along the respective axis of the filter element, wherein the coupling element combines signals from the high-band notch resonator and the low-band notch resonator to produce a filtered signal that has the dual stop bands disposed symmetrically on either side of a central frequency, and wherein the coupling element has a length substantially equal to an integral multiple of a quarter wavelength of the central frequency. 
     In various exemplary embodiments, the central conductor may be a transmission line. Alternatively, the central conductor may be a stripline. In a further exemplary embodiment, the central conductor may be a coaxial line. In yet another exemplary embodiment, the central conductor may be a microstrip line. 
     In various exemplary embodiments, the coupling element may comprise a loop wire, the loop wire extending from the high-band notch resonator to the low-band notch resonator. The loop wire may extend through a first open slot in a cavity wall of the high-band notch resonator to the central conductor and extend from the central conductor through a second open slot in a cavity wall of the low-band notch resonator. 
     In various exemplary embodiments, a tuner for a band stop filter may comprise a coupling element that combines signals from a high-band notch resonator and a low-band notch resonator to produce a filtered signal that has dual stop bands disposed symmetrically on either side of a central frequency; and a central conductor that receives the filtered signal from the coupling element, wherein the coupling element may have a length equal to an integral multiple of a quarter wavelength of the central frequency and the coupling element is soldered to be substantially perpendicular to the central conductor. 
     In various exemplary embodiments, a method of tuning a signal to produce dual stop bands may comprise: using a plurality of high-band notch resonators to produce a first notch in a signal characteristic; using a plurality of low-band notch resonators to produce a second notch in the signal characteristic; using a plurality of coupling elements to combine signals from the plurality of high-band notch resonators and the plurality of low-band notch resonators to produce a filtered signal that has dual stop bands disposed symmetrically on either side of a central frequency; and sending the filtered signal from the coupling elements to a central conductor, wherein each of the coupling elements may have a length equal to an integral multiple of a quarter wavelength of the central frequency and each of the coupling elements is soldered to be substantially perpendicular to the central conductor. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In order to better understand the various exemplary embodiments, reference is made to the accompanying drawings, wherein: 
         FIG. 1  is a perspective view of an exemplary tunable filter; 
         FIG. 2  is a top view of an exemplary filter element; 
         FIG. 3  is a top view of an exemplary loop wire; 
         FIG. 4  is a diagram of an exemplary filter response for the tunable filter; 
         FIG. 5   a  is a flow chart of an exemplary method of tuning a signal to produce dual stop bands; and 
         FIG. 5   b  is a flow chart of another exemplary method of tuning a signal to produce dual stop bands. 
     
    
    
     DETAILED DESCRIPTION 
     Referring now to the drawings, in which like numerals refer to like components or steps, there are disclosed broad aspects of various exemplary embodiments. 
       FIG. 1  is a perspective view of an exemplary tunable filter  100 . Tunable filter  100  may comprise four high-band notch resonators  110   a ,  110   b ,  110   c ,  110   d , four low-band notch resonators  120   a ,  120   b ,  120   c ,  120   d , at least one coupling element  130 , and a central conductor  140 . These elements are described in detail below. 
     Tunable filter  100  may comprise a plurality of high-band notch resonators  110   a ,  110   b ,  110   c ,  110   d  disposed along a first axis. High-band resonators  110   a ,  110   b ,  110   c ,  110   d  may have metallic walls to prevent leakage of electromagnetic fields between respective cavities inside high-band resonators  110   a ,  110   b ,  110   c ,  110   d . While four high-band resonators  110   a ,  110   b ,  110   c ,  110   d  are depicted in  FIG. 1 , the number of high-band resonators  110   a ,  110   b ,  110   c ,  110   d  may vary depending upon their desired application, as will be apparent to those having ordinary skill in the art. 
     High-band resonators  110   a ,  110   b ,  110   c ,  110   d  may be box-shaped, having rectangular cross-sections. Alternatively, high-band resonators  110   a ,  110   b ,  110   c ,  110   d  may be cylindrical, having circular cross-sections. Other implementations of high-band resonators  110   a ,  110   b ,  110   c ,  110   d , such as a spherical configuration, may be used as will be apparent to those having ordinary skill in the art. 
     High-band resonators  110   a ,  110   b ,  10   c ,  10   d  may be fabricated from a metal having a high thermal conductivity. For example, as will be apparent to those having ordinary skill in the art, aluminum, a metal with a thermal conductivity value of 221 W/mK, could be used. Alternatively, a non-metallic material, such as ceramic, may be used so long as high-band resonators  110   a ,  110   b ,  110   c ,  110   d  are disposed within a housing that can evacuate heat at a sufficient rate. 
     The tunable filter  100  may also comprise a plurality of low-band notch resonators  120   a ,  120   b ,  120   c ,  120   d  disposed along a second axis. Unlike conventional techniques that have collinear cavities, the second axis may be separated from and parallel to the first axis in this arrangement. Low-band resonators  120   a ,  120   b ,  120   c ,  120   d  may have metallic walls to prevent leakage of electromagnetic fields between respective cavities inside low-band resonators  120   a ,  120   b ,  120   c ,  120   d . While four low-band resonators  120   a ,  120   b ,  120   c ,  120   d  are depicted in  FIG. 1 , the number of low-band resonators  120   a ,  120   b ,  120   c ,  120   d  may vary depending upon their desired application, as will be apparent to those having ordinary skill in the art. 
     As with high-band notch resonators  110   a ,  110   b ,  110   c ,  110   d , low-band resonators  120   a ,  120   b ,  120   c ,  120   d  may be box-shaped, having rectangular cross-sections. Alternatively, low-band resonators  120   a ,  120   b ,  120   c ,  120   d  may be cylindrical, having circular cross-sections. Other implementations of low-band resonators  120   a ,  120   b ,  120   c ,  120   d , such as a spherical configuration, may be used as will be apparent to those having ordinary skill in the art. 
     Low-band resonators  120   a ,  120   b ,  120   c ,  120   d  may be fabricated from a metal having a high thermal conductivity. For example, as will be apparent to those having ordinary skill in the art, aluminum, a metal with a thermal conductivity value of 221 W/mK, could be used. Alternatively, a non-metallic material, such as ceramic, may be used so long as low-band resonators  120   a ,  120   b ,  120   c ,  120   d  are disposed within a housing that can evacuate heat at a sufficient rate. 
     The tunable filter  100  may further comprise a coupling element  130  that combines signals from a single high-band notch resonator  110   a  and a single low-band notch resonator  120   a  to produce a filtered signal that has a dual stop band characteristic. Coupling element  130  may be a wire made of a metal that is sufficiently malleable, ductile, and electrically conductive. As will be apparent to those of ordinary skill in the art, an inexpensive design choice for coupling element  130  may be copper. However, any suitable material may be used for coupling element  130 , provided that the material is both capable of electrically coupling high-band resonator  110   a  to low-band resonator  120   a  and bendable so that the amount of coupling between high-band resonator  110   a  and low-band resonator  120   a  is easily tunable. 
     While only a single coupling element  130  is marked in  FIG. 1 , tunable filter  100  may use a plurality of coupling elements  130 . In such a case, each coupling element  130  may correspond to a respective pair of high-band  110   a ,  110   b ,  110   c ,  110   d  and low-band  120   a ,  120   b ,  120   c ,  120   d  notch resonators. Each coupling element  130  may be regularly spaced to provide a more symmetric signal. 
     The total length of coupling element  130  may be designed to provide a desired central frequency. The central frequency may be a frequency directly between the high stop band and the low stop band. The length of coupling element  130  may be an integral multiple of one-quarter wavelength of the central frequency. 
     The tunable filter  100  may additionally comprise a central conductor  140  that receives the filtered signal from coupling element  130 . Central conductor  140  may be a transmission line. Alternatively, central conductor  140  may be a stripline. In a further exemplary embodiment, central conductor  140  may be a coaxial line. In yet another exemplary embodiment, central conductor  140  may be a microstrip line. 
       FIG. 2  is a top view of an exemplary filter element  200 . Filter element  200  may comprise a loop wire  210 , a high-band notch resonator  220 , a low-band notch resonator  230 , a first open slot  240 , a central conductor  250 , and a second open slot  260 . These elements are described in detail below. 
     Filter element  200  may comprise a loop wire  210  made of a bendable metal such as copper. Copper may also be a good design choice for coupling element  200  because copper has an electrical conductivity of 60 mmhos/m, the second highest electrical conductivity of any element after silver. Loop wire  210  may extend from a high-band notch resonator  220  to a low-band notch resonator  230 . Loop wire  210  may extend through a first open slot  240  in a cavity wall of high-band notch resonator  220  to a central conductor  250  and extend from central conductor  250  through a second open slot  260  in a cavity wall of low-band notch resonator  230 . 
     First open slot  240  and second open slot  260  may be fabricated to be of minimal size. As will be apparent to those having ordinary skill in the art, electromagnetic waves may leak out of a cavity resonator having an aperture such as open slot. Consequently, a designer may plug first open slot  240  and second open slot  260  with respective metallic blocks to reduce leakage after loop wire  210  is inserted through both first open slot  240  and second open slot  260 . 
     Filter element  200  may act as a tuner, combining signals from high-band notch resonator  220  and low-band notch resonator  230  to produce a filtered signal that has dual stop bands. Central conductor  250  may receive this filtered signal from both resonators  220 ,  230 . For efficient coupling, loop wire  210  may be perpendicular to central conductor  250  to maximize energy transfer. Alternative coupling arrangements are also possible, as will apparent to those having ordinary skill in the art. 
     In various exemplary embodiments, central conductor  250  may be a transmission line. Alternatively, central conductor  250  may be a stripline. In a further exemplary embodiment, central conductor  250  may be a coaxial line. In yet another exemplary embodiment, central conductor  250  may be a microstrip line. 
       FIG. 3  provides a top view of an exemplary loop wire  300 , which may correspond to loop wire  210  in  FIG. 2 . Loop wire  300  may comprise a first end  310 , a second end  315 , a first bent portion  320 , a second bent portion  325 , a first coupling portion  330 , a second coupling portion  335 , a third bent portion  340 , a fourth bent portion  345 , a first wall portion  350 , a second wall portion  355 , and an energy transfer portion  360 . These elements are described in detail below. 
     A first end  310  of the loop wire  300  may be mounted on a wall of a first cavity resonator, such as high-band resonator  110   a  depicted in  FIG. 1 . A second end  315  of the loop wire  300  may be mounted on a wall of a second cavity resonator, such as low-band resonator  120   a  depicted in  FIG. 1 . Both the first end  310  and the second end  315  of the loop wire  300  may be disposed perpendicularly to the respective walls of the cavity resonators  110   a ,  120   a.    
     A first bent portion  320  of the loop wire  300  may be orthogonal to the first end  310  of the loop wire  300 . Similarly, a second bent portion  325  of the loop wire  300  may be orthogonal to the second end  315  of the loop wire  300 . Both the first bent portion  320  and the second bent portion  325  may be respectively directed toward central conductors of the cavity resonators  110   a ,  120   a.    
     A first coupling portion  330  of the loop wire  300  may be parallel to a central conductor within high-band cavity resonator  110   a . A second coupling portion  335  of the loop wire  300  may be parallel to a central conductor within low-band cavity resonator  120   a . Bending loop wire  300  may alter the respective lengths of first coupling portion  330  and second coupling portion  335 , thereby respectively tuning the amount of electrical energy coupled from resonators  110   a ,  120   a . While such bending may occur in first bent portion  320  and second bent portion  325 , a user may bend other portions of loop wire  300  to change the effective amount of coupling from first coupling portion  330  and second coupling portion  335 , as will be apparent to those having ordinary skill in the art. 
     A third bent portion  340  of the loop wire  300  may be orthogonal to the first coupling portion  330  of the loop wire  300 . Similarly, a fourth bent portion  345  of the loop wire  300  may be orthogonal to the second coupling portion  335  of the loop wire  300 . Both the third bent portion  340  and the fourth bent portion  345  may be respectively directed away from central conductors of the cavity resonators  110   a ,  120   a.    
     A first wall portion  350  of the loop wire  300  may be disposed substantially along a wall of the high-band cavity resonator  110   a . Similarly, a second wall portion  355  of the loop wire  300  may be disposed substantially along a wall of the low-band cavity resonator  120   a . Because first wall portion  350  and second wall portion  355  are relatively distant from the central conductors of cavity resonators  110   a ,  120   a  and located near a conductive wall, they couple an insignificant amount of energy compared to first coupling portion  330  and second portion  335 . First wall portion  350  and second wall portion  355  may be respectively orthogonal to third bent portion  340  and fourth bent portion  345 . 
     The energy transfer portion  360  of the loop wire  300  may be disposed perpendicular to a transmission line, such as central conductor  140  in  FIG. 1 . Energy transfer portion  360  may also be orthogonal to both first wall portion  350  and second wall portion  355 . Energy transfer portion  360  may be directly soldered onto central conductor  140 , using an appropriate soldering technique, as will be apparent to those having ordinary skill in the art. 
     The structure described for loop wire  300  above is intended to be exemplary and illustrative, not limiting in scope. As will be apparent to those having ordinary skill in the art, loop wire  300  may be fabricated with other shapes, depending upon the applicable resonator filter environment. Such shapes may be designed so that the total length of loop wire  300  is substantially an integral multiple of a quarter wavelength corresponding to a central frequency between the dual stop bands. 
       FIG. 4  depicts an exemplary filter response  400  for the tunable filter  100  of  FIG. 1 . Filter response  400  may comprise a first notch  410 , a pass band  420 , and a second notch  430 . These elements are described in detail below. 
     As shown in  FIG. 4 , filter response  400  displays the frequency characteristics of a dual notch filter. A first notch  410  may occur in a stop band of frequencies extending from roughly 1695 MHz to 1720 MHz. A pass band  420  may occur next, defined by the 0 dB magnitude between roughly 1730 and 1740 MHz. A second notch  430  may appear on the opposite side of pass band  420  from first notch  410 . Second notch  430  may encompass frequencies ranging from roughly 1750 to 1770 MHz. 
     First notch  410  and second notch  430  may be disposed symmetrically on either side of a central frequency within pass band  420 . The central frequency within pass band  420  may be used to design the length of loop wire  300 , as depicted in  FIG. 3 . While loop wire  300  may have a length of one quarter wavelength of the central frequency, loop wire  300  could also have a length of an integral multiple of the same quarter wavelength in order to achieve similar electrical characteristics. 
     As described above, frequency response  400  is intended to be exemplary and illustrative, not limiting in scope. As will be evident to those having ordinary skill in the art, first notch  410  and second notch  420  may be designed to occur at different frequency values. The widths of both first notch  410  and second notch  420  may vary to encompass broader or narrower frequency spectra, depending upon applicable resonator designs. A designer may also change the depths of both first notch  410  and second notch  420 , depending upon the desired rejection level of the stop bands. 
       FIG. 5   a  depicts an exemplary method  500  of tuning a signal to produce dual stop bands. Method  500  starts in step  505 . It then proceeds to step  510 , where a plurality of high-band notch resonators  110   a ,  110   b ,  110   c ,  110   d  produce a first notch in a signal characteristic. Next, in step  520 , a plurality of low-band notch resonators  120   a ,  120   b ,  120   c ,  120   d  create a second notch in the signal characteristic. The first and second notches may be symmetrically disposed on either side of a central pass band. 
     In step  530 , at least one coupling element  130  combines signals from the high-band notch resonators  110   a ,  110   b ,  110   c ,  110   d  and low-band notch resonators  120   a ,  120   b ,  120   c ,  120   d  to produce a filtered signal that has dual stop bands. In step  540 , the at least one coupling element  130  transmits this filtered signal into a central conductor  140 . Such transmission may be most efficient when the coupling element  130  is soldered to be substantially perpendicular to the central conductor  140 . The method stops in step  545 . 
       FIG. 5   b  depicts another exemplary method  550  of tuning a signal to produce dual stop bands. Exemplary method  550  resembles exemplary method  500  but uses a parallel approach instead of a serial technique. Thus, in method  550 , steps  510  and  520 , instead of occurring in succession, may be substantially simultaneous. Parallel production of a high-band notch and a low-band notch may result in faster operation of exemplary tunable filter  100  and simplify its operation. 
     Although the various exemplary embodiments have been described in detail with particular reference to certain exemplary aspects thereof, it should be understood that the invention is capable of other embodiments and its details are capable of modifications in various obvious respects. As is readily apparent to those skilled in the art, variations and modifications may be implemented while remaining within the spirit and scope of the invention. Accordingly, the foregoing disclosure, description, and figures are for illustrative purposes only and do not in any way limit the invention, which is defined only by the claims.