Abstract:
A radio-filter of combline structure with a capacitor compensation circuit, having a transmission line filter with at least one pair of transmission lines arranged between input and output terminals for filtering the input signals through the input terminal to select signals of a given frequency band delivered to the output terminal, each of the transmission lines having a via-hole at each of its ends, a capacitor compensator of lumped element connected through one of the via-holes to one of the transmission lines for providing capacitance between the transmission line and ground, and a ground layer connected to the other via-hole not connected with the transmission line to ground the transmission line.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention relates to a radio filter employing transmission lines, and more particularly a radio filter of combline structure with a capacitor compensation circuit connecting multiple filter layers through via-holes.  
           [0003]    2. Description of the Related Art  
           [0004]    It is generally desired to manufacture a portable radio communications system, like a mobile phone, being of small size and low cost. Of course, this applies to other kinds of equipment and therefore development of various technologies is required.  
           [0005]    In order to reduce the size of the portable radio communications system having a radio filter for receiving or transmitting signals of a desired frequency band only, while blocking other noise signals, transmission lines (stripline or micro stripline) are used which occupy much less space than passive elements.  
           [0006]    Such a strip-line filter is disclosed in U.S. Pat. No. 4,963,843 granted to Motorola on Oct. 16, 1990, in reference to which the conventional combline stripline filter is briefly described as follows:  
           [0007]    The conventional combline stripline filter comprises a substrate having top and bottom surfaces each with a conductive material thereon forming a respective ground plane. The substrate also has an inner circuitry layer, which includes a ground area consisting of a conductive material having a plurality of angled edges, and coupled to at least one of the ground planes, and at least two combline resonators. Each resonator comprises a strip of conductive material. The strips are substantially parallel to one another. Each strip has adjacent first ends coupled to at least one of the ground planes, and adjacent second ends capacitively coupled to the ground area. Each of the adjacent second ends further has at least one angled edge disposed opposite a corresponding angled edge of the ground area. Each strip further has a respective extension portion at the second ends disposed substantially at right angles to the parallel strips. Each extension portion has a respective edge disposed opposite and capacitively coupled to the ground area. One of the extension portions is coupled to the radio signal.  
           [0008]    However, such a conventional stripline filter employing the pattern capacitor as described above suffers from increased layout size and increased error of the pattern capacitor caused by electrical interference. In addition, it is hard to connect with other devices, and its capacitance cannot be accurately measured because of its being capacitively coupled to ground.  
           [0009]    Furthermore, its manufacture is complicated because the type of material used for the substrate determines the capacitance with respect to the ground area. Moreover, since the connections with the input pads, output pads, and ground are made at the ends of the substrate, the sizes and positions of other devices connected thereto are very limited.  
         SUMMARY OF THE INVENTION  
         [0010]    It is an object of the present invention to provide a radio filter of combline structure with a capacitor compensation circuit connecting multiple layers through via-holes.  
           [0011]    It is another object of the present invention to provide a radio filter of combline structure with a capacitor compensation circuit employing the capacitor of lumped element as the capacitor compensator.  
           [0012]    It is still another object of the present invention to provide a radio filter of combline structure with a capacitor compensation circuit, which can be embodied on an ordinary substrate.  
           [0013]    According to the present invention, a radio-filter of combline structure with a capacitor compensation circuit, comprises a transmission line filter having at least a pair of transmission lines arranged between input and output terminals for filtering the input signals through the input terminal to select signals of a given frequency band delivered to the output terminal. Each of the transmission lines has a via-hole at each of its ends, a capacitor compensator of lumped element connected through one of the via-holes to one of the transmission lines for providing capacitance between the transmission line and ground, and a ground layer connected to the other via-hole which is not connected to the transmission line in order to ground the transmission line.  
           [0014]    The present invention will now be described more specifically with reference to the attached drawings which are shown for exemplary purposes only. 
       
    
    
     BRIEF DESCRIPTION OF THE ATTACHED DRAWINGS  
       [0015]    [0015]FIG. 1 is a schematic diagram illustrating the structure of a stripline filter according to an embodiment of the present invention.  
         [0016]    [0016]FIG. 2 is a view similar to FIG. 1 but with an additional pair of micro striplines arranged between the first pair of striplines of FIG. 1.  
         [0017]    [0017]FIG. 3 is a partial exploded view for illustrating the inventive stripline filter arranged in an ordinary multi-layer substrate. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0018]    Referring to FIG. 1, the inventive radio filter comprises a top filter layer  100  provided with a pair of micro striplines  108   a ,  108   b  forming the filter, and a bottom ground layer  120  supporting the top filter layer. Namely, the radio filter has the combline structure with the micro striplines  108   a  and  108   b  arranged on an ordinary copper clad laminate (CCL) substrate. The micro striplines  108   a  and  108   b  are coupled through via-holes  102   a ,  102   b ,  104   a , and  104   b  to the bottom ground layer  120 .  
         [0019]    Namely, the micro striplines  108   a  and  108   b  are respectively coupled through the via-holes  102   a  and  102   b  to capacitor compensators  110   a  and  110   b  which are grounded, and also grounded through the via-holes  104   a  and  104   b  to the bottom ground layer  120 . This is called a “blind via-hole arrangement”.  
         [0020]    Alternatively, the via-holes  102   a  and  102   b  are extended to the bottom ground layer  120  to connect with the capacitor compensators  110   a  and  110   b , which is called “through via-hole arrangement”.  
         [0021]    In the present embodiment, the inventive radio filter is described in connection with the blind via-hole arrangement.  
         [0022]    A first  108   a  of the pair of the micro striplines  108   a  and  108   b  constituting the radio filter is connected with the input terminal  106   a , and the second  108   b  of the pair of the micro striplines is connected with the output terminal  106   b . Each of the micro striplines  108   a  and  108   b  has via-holes  102   a  and  104   a  or  102   b  and  104   b  at respective ends. The via-holes  104   a  and  104   b  respectively connect the micro striplines  108   a  and  108   b  with the bottom ground layer  120 , while the via-holes  102   a  and  102   b  respectively connect them with the capacitor compensators  110   a  and  110   b.    
         [0023]    The capacitor compensator  110   a ,  110   b  should have such a capacitance value that the length of the micro stripline  108   a ,  108   b  electrically meets the half wavelength of the center frequency of the radio filter. Thus capacitor compensators  110   a  and  110   b  reduce the length of the micro striplines  108   a  and  108   b , and easily adjust the impedance matching and tuning. To this end, using a capacitor of lumped element, the capacitance is easily adjusted without adjusting the width or distance as in the conventional filter arrangement.  
         [0024]    Although the capacitor compensators  110   a  and  110   b  are shown in FIG. 1 respectively arranged at the ends of the micro striplines  108   a  and  108   b , each on the same side, their positions may be varied. The capacitances of the capacitor compensators  110   a  and  110   b  should be determined considering the capacitances of the via-holes  102   a  and  102   b . Of course, the capacitances of the via-holes  102   a  and  102   b  vary from the blind via-hole arrangement to the through via-hole arrangement.  
         [0025]    The radio filter thus obtained filters the input signals from the input terminal  106   a  to only select the signals of a given frequency band which are delivered to the output terminal  106   b . In this case, the given frequency band is determined by the length of and the space between the micro striplines  108   a  and  108   b , and the capacitances of the capacitor compensators  110   a  and  110   b  connected through the via-holes  102   a  and  102   b  to them. Further, additional via-holes may be provided in the input and output terminals  106   a  and  106   b  in order to connect other devices with the radio filter. For example, an additional via-hole in the input terminal  106   a  may be used for connecting with an antenna, with others in the output terminal  106   b  for connecting with another signal processing device.  
         [0026]    Referring to FIG. 2, a pair of additional micro striplines  208   b  and  208   c  are arranged between a pair of micro striplines  208   a  and  208   d  which are respectively connected with the input and output terminals  206   a  and  206   b , as shown in FIG. 1. Of course, the number of the micro striplines arranged between the outer micro striplines  208   a  and  208   d  may be increased.  
         [0027]    Referring to another embodiment shown in FIG. 3, the radio filter of multi-layer structure includes an additional layer  340  which is placed over the filter layer  300 , as compared to the previous embodiment. Namely, the filter layer  300  containing a radio filter consisting of the striplines  308   a  and  308   b  is interposed between the top and bottom ground layers  320  and  340 .  
         [0028]    The filter layer  300  comprises an ordinary CCL substrate upon which the striplines  308   a  and  308   b  are arranged to construct the radio filter. In addition, the striplines  308   a  and  308   b  are respectively provided with a plurality of via-holes  302   a ,  304   a ,  310   a ,  302   b ,  304   b  and  310   b . Thus, the striplines  308   a  and  308   b  are respectively connected through the via-holes  304   a  and  304   b  to the bottom ground layer  320 , and through the via-holes  302   a ,  302   b ,  310   a ,  310   b  to the top ground layer  340 .  
         [0029]    Namely, striplines  308   a  and  308   b  are respectively connected through via-holes  302   a  and  302   b , and via-holes  348   a  and  348   b  to the capacitor compensators  350   a  and  350   b  which are grounded, and through via-holes  310   a  and  310   b , and via-holes  344   a  and  344   b  to the striplines  342   a  and  342   b . This is the blind via-hole arrangement because the capacitor compensators  350   a  and  350   b  and striplines  342   a  and  342   b  are not connected through the via-holes to the bottom ground layer  320 . Of course, they may be designed to connect through via-holes with the bottom ground layer by using the through via-hole arrangement.  
         [0030]    Describing more specifically the radio filter thus obtained, one  308   a  of the pair of the micro striplines  308   a  and  308   b  constituting the radio filter is connected with the input terminal  306   a , and the other  308   b  of the pair of the micro striplines is connected with the output terminal  306   b . Each of the micro striplines  308   a  and  308   b  has via-holes  302   a  and  304   a  or  302   b  and  304   b  at respective ends.  
         [0031]    The respective via-holes  310   a  and  310   b  of the input and output terminals  306   a  and  306   b  are respectively connected with the via-holes  344   a  and  344   b  of the top ground layer  340 . The via-holes  304   a  and  304   b  connect the respective striplines  308   a  and  308   b  with the bottom ground layer  320 , while the via-holes  302   a  and  302   b  respectively connect them through the via-holes  348   a  and  348   b  of the top ground layer  340  to the capacitor compensators  350   a  and  350   b.    
         [0032]    The top ground layer  340  is provided with a pair of closed loop striplines  342   a  and  342   b  enclosing a given area corresponding to the striplines  308   a  and  308   b  of the filter layer  300 . The areas of the closed loop striplines  342   a  and  342   b  are respectively provided with via-holes  344   a  and  344   b  connected to the via-holes  310   a  and  310   b  of the input and output terminals  306   a  and  306   b  which are connected with striplines  308   a  and  308   b . The striplines  342   a  and  342   b  serve the input and output terminals of the radio filter.  
         [0033]    An additional pair of closed loop striplines  346   a  and  346   b  are provided in the top ground layer  340  to connect with the via-holes  302   a  and  302   b  formed at one-side ends of the striplines  308   a  and  308   b  of the filter layer  300 . Namely, the areas enclosed by the closed loop striplines  346   a  and  346   b  are respectively provided with via-holes  348   a  and  348   b  connected to the via-holes  302   a  and  302   b . In addition, the via-holes  302   a  and  302   b  are respectively connected with the capacitor compensators  350   a  and  350   b.    
         [0034]    The capacitor compensators  350   a  and  350   b  consist of capacitors of lumped element as described in the previous embodiment with capacitances proper for the frequency band filtered. Namely, the capacitor compensator  350   a ,  350   b  should have such a capacitance that the length of the micro stripline  308   a ,  308   b  electrically meets the half wavelength of the center frequency of the radio filter. The purpose of the capacitor compensators  350   a  and  350   b  is to reduce the length of the micro striplines  308   a  and  308   b , and to easily adjust the impedance matching and tuning. To this end, using a capacitor of lumped element, the capacitance is easily adjusted without adjusting the width or distance as in the conventional filter arrangement. Although the capacitor compensators  350   a  and  350   b  are shown in FIG. 1 respectively arranged at the ends of the micro striplines  308   a  and  308   b  on the same side, their positions may be varied.  
         [0035]    The capacitance values of the capacitor compensators  350   a  and  350   b  should be determined considering the capacitances of the via-holes  348   a  and  348   b . Of course, the capacitances of the via-holes  308   a  and  308   b  vary from the blind via-hole arrangement to the through via-hole arrangement.  
         [0036]    The radio filter thus obtained filters the input signals from the input stripline  342   a  through the via-holes  344   a  and  310   a  to the input terminal  306   a  to select the signals of a given frequency band only which is delivered to the output terminal  306   b , which then transfers the signals through the via-hole  310   b  to the via-hole  344   b  of the top ground layer  340 . In this case, the given frequency band is determined by the lengths of and the space between the striplines  308   a  and  308   b , and the capacitance values of the capacitor compensators  110   a  and  110   b  connected through the via-holes  302   a ,  348   a  and  302   b ,  348   b  to them. Further, the striplines  342   a  and  342   b  provided in the top ground layer  340  may be used to connect other devices to the radio filter. For example, the stripline  342   a  may be used to connect an antenna, and the stripline  342   b  may be used to connect with another signal processing device.  
         [0037]    Each layer of the inventive radio filter is composed of a CCL of epoxy resin, so that the conventional process may be used to construct the striplines. This facilitates the production of the radio filter with reduced cost.  
         [0038]    Further, the capacitor compensates for the length of the stripline to electrically select the half wavelength of the center frequency of the radio filter. While the present invention has been described in connection with specific embodiments accompanied by the attached drawings, it will be readily appreciated that various changes and modifications may be made thereto without departing the gist of the present invention.