Abstract:
A slot line band reject pass filter including a substrate of insulating material having slot line primary conductors formed thereon. One or more supplemental conductors are preferably coupled to the slot line primary conductors to achieve rejection of a desired frequency. Several embodiments of supplemental conductors are disclosed including substantially closed loop and non loop segments that extend in a range from parallel to perpendicular from the primary conductors. The supplemental conductors may be directly or electromagnetically coupled, or both.

Description:
FIELD OF THE INVENTION 
     The present invention relates to slot line band reject filters. 
     BACKGROUND OF THE INVENTION 
     The prior art provides several types of filters for use with radio frequency signals including high pass, low pass, band pass, notch and other types of filters fabricated in lumped or distributed form. Filters of these types have been formed in a variety of transmission media. 
     To accommodate higher frequency signals some filters have been fabricated in microstrip transmission media using distributed elements. Microstrip transmission media generally consists of one or more thin conducting strips of finite width that are arranged parallel with a single extended conducting ground plan. In its common form, the strips are fixed to one side of an insulating substrate and the ground plane is attached to the other side. While microstrip transmission media have been recognized as possible conductors for higher frequency signals, microstrip transmission media also have disadvantageous aspects. These aspects include that the fabrication of microstrip circuits is process intensive, involving (1) metalization on two sides of a substrate and (2) the formation of interconnecting vias between the two surface materialization layers to achieve proper grounding. 
     Coplanar waveguide (CPW) and slot line are alternative types of transmission media. Both CPW and slot line support uniplanar fabrication, though they have not been used widely for high frequency signal propagation. 
     To provide less expensive and more efficient circuit construction, a need exists to form circuits that support high frequency operation (approximately &gt;1 GHz) in a uniplanar transmission media. To provide necessary signal processing, a need exists to provide circuit components such as band reject filters and the like in such media. Suitable band reject filters will provide LO, image reject and spurious frequency filtering. 
     SUMMARY OF THE INVENTION 
     Accordingly, it is an object of the present invention to provide a slot line band reject filter. 
     It is another object of the present invention to provide a slot line band reject filter that affords flexibility in the design of performance characteristics. 
     It is another object of the present invention to provide a band reject filter that is compact in size. 
     It is also an object of the present invention to provide a uniplanar implemented image reject filter that is suitable for use in a radio system. 
     These and related objects of the present invention are achieved by use of a slot line band reject filter as disclosed herein. 
     The attainment of the foregoing and related advantages and features of the invention should be more readily apparent to those skilled in the art, after review of the following more detailed description of the invention taken together with the drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a diagram of a slot line band reject filter in accordance with the present invention. 
     FIGS. 2A-2C are diagrams of alternative embodiments of the slot line band reject filter of FIG. 1 in accordance with the present invention. 
     FIGS. 3A-3B are diagrams of other embodiments of a slot line band reject filter in accordance with the present invention. 
     FIG. 4 is a diagram of other embodiments of a slot line band reject filter in accordance with the present invention. 
     FIG. 5 is a diagram of another embodiment of a band reject slot line filter in accordance with the present invention. 
     FIG. 6 is a diagram of another embodiment of a band reject slot line filter in accordance with the present invention. 
    
    
     DETAILED DESCRIPTION 
     Slot line transmission media generally consists of two semi-infinite coplanar conducting planes affixed to the same side of an insulating substrate of arbitrary thickness and separated by a finite gap. In the present invention, the slot line transmission media is preferably implemented in strip format. Amongst other benefits, slot line transmission media provides significant flexibility in component layout and the benefits of uniplanar fabrication. 
     The filters described herein are preferably formed on a substrate that may include fused silica, ceramic, plastic, Teflon, glass, air or the like. Though preferably formed with slot line strips, filters of the present invention may also be formed with infinite or semi-infinite ground planes. 
     Referring to FIG. 1, a diagram of a band reject filter in accordance to the present invention is shown. The band reject filter  10  includes a positive signal line  30  and a negative signal line  40 . Positive signal line  30  is comprised of a principal positive conductor  51  and three conducting segments  54 - 56  (which form a supplemental conductor) arranged to form a loop  52  in conjunction with a section of conductor  51 . Similarly, negative signal line  40  is comprised of a principal negative conductor  61  and three conducting segments  64 - 66  (which form a supplemental conductor) arranged to form a loop  62  in conjunction with a section of conductor  61 . 
     Segments  54 - 56  have a combined length termed L 2 , while the section of conductor  51  defined by the intersection of segments  54 , 56  has a length termed L 1 . A similar conductor arrangement is provided in negative signal line  40 . The rejection center frequency of filter  10  is inversely proportional to the difference between L 1  and L 2 . Rejection of a desired frequency is achieved through destructive interference. 
     It should also be recognized that although members  54 - 56  are straight and orthogonally arranged, these members (and the principal conductors to which they attach) can be curved, zigzag, trapezoidal, circular, amorphous or otherwise shaped. 
     With respect to design criteria, it has been recognized that the center frequency, fc, of filter  10  relates to L 1  and L 2  as follows:        fc   ≅     C     2          (     ε   r     )       1   /   2            (       (     L1   -   L2     )     /   2.91     )                                
     where C is the speed of light, L 1  and L 2  are as shown in FIG. 1, and ε r  is the dielectric constant of the substrate. It should be recognized that fc is proportional to 1/(L 1 −L 2 ) because fc generally increases as L 2  increases. 
     Referring to FIGS. 2A-2C, diagrams of alternative embodiments of the band reject filter of FIG. 1 in accordance with the present invention are shown. FIG. 2A illustrate a filter in which the loops  52 , 62  are configured such that the long dimension of L 2  is disposed substantially perpendicular to the center line of the filter. 
     FIG. 2B illustrates the formation of loops  52 , 62  in a circular, oval or elliptical pattern. In this filter, L 2  may approach a maximum while L 1  may approach a minimum, depending on the final design. FIG. 2C illustrates generally circular loops  52 , 62  that are electromagnetically coupled to primary conductors  51 , 61 . Though loops  52 , 62  of FIGS. 2B-2C are substantially circular as illustrated, other shapes may be utilized. 
     Referring to FIG. 3A, a diagram of another embodiment of a band reject filter  110  in accordance with the present invention is shown. Band reject filter  110  includes positive and negative signal lines  130 , 140 , respectively. Supplemental conductors (or resonators)  171 , 181  are respectively coupled through connecting conductors  172 , 182  and through gaps  173 , 183  to the positive and negative signal lines  130 , 140 . The supplemental conductors  171 , 181  each have a length of approximately one-quarter wavelength of the rejection center frequency. Though conductors  172 , 182  are shown connecting the supplemental conductors to signal lines  130 , 140  proximate an input  121  of filter  110 , one or both of connecting conductors  172 , 182  could alternatively be provided proximate an output  122  of filter  110  (i.e., connected at the other end of the supplemental conductor from the end shown). Frequency cancellation occurs by presenting a short circuit at the rejection center frequency to both the positive and negative signal lines  130 , 140 . The short circuit is due to the open circuit at the end of supplemental conductors  171 , 181  transformed through a quarterwave. 
     The impedance of the transmission line can be varied to optimize filter characteristics by modifying the width of supplemental conductors  171 , 181  and their respective spacing from the positive and negative signal lines. 
     Referring to FIG. 3B, a diagram of another embodiment of a slot line band reject filter in accordance with the present invention is shown. The filter arrangement shown in FIG. 3B is similar to that shown in FIG. 3A, however, the supplemental conductors  171 , 181  are staggered as compared to being generally symmetrically positioned as shown in FIG.  3 A. The left most pair of supplemental conductors  171 ′, 181 ′ overlapped, while the right most pair of supplemental conductors  171 ′, 181 ′ do not overlap. While the conductors  171 , 181  are shown paired, it should be recognized that the present invention includes non-pair supplemental conductors. 
     Referring to FIG. 4, a diagram of another embodiment of a slot line band reject filter  210  in accordance with the present invention is shown. Filter  210  comprises positive and negative principal conductors  230 , 240 , respectively. A pair of resonators (or supplemental conductors)  235 , 245 , are coupled to the positive and negative signal lines. Each of these resonators is preferably a quarter wavelength (or multiple thereof) of a center frequency (of the rejection frequency) in length and open circuited such that each presents a short circuit at the principal conductor to signals approximately at the rejection center frequency. The short circuit attenuates these signals. 
     A second pair of resonators  270 , 280  may also be coupled to positive and negative signal lines  230 , 240 . These resonators  270 , 280  are preferably a quarter wavelength of a center frequency in length and their spacing from resonator  235 , 245  is preferably approximately a half wavelength of the center frequency. The spacing is also preferably optimized to achieve a required rejection profile (band rejection depth and width). 
     It should be recognized that the band reject filter of FIG. 4 can be constructed by using only a single resonator, such as resonators  235  or  245 , a plurality of staggered single resonators, a single pair of resonators or a plurality of pairs of resonators, or a combination thereof. Furthermore, supplemental conductors (resonators) of the types shown in FIGS. 3 and 4 could be combined. Considerations in filter design include providing a sufficient number and arrangement of resonators to achieve a desired rejection profile, while minimizing circuit size. Two single, staggered (asymmetrically arranged) resonators  291 , 292  are shown in dashed lines to achieve a desired band rejection filter profile. 
     It should further be recognized that while rectilinear edged supplemental conductors are shown herein, these conductors may have a non-rectilinear shape, including amorphous shapes that are empirically or otherwise determined to provide a desired profile. In addition, the performance of the filters described herein may be modified (optimized) by modifying the width of the supplemental conductors that achieve signal rejection. 
     Referring to FIG. 5, a diagram of another embodiment of a slot line band reject filter  310  in accordance with the present invention is shown. Filter  310  includes positive and negative supplemental conductors  316  and  318  that respectively extend from and return to the positive and negative principal conductors  312  and  314  in such a manner as to form transmission line (slot line) segments. The length of these transmission line segments  316 , 318  is preferably one-quarter wavelength of the rejection frequency such that a voltage minima is returned to the principal conductors for that frequency. 
     Referring to FIG. 6, a diagram of another embodiment of a slot line band reject filter  410  in accordance with the present invention is shown. Filter  410  includes a supplemental conductor  415  which is connected to the positive and negative principal conductors  412 , 414  and forms a loop that is approximately an integer multiple of a wavelength of the rejection frequency. Inductive traces  417 , 418  and interdigitated capacitor  421  provide impedance matching. Leads  423  provide propagation of non-rejected frequencies through to output positive and negative single conductors  412 ′, 414 ′. 
     While the invention has been described in connection with specific embodiments thereof, it will be understood that it is capable of further modification, and this application is intended to cover any variations, uses, or adaptations of the invention following, in general, the principles of the invention and including such departures from the present disclosure as come within known or customary practice in the art to which the invention pertains and as may be applied to the essential features hereinbefore set forth, and as fall within the scope of the invention and the limits of the appended claims.