Abstract:
A filter filtering microwaves, the filter comprising a cavity and at least one metal comb-line resonator located within the cavity and at least one dielectric member surrounding the at least one metal comb-line resonator.

Description:
FIELD OF THE INVENTION 
       [0001]    This invention relates to a filter. In particular, the invention relates to a microwave filter for use in the telecommunications industry and therefore will be described in this context. However, it should be appreciated that the microwave filter may be used for other applications such as military, satellite, WIFI and television. 
       BACKGROUND OF THE INVENTION 
       [0002]    People throughout the world are demanding an increase in information to remote locations. This is achievable through the use of wireless telecommunication systems that are able to provide the necessary infrastructure to provide access to this information. One necessary component of wireless telecommunication base stations are microwave filters. 
         [0003]    In any telecommunication base station, the amount of space available for equipment is limited. Accordingly, the dimensions of a microwave filter are an important consideration when designing a telecommunication base station. Unfortunately, due to the physics involved, the dimensions of the microwave filter is directly related to the frequencies being filtered. For example, in certain types of filter, the lower the frequency being filtered, the larger is the height required. With telecommunication towers filtering signals and frequencies in the range of 400 MHz to 4 GHz, the number and size of a microwave filter can vary considerably. 
         [0004]    Further, with any microwave filter, there is the cost of manufacture which relates to both the materials used and the labour required. Even after a microwave filter has been assembled, it may require several hours of tuning. 
       OBJECT OF THE INVENTION 
       [0005]    It is an object of the invention to overcome or alleviate one or more of the above disadvantages or to provide the consumer with a more useful or commercial choice. 
       SUMMARY OF THE INVENTION 
       [0006]    In one form, although not necessarily the only or broadest form, the invention relates to a filter filtering microwaves, the filter comprising: 
         [0007]    a cavity; and 
         [0008]    at least one metal comb-line resonator located within the cavity; and 
         [0009]    at least one dielectric member surrounding the at least one metal comb-line resonator. 
         [0010]    Preferably, the filter includes a housing having a base and a lid. 
         [0011]    Preferably, the resonator is located within the base. Preferably, the resonator is integrally formed with the base. However, the resonator may be detachable and may be formed from a different material, such as copper, to better control the frequency drift of the filter over a larger temperature range. The resonator may be screwed, soldered or fixed to the base using any process known in the art. The base is typically made from aluminum and coated with silver. However, it should be appreciated that any other materials may be used that would be evident to a person skilled in the art. For example, a gold coated plastic may be used. 
         [0012]    The base may also include an inlet to allow microwaves to pass into the cavity in the housing and an outlet to allow microwaves to pass out of the cavity in the housing. A coupling element may form part of the inlet and/or outlet to connect the inlet and/or outlet to an adjacent resonator. 
         [0013]    The dielectric member is preferably attached to the lid. The dielectric member may be soldered to the lid using a conductive material and the conductive material may be silver. The lid is preferably made from a material with a coefficient of thermal expansion well matched to that of the dielectric member to avoid the latter cracking with variation in ambient temperature. 
         [0014]    Flux holes may be located within the lid to allow flux to pass through the lid during soldering to ensure a solid conductive metal connection between the lid and the dielectric member. 
         [0015]    The dielectric member may be in the form of a hollow cylinder. Preferably, the dielectric member is made from a high Q ceramic material. The Q value of the dielectric member material should be in the range of 4000 to 35,000 at 1 GHz and preferably above 75,000 at 1 GHz. Additionally, the dielectric member material has a preferred dielectric constant in the range of 10 to 120. 
         [0016]    The filter may further include at least one tuning screw for adjustment of coupling levels between various parts of the filter. Normally these coupling tuning screws are located in the lid of a filter. Preferably, the at least one coupling tuning screw is located between two resonators to control the coupling strength between the two resonators. 
         [0017]    The filter may further include at least one frequency tuning screw. Normally the frequency tuning screw is located in the lid of the filter. However, persons skilled in that art may position the tuning screw in a different position. For example, the tuning screw may form part of the resonator. Preferably, the at least one frequency tuning screw is located within the dielectric material to vary the frequency of the resonator. 
         [0018]    In another form, the invention resides in a method of manufacturing a filter, the method including the steps of: 
         [0019]    locating at least one dielectric member over at least one metal comb-line resonator located within a cavity. 
         [0020]    The method may further include one or more of the steps of: 
         [0021]    forming at least one resonator within a cavity of a housing; 
         [0022]    soldering at least one dielectric member onto a lid; and 
         [0023]    placing tuning screws within a lid. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0024]    An embodiment of the invention, by way of example only, will now be described with reference to the accompanying figure in which: 
           [0025]      FIG. 1  is a perspective view of a filter according to an embodiment of the invention; and 
           [0026]      FIG. 2  is a further perspective view of a filter according to an embodiment of the invention. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0027]      FIGS. 1 and 2  show a perspective view of a filter  10  for filtering microwaves. The filter  10  includes a housing  20 , a series of resonators  30  and a series of dielectric members  40 . 
         [0028]    The housing  20  is manufactured from aluminum and has a base  21  and a lid  22 . The base  21  includes a cavity  50  with a series of resonators  30  extending upwardly within the cavity  50 . The resonators  30  and the base  21  are integrally molded. An inlet  60  is located at one side of the cavity  50  whilst an outlet  70  is located at an opposite end of the cavity  50 . An inlet coupling element  61  forms part of the inlet whilst an outlet coupling element  71  forms part of the outlet  70 . A series of fastening holes  23  are located are located through the base  21 . 
         [0029]    The lid  22  is a flat plate and has a series of fastening holes to hold the lid  22  to the base  21 . Appropriate fasteners  24 , such as screws, are located through the lid  22  and the base  21  to hold the lid  22  and the base  21  together. Dielectric members  40  are soldered to the lid  22  using a silver solder. Each dielectric member  40  is of a hollow cylindrical shape. The dielectric material used to produce each dielectric member  40  has a dielectric constant of 35. This material also has a Q of 35000 at 1 GHz. It should be appreciated that other suitable materials with different Qs may be utilized as would be recognised by a person skilled in the art. 
         [0030]    Frequency tuning screws  80  are located through the lid  22  and within each dielectric member  40 . The tuning screws  80  are of a standard format as will be recognised by a person skilled in the art. Coupling tuning screws  81  are also located through the lid  22  so that when the lid  22  is placed on the base  21 , the coupling tuning screws  81  are located between adjacent resonators  30 . 
         [0031]    When the lid  22  is attached to the base  21 , each dielectric member  40  is located over a respective resonator  30 . Accordingly, each tuning screw  80  is also located adjacent a top of a respective resonator  30  and each coupling tuning screw  81  is between adjacent resonators  30 . The height of the cavity  50  is greater than the height of each dielectric member  40 . Therefore, when the lid  22  is placed on the base  21 , an expansion gap is left between the dielectric members  40  and the base  21 . Further, an expansion gap exists between each dielectric member  40  and the respective resonator  30 . 
         [0032]    In order to complete manufacture of the filter  10  the base  21  and lid  22  must be produced. The base  21  starts as a block of aluminum that is machined to form resonators  30  and an associated cavity  50 . As the base  21  is made from aluminum, it is very quick and easy and hence cost effective machine the base  21 . Once the base  21  is machined, a silver coating is placed over the base  21 . It should be appreciated that the base  21  may be formed by other means than machining. For example, the base  21  may be made from plastic and injection molded. An inlet hole and outlet hole are then drilled through the base  21 . The inlet  60  with associated coupling element  61  and outlet  70  with associated coupling element  71  are inserted to complete the base  21 . Fastening holes are then drilled through the base  21  adjacent to the cavity  50 . It should be appreciated that the sequence of steps to produce the base may be varied as would be evident to a person skilled in the art. 
         [0033]    The lid  22  starts as a flat sheet of stainless steel 410. Stainless steel 410 is used to match the dielectric members  40  coefficient of thermal expansion so that the dielectric members  40  expand at the same rate during variations in temperature and the ceramic does not crack. Alternatively the lid  22  can be made out of a Printed Circuit Board (PCB) material, such as FR4, with a copper conductor plating to match the coefficient of thermal expansion of the lid  22  to the dielectric members  40 . The lid  22  however can be made of any suitable material. In this case, the coefficient of thermal expansion of the stainless steel 410 material is 9.5 ppm/° C. and the coefficient of thermal expansion for the dielectric member  50  is 10 ppm/° C. Fastening holes, frequency tuning screw holes, coupling tuning screw holes and flux holes are drilled through the top of the lid  22 . The whole lid  22  or masked areas of the lid  22  are then plated with silver to further improve the conductivity of the cavity  50 . 
         [0034]    In order to attach the dielectric members  40  to the lid  22 , the lid  22  is heated to an appropriate temperature for soldering. Flux is located on the lid  22  adjacent to the flux holes  90 . That is, at locations on which silver solder and the dielectric members  40  will be placed on the lid  22 . Silver solder and the dielectric members  40  are then placed on the lid  22  to attach the lid  22  to the dielectric members  40 . Any excess flux located between the lid  22  and the dielectric members  40  is forced through the flux holes so that the flux does not interfere with the bonding of the dielectric members  40  to the lid  22  by the silver solder. All soldering of all of the dielectric members  40  are contacted at one time. Minimum heating output is required as the lid  22  is a single sheet of stainless steel. 
         [0035]    Once the lid  22  has cooled, coupling tuning screws  81 , frequency tuning screw  80  and fastening screws  24  are located within respective holes in the lid  22 . 
         [0036]    Once the base  21  and lid  22  have been completed, the lid  22  is placed on the base  21  so that the dielectric members  40  are located over the resonators  30 . The fastening screws  24  are screwed into the fastening holes  27  located within the base  21  to hold the lid  22  and the base  21  firmly together. At this point, the frequency tuning screws  80  are located adjacent respective resonators  30  and the coupling tuning screws  81  are located between adjacent resonators  30 . 
         [0037]    The final step to complete the manufacture of the filter  10  is to tune the filter  10 . This is completed by adjusting the coupling tuning screws  81  to achieve the desired bandwidth and to adjust the frequency tuning screws  80  to achieve the desired frequency. 
         [0038]    In use, the filter  10  is placed in a system in which microwaves are fed through the inlet  60 . The microwaves pass through the cavity  50  where their frequency is changed to a desired frequency, e.g. 900 MHz. The microwaves then pass through the cavity  50 , to the outlet  70  which is in turn connected to a receiver or transmitter. 
         [0039]    It should be appreciated that depending on the required output frequency, the number of resonators  30  and the size of the cavity  50  may be increased or decreased as would be clear to a person skilled in the art. For example, there may be as little as a single resonator or as many as fifty resonators. 
         [0040]    The above filter  10  provides many benefits including quick and easy tuning of the filter; quick, easy and inexpensive manufacture of the filter; low insertion loses and the ability to operate in TEM 01  mode. Further, the filter has a larger power handling capacity compared to a conventional comb-line resonator. 
         [0041]    Furthermore the filter  10  described was designed for use in the GSM 900 band (935-960 MHz) and the Q value was 4000. If the dielectric member is removed, the resonant frequency of the resonators  30  increases to 2400 MHz. Alternatively, if the resonator  30  geometry is modified in order to operate in the GSM 900 band without the dielectric members  40 , the power handling would be much reduced and the Q value would be reduced by 25%. 
         [0042]    It will be appreciated that various other changes and modifications may be made to the embodiment described without departing from the spirit and scope of the invention.