Abstract:
A cross-coupled bandpass filter for a microwave electromagnetic signal which utilizes a housing which has formed therein a plurality of sequentially located resonator cavities with these cavities being interconnected by in-line couplers. A resonator is mounted within each cavity. A cross-coupler is disposed between a pair of the cavities that are not sequentially located. The cross-coupler takes the form of a printed circuit board upon which are mounted at least one manually movable screw access to which is permitted exteriorly of the cavities.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates generally to the field of microwave filters and more particularly to a bandpass filter which is to be used in microwave communication systems, such as cellular phones, cellular phone base stations, satellites and the like. 
     2. Description of the Related Art 
     In the microwave communications market, the microwave frequency spectrum has become severely crowded and has been subdivided into a vast number of different frequency bands. There is a need to design microwave filters that have an output signal only at a precise (narrow) frequency band. Also, it is necessary that this filter can be tuned to a precise frequency band with there being a separate filter for each precise frequency band. 
     In the field of microwave bandpass filters, it is known that the frequency band of the signal of the filter is a function of the resonant frequency of resonators that are incorporated within the filter and respective coupling coefficients between each of these resonators. Typically, in order to achieve a specific precise bandwidth, the resonators are longitudinally spaced in a sequential manner. The bandwidth is a function of the coupling between the resonators and the frequency of the resonance of the resonators. Varying of the spacing between the resonators results in variations in the bandwidth. Accordingly, overall filter dimensions, such as the filter length, typically must be varied in order to tune a filter to a precise bandwidth. Therefore, in the past in order to divide a microwave communications band into the many different frequency bands of operation, a multitude of different filter dimensions are necessary. However, because there is a need to minimize the size of such filters, and the fact that such filters may be located in very remote locations, such as satellites, a non-uniform filter dimension is just not acceptable. 
     The constructing of a filter that can be tuned to a selected microwave frequency has long been known. It has been discovered that if there is included in the filter a cross-coupler that connects between a pair of non-sequential resonators, a variation in the response of the filter is obtained. A slight position variation of that cross-coupler will result in a mismatch of the microwave signal inside the filter. Therefore, changing the position of the cross-coupler can produce filters that more or less mismatched depending on cross-coupler coupling valve. 
     A typical cross-coupler constitutes an electrically conductive wire like member with a small plate being fixedly mounted at each end of the member. The member is then mounted across a vertical wall located in the filter that separates two of the non-sequential resonating cavities. The filter is covered by a removable cover. A technician whom has been instructed to produce a filter at a precise frequency, connects the filter to a piece of test equipment. If the coupling is not at the precise value, then the technician is to remove the cover, manually alter the position of one end or both ends of the wire type member cross-coupler, then replace the cover in position on the housing of the filter and then retest to determine if the coupling value is correct. If it is not the desired specific value, then the adjustment procedure is performed again and continues until the desired coupling is obtained. At times, it can literally take hours for a filter to be tuned to the precise coupling value because of the time involved in removing of the cover and reinstalling same. 
     SUMMARY OF THE INVENTION 
     The first embodiment of the present invention is to construct a tunable, cross-coupled bandpass filter which is formed of an enclosing housing which has a plurality of sequentially located resonator cavities. An input port is connected to a beginning cavity and an outlet port is connected to an ending cavity. A resonator is mounted within each of the resonator cavities. Each of the resonator cavities have an in-line coupler for coupling the electromagnetic signal between each sequential pair of resonators. A cross-coupler is disposed between a pair of non-sequential cavities. The cross-coupler includes a printed circuit (PC) board. 
     A further embodiment of the present invention is where the first basic embodiment is modified by the cavities being divided into a pair of side-by-side rows. 
     A further embodiment of the present invention is where the first basic embodiment is modified by there being located a vertical wall between at least two in number of the cavities that are not in direct sequence. 
     A further embodiment of the present invention is where the first basic embodiment is modified by each of the cavities being of a square shape in transverse cross-section. 
     A further embodiment of the present invention is where the first basic embodiment is modified by each resonator being cylindrical. 
     A further embodiment of the present invention is where the first basic embodiment is modified by the PC board including a dielectric compression board. 
     A further embodiment of the present invention is where the first basic embodiment is modified by the PC board being formed of a dielectric layer and an electrically conductive layer. 
     A further embodiment of the present invention is where the first basic embodiment is modified by the PC board including at least one tuning screw passing through a hole in the PC board. 
     However, it is important that the copper layer  74  form edge layers at each longitudinal end of the fiberglass layer  72  such as edge layer  75 . Edge layer  75  will alter the inductance of the magnetic field passing through the filter  10  by the close proximity of each edge layer  75  to a resonator  28 . Each edge layer  75  covers the edge of fiberglass layer  72  but not the edge of the compression board  66 . 
     A further embodiment of the present invention is where the first basic embodiment is modified by a cover being mounted on the housing of the filter with the cover being removable. 
     A second basic embodiment of the present invention comprises a cross-coupled bandpass filter for a microwave electromagnetic signal which takes the form of an enclosing housing that has a plurality of resonator cavities located in a sequential arrangement. Directly between each pair of cavities in sequence there is located an in-line coupler. A resonator is located within each of the cavities. A cross-coupler is disposed between a pair of the cavities that are not in sequence with a first portion of the cross-coupler being located within one cavity and a second portion of the cross-coupler being located within another cavity. A cross-coupler is mounted between those cavities with the cross-coupler including a tuning screw that is manually turnable relative to the cross-coupler. 
     A further embodiment of the present invention is where the second basic embodiment is modified by the cavities being located in a pair of side-by-side rows. 
     A further embodiment of the present invention is where the second basic embodiment is modified by there being a vertical wall located between a pair of cavities which are not in direct sequence. 
     A further embodiment of the present invention is where the second basic embodiment is modified by the resonator cavities each being formed square in transverse cross-section. 
     A further embodiment of the present invention is where the second basic embodiment is modified by each resonator that is mounted within each cavity being cylindrical. 
     A further embodiment of the present invention is where the second basic embodiment is modified by the cross-coupler including a PC board which is formed by a dielectric layer and an electrically conductive layer. 
     A further embodiment of the present invention is where the second basic embodiment is modified by the tuning screw being mounted in conjunction with the PC board. 
     A further embodiment of the present invention is where the second basic embodiment is modified by there being a pair of tuning screws mounted in conjunction with the PC board with these tuning screws being located in a spaced apart arrangement. 
     A further embodiment of the present invention is where the second basic embodiment is modified by there being mounted a removable cover in conjunction with the housing with the tuning screws protruding exteriorly of the cover. 
     A further embodiment of the present invention is where the second basic embodiment is modified by the cover being spaced from both the electrically conductive layer and the resonators. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     For a better understanding of the present invention, reference is to be made to the accompanying drawings. It is to be understood that the present invention is not limited to the precise arrangement shown in the drawings. 
     FIG. 1 is an isometric view of the bandpass filter of the present invention showing the cover of the bandpass filter being located in.a disengaged position from the housing; 
     FIG. 2 is transverse cross-sectional view taken along line  2 — 2  of FIG.  1  through the housing of the filter of the present invention showing the cover mounted on the housing; 
     FIG. 3 is an isometric view of the cross-coupler that is usable in conjunction with the bandpass filter of the present invention; 
     FIG. 4 is an exploded isometric view of FIG. 3 showing the compression board removed and spaced from the printed circuit board of the cross-coupler; and 
     FIG. 5 is a plan view of the cross-coupler included within the bandpass filter of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring particularly to the drawings, there is shown in FIG. 1 a tunable, cross-coupled, bandpass filter  10 . The filter  10  utilizes a rectangularly shaped housing  12  which has an internal chamber which is divided into a plurality of cavities  14 . Preferable material of construction for housing  12  would be aluminum. The cavities  14  include a beginning cavity  16  and an ending cavity  18 . Each cavity  14 ,  16  and  18  is basically of the same size. In transverse cross-section, each cavity  14 ,  16  and  18  is basically square in configuration. However, it is considered to be within the scope of this invention that other shapes for the cavities  14 ,  16  and  18  could be utilized. Connecting with the beginning cavity  16  is an input port  20 . An output port  22  connects with the ending cavity  18 . 
     Between the beginning cavity  16  and the directly adjacent cavity  14  there is an iris in the form of a partial wall  24 . The partial wall  24  includes an opening  26 . The opening  26  functions as an in-line coupler for the electromagnetic signal which is being transmitted through the input port  20  into the beginning cavity  16  and into directly adjacent cavity  14 . Mounted within the beginning cavity  16  is a resonator  28  which is in the form of an aluminum cylindrical tube. The resonator  28  is centrally located within the cavity  16  and extends from the bottom wall  30  of the housing  12 . It is to be understood that each cavity  14  has a similar partial wall  24  and a similar opening  26  and also a similar resonator  28 . 
     The cavities  14  that are located furthest from the input port  20  and the output port  22  are known as the corner cavities  32  and  34 . Located directly adjacent the corner cavities  32  are a pair of connecting cavities  36  and  38 . In between the connecting cavities  36  and  38  is a bridge coupler in the form of an opening  40 . There is also an opening  26  that connects between corner cavity  34  and connecting cavity  38 . In other words, the electromagnetic signal is being transmitted through both the inline coupler of opening  26  and the bridge coupler of opening  40  prior to transmittal through the remaining cavities  14  to the ending cavity  18  and out through the outlet port  22 . 
     Planar upper edge  42  of the housing  12  includes a mass of spaced apart threaded holes  44 . Threaded holes  44  are to be engageable with threaded bolts  46  which are mounted within a planar cover  48 . The cover  48  is to be tightly sealed onto the housing  12  so that the cavities  14  are completely closed relative to ambient. It is to be noted that the cavities  14  within the housing  12  is formed in essence into one row and a second row which is parallel to the first row. Separating these rows is a vertical wall  50 . The vertical wall  50  also includes a series of threaded holes  52  with which there is mounted in the cover  48  a series of threaded bolts  54  which threadably connect with the holes  52 . 
     Threadably mounted within the cover  48  are a plurality of threaded set screws  56 . Each set screw  56  is to be locatable within the internal chamber  58  of a resonator  28 . Therefore, there is a threaded set screw  56  for each resonator  28 . However, there may not be utilized set screw  56  for each resonator  28  with only some resonators  28  having a set screw. The threaded set screws  56  can be manually adjusted in order to vary the frequency of the electromagnet signal being received at the outlet port  22 . Generally, the set screws  56  will be turned so that the frequency of the signal being emitted from the outlet port  22  is close to the precise frequency that is desired. Then to achieve the exact frequency, there is used the cross-coupler  60 . The cross-coupler  60  is fixedly mounted as with adhesive within a chamfered recess  62  formed within the vertical wall  50 . The chamfered recess  62  connects between two cavities  14  that are not directly in sequence. The cross-coupler  60  is to be constructed of a PC board  64  and a compression board  66 . The cross-coupler  60  has a pair of inward cuts  68  and  70  which matingly connect with the chamfered recess  62  formed within the vertical wall  50 . This means that the cross-coupler  60  is fixedly positioned in a precise position on the vertical wall  50 . 
     The printed circuit board  64  is formed of a fiberglass layer  72  upon which is adhered an electrically conducting layer  74 . The fiberglass layer  72  is dielectric and the conducting layer  74  could be of copper or other suitable metallic electrically conductive substance. Generally, the thickness of the layer  74  would be 1.4 mils. The cross-coupler  60  has a “bow tie” configuration due to the forming of an inward cut  68  and  70 . The layer  74  also includes inner cuts  76  and  78  which are spaced respectively from the inward cuts  68  and  70 . This is so that the copper layer  74  will not physically come into contact with the wall  50  which may affect the transmitting of the electromagnetic signal. However, it is important that the copper layer  74  form edge layers at each longitudinal end of the fiberglass layer  72  such as edge layer  75 . Edge layer  75  will alter the inductance of the magnetic field passing through the filter  10  by the close proximity of each edge layer  75  to a resonator  78 . Each edge layer  75  covers the edge of fiberglass layer  72  but not the edge of the compression board  66 . 
     Formed within the copper layer  74  and the fiberglass layer  72  are a pair of holes  80  and  82 . Formed within the compression board  66  are a similar pair of holes  84  and  86 . Hole  86  is to align with hole  80  and hole  84  aligns with hole  82 . All holes  80 ,  82 ,  84  and  86  are of the same size. A tuning screw  88  is to be mounted within the cover  48  and is to be located within the aligned holes  80  and  86 . A similar tuning screw  90  is to be mounted within the cover  48  and is to be located within aligned holes  82  and  84 . Both the tuning screws  88  and  90  are to be in physical contact with the copper layer  74 . The function of the compression board  66  is to keep the PC board  64  spaced from the cover  54  with this spacing occurring by means of a dielectric with the general material of construction for the compression board  66  also being fiberglass. It is also to be noted that the free end of each of the resonators  28  is of a length so that it will be spaced from the cover  48 . The spacing of the PC board  64  from the cover  48  and the spacing of each of the resonators  28  from the cover  48  is to insure the maximum transmission of energy of the electromagnetic signal from the input port  20  to the output port  22  over operating temperatures. 
     With the filter  10  of this invention connected to a piece of test equipment, which is not shown, such as an network. analyzer, the frequency of the signal being emitted from the output port  22  is ascertained. To fine tune that frequency, the technician can manually adjust the position of the screws  88  and  90  relative to the cross-coupler  60 . Once the desired precise frequency is obtained, the position of the screws  88  and  90  is maintained as well as each-of the screws  56 . The filter  10  is then ready for installation. It is important to note that by utilizing of the screws  56 ,  88  and  90  that tuning of the filter  10  is accomplished without removal of the cover  48  from the housing  12 . Obviously, by the sheer number of the threaded bolts  46  and  54 , it would constitute a rather time consuming procedure to be constantly removing of the cover  48  and replacing the cover  48  in order to achieve tuning of the filter  10 . This removal of the cover  48  has been eliminated. By using of the cross-coupler  60 , a precise frequency can be obtained for each filter  10 . It is to be understood that in a given installation there will generally be only one filter  10  for a precise frequency. A typical satellite will have installed several hundred of the filters  10 . It is to be understood that the turning of tuning screws  88  and  90  is accomplished individually as well as the turning of the set screws  56 . Tuning screws  88  and  90  function to interrupt the magnetic field passing through the trace copper layer  74  which changes the overall susceptance of the electromagnetic field that is being conducted through the filter  10 .