Abstract:
A multiple-section bandpass filter for broadcast communications includes adjacent waveguide segments with a perpendicular connecting segment between them to form a U-shaped signal path. The waveguide cavities of the segments may be extruded and rectangular in cross section, and have a groundplane spacing that allows signal propagation between filter sections by evanescent coupling. Resonators in each of the adjacent segments have a separation that establishes the coupling bandwidths without the need for passive decoupling structures. A cross coupling conductor between the adjacent segments provides a capacitive or inductive coupling between them. A decoupling structure may be located in the connecting segment.

Description:
FIELD OF THE INVENTION 
   This invention relates generally to the field of electromagnetic signal communication and, more specifically, to the filtering of high power signals for broadcast communications. 
   BACKGROUND OF THE INVENTION 
   In the field of broadcast communications, electrical filters are required to separate a desired signal from energy in other bands. These bandpass filters are similar to bandpass filters in other fields. However, unlike most other electrical bandpass filters, filters for broadcast communication must be capable of handling a relatively high input power. For example, a signal input to a broadcast communications filter might have an average power between 5 and 100 kilowatts (kW). Many electronic filters do not have the capacity for such large signal powers. 
   For many years, high power electrical bandpass filtering has included the use of waveguide cavity filters. A variety of different waveguide filter types have evolved, each having its particular benefits and drawbacks. One popular class of filter in the industry is based on a pseudo-elliptical filter function. This type of filter function may be achieved in a number of different ways. Some waveguide bandpass filters make use of the “evanescent mode” to provide coupling between the separate resonators of a filter. In an evanescent mode filter, the waveguide is “below cutoff” (i.e., having a cross-sectional dimension small enough that frequencies within a desired passband cannot proceed normally from one end of the cavity to the other). In such a filter, resonances are formed between the inductance of a section of the waveguide, and the capacitance of a resonator, typically in the form of an adjustable length element projecting into the cavity, such as a tuning screw. 
   SUMMARY OF THE INVENTION 
   In accordance with the present invention, a multiple-section bandpass filter is provided for filtering broadcast communications in a predetermined frequency band. The filter operates in evanescent mode and has coupling bandwidths between adjacent filter sections that establish a frequency band for the filter between f L  and f H . The filter has a waveguide that includes a first segment and a second segment adjacent to each other in a direction perpendicular to the signal propagation direction of each segment, and a connecting segment that has a perpendicular orientation to that of the first and second segments. The connecting segment connects a cavity of the first segment with a cavity of the second segment to form a continuous cavity through which a signal propagates along a substantially U-shaped path. As evanescent mode cavities, each of the waveguide segments has a predetermined groundplane spacing that creates a lower cutoff frequency f C =c/2a that is higher that f H , where “c” is the speed of light and “a” is the groundplane spacing. 
   Within each of the first and second waveguide segments are resonators, each of which comprises a conductor that extends into the waveguide in a direction substantially perpendicular to the direction of signal propagation. Coupling bandwidths in the filter are established by the physical separation between adjacent resonators in each of the first and second waveguide segments, without the need for a passive decoupling structure located between them. The filter also includes a cross coupling conductor, for example, a coaxial conductor, that is connected between the first and second waveguide segments and that provides capacitive coupling between a resonator of the first waveguide segment and a resonator of the second waveguide segment to create additional transmission zeroes for the filter. An inductive coupler could also be used that would provide delay equalization to the filter. 
   The first and second waveguide segments may have a physical separation between them, and may have a rectangular outer shape, although other shapes are also possible. The waveguide cavities may be formed by an extrusion process which provides a low-cost means of production. Adjustable coupling screws located between adjacent resonators may be provided to allow adjustment of the relative coupling between them. In addition, a decoupling structure, which may be adjustable, can be provided in the connecting segment to allow a certain amount of decoupling between resonators of the first and second waveguide segments. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above and further advantages of the invention may be better understood by referring to the following description in conjunction with the accompanying drawings in which: 
       FIG. 1  is a perspective view of a filter according to the present invention; 
       FIG. 2  is a schematic cross-sectional view of a filter such as that shown in  FIG. 1 ; 
       FIG. 3  is a schematic cross-sectional view of one segment of a filter such as that shown in  FIG. 1 ; and 
       FIG. 4  is a schematic cross-sectional view of a filter such as that shown in  FIG. 1  in the vicinity of a connecting waveguide segment of the filter. 
   

   DETAILED DESCRIPTION 
     FIG. 1  is a perspective view of an evanescent mode waveguide cavity bandpass filter  10  that might be used for suppression of signals outside of the pass band at the output of a high power transmitter. The filter is a six-section filter having an input coaxial terminal  12  and an output coaxial terminal  14 . The filter cavity has a rectangular cross section, although those skilled in the art will recognize that other filter shapes are also possible. In the filter shown in  FIG. 1 , the cavity structure is extruded aluminum, which provides good performance while maintaining a relatively low cost of manufacture. The overall shape of the filter is “folded” in the sense that the filter path follows roughly a “U-shape,” so that the major segments are adjacent to each other and the input and output ports are on the same physical side of the filter. In the arrangement shown in the figure, the waveguide cavities are rigidly fixed to a support bracket  16  for support and mounting purposes. 
   A cross-sectional, schematic side view of the filter is shown in  FIG. 2 . The filter includes waveguide segments  30  and  32 , each of which contains three filter sections, and a connecting waveguide segment  34  that connects segments  30  and  32 . In each of the filter sections is located a “resonator”  18 , which is a conductive post that forms the filter section along with the waveguide. Each resonator  18  includes a tuning screw  20  that is threaded and that may consist of a material having a low coefficient of thermal expansion, such as a threaded rod of INVAR® (a registered trademark of Imphy, S. A., Paris, France). When turned, the screw changes the distance that a conductive portion of the resonator extends into the cavity, allowing fine tuning of the resonant frequency of that cavity. Between each pair of adjacent resonators  18  are coupling screws used to fine tune the coupling between those resonators. A signal input at the input terminal  12  is coupled from one filter section to the next along the length of the filter until finally reaching the output terminal  14 . The basic structure of the resonators and tuning screws is more clearly shown in  FIG. 3 , which is a cross section of one portion of the filter as indicated by the section line III—III shown in  FIG. 2 . 
   As shown in  FIG. 3 , each resonator  18  consists of a copper tube  22  within which is located a copper plunger  24  attached to the tuning screw  20  for that resonator. Spring fingers  26  attached to the plunger provide electrical contact between the plunger  24  and the tube  22 . Threads on the tuning screw  20  mate with a bracket on the waveguide surface, such that rotation of the screw  20  controls the degree to which the plunger  24  extends into the cavity, thereby changing the effective “length” of the conductive segment. This, in turn, causes slight changes in the resonant frequency, allowing fine tuning of the filter sections. 
   The filter uses evanescent mode coupling between resonators. That is, the waveguide sections have a groundplane spacing that is small enough that they have a cutoff frequency higher than the operating frequency of the filter, so that the signal propagation through the filter is via evanescent modes. For example, for a filter operating at a frequency of 500 MHz, the waveguide cavity may have a spacing of 7.75 inches, which establishes a cutoff frequency of approximately 762 MHz. The operation of the filter “below cutoff” creates a reactance in each filter section that, together with the capacitance of an adjacent resonator  18 , forms a resonant circuit having a particular resonant frequency. This resonance may be adjusted by adjusting the tuning screw attached to the plunger of the resonator. 
   With the spacing of the resonators along the length of the waveguide, there is coupling of the resonances from one filter section to the next. The degree of coupling between adjacent sections is controlled through the use of coupling screws  28 , each of which is positioned between two adjacent resonators  18 . Threads of each screw  28  mesh with a bracket on the waveguide surface, so that rotation of the screw changes the extent to which it extends into the waveguide cavity and inhibits capacitive coupling between the adjacent resonators. In this way, the relative coupling from one resonator to the next may be controlled. 
   Vanes separating one filter section from the next are common in the prior art for decoupling one section from the other. However, in the waveguide segments  30 ,  32 , spacing of the resonators themselves is used to establish a default level of decoupling. That is, the physical distance from one resonator to the next is used to establish the degree of coupling between adjacent resonators. While this results in a longer waveguide for the given number of sections, the filter benefits from a substantially higher quality factor “Q” than exists in similar filters having vanes separating the sections. 
   The use of increased resonator spacing to establish a desired decoupling between filter sections is also notable with regard to the two resonators furthest from the input and output terminals. Referring again to  FIG. 2 , if the resonators are numbered from input to output along the length of the waveguide, with the resonator adjacent to the input terminal being the first, and the resonator adjacent to the output terminal being the sixth, the third and fourth resonators reside at the side of the filter opposite the input and output terminals. The two waveguide sections could be placed directly next to each other, and a vane used between the third and fourth resonators to provide the necessary decoupling. However, in the embodiment of  FIG. 2 , a larger separation between the third and fourth resonators is used to maximize the Q factor of the filter. 
   Because of the spacing between the third and fourth resonators, it is necessary to separate the two waveguide sections from each other. As shown in  FIG. 2 , the first segment  30  is physically separated from the second segment  32 , and additional connecting waveguide segment  34  is located between the third and fourth resonators to provide continuity. A cross-sectional view of this segment, taken along line IV—IV of  FIG. 2 , is shown schematically in  FIG. 4 . As shown, this segment of the waveguide is similar to the two larger segments, and provides additional separation between the third and fourth resonators of the filter. However, in the embodiment shown, the separation between the third and fourth resonators is not as great as between the other adjacent resonators. The reason for this is discussed in more detail below 
   In this particular filter embodiment, it is desirable to have a cross coupling between non-adjacent resonators of the first waveguide segment  30  and the second waveguide segment  32 . To this end, a coupling path  38  (as shown in  FIG. 2 ) is provided that couples the first filter section to the sixth filter section. The coupling path  38  might also be provided between other resonators, such as between the second and the fifth. In the embodiment shown, the coupling path  38  contains a coaxial conductor  40  that extends from the first filter section to the sixth, and provides the necessary capacitive coupling. This cross coupling introduces additional transmission zeroes into the filter function, which provide the filter with a greater number of rejection points. However, the cross coupling conductor can not be too long, or it will create a resonance in the filter that comes too close to the filter pass band. Therefore, the separation between the waveguide segments  30 ,  32  is limited, and does not reach the point at which the separation between the third and fourth resonators is, by itself, sufficient to provide the desired degree of decoupling. Therefore, as shown in  FIG. 4 , a post  42  is located between the third and fourth resonators to provide an additional degree of decoupling, while still keeping the separation between the resonators, and therefore the Q factor, as high as possible. A tuning screw  28  is also provided between the third and fourth resonators to allow fine tuning of the coupling between them. In addition, an access hole  36  is provided in the surface of this waveguide segment furthest from the location of the input and output terminals. 
   The following is an example of a broadcast waveguide filter according to the present invention. Those skilled in the art will recognize that this is an example for descriptive purposes only, and should not be considered limiting of the overall scope of the invention. In this example, the filter has the form shown in  FIGS. 1–4 . The filter is designed to provide filtering for a selected pass band of approximately 6 MHz within a a range of 470 MHz to 488 MHz. Different filter dimensions would be used for pass bands in other frequency ranges. The waveguide is extruded aluminum having a square cross section and a groundplane spacing of 7.75 inches. Each of the resonators of the filter has a copper tube with a diameter of 2 inches, and the separation between the resonators, from the center of one to the center of the next, varies depending on their position in the cavity. The separation between the first and second resonators is 12.3 inches, as is the separation between the fifth and sixth resonators. The separation between the second and third resonators is 13.8 inches, as is the separation between the fourth and fifth resonators. The tuning screws  28  located between the resonators are equidistant from each resonator. 
   In this example, the third and fourth resonators are separated by a distance of 13 inches, with the tuning screw located between them and equidistant to each. The post for providing additional decoupling is equidistant from the third and fourth resonators, and positioned 2.625 inches from the waveguide wall  35 . Like the waveguide portions  30 ,  32 , waveguide portion  34  has a groundplane spacing of 7.75 inches. With this separation between the third and fourth resonators, the length of the cross coupling conductor  40  is 5.62 inches. At this length, there is no risk of the conductor having a resonance too close to the desired pass band. The spacing between all of the resonators also contributes to a relatively high unloaded quality factor “Q U .” For this particular design, the Q U  of the filter is approximately 10,500. 
   While the invention has been shown and described with reference to a particular embodiment thereof, it will be recognized by those skilled in the art that various changes in form and detail may be made herein without departing from the spirit and scope of the invention as defined by the appended claims.