Abstract:
Disclosed is a high-temperature superconductor low-pass filter for removing broadband harmonics in a wireless communication system. The high-temperature superconductor low-pass filter includes a coupled line section and a transmission line section, in which the coupled line section is connected in parallel with the transmission line section. The coupled line section has two microstrip open-stub type parallel stripe lines stacked on a high-temperature superconductor, and the transmission line section has one stripe line. Since the high-temperature superconductor low-pass filter has attenuation poles at a stopband, it has stopband characteristics to 7-8 times wider than a cutoff frequency. The high-temperature superconductor low-pass filter can easily remove sub-harmonics which are inevitably occurred in the wireless communication system.

Description:
FIELD OF THE INVENTION  
         [0001]    The present invention relates to a low-pass filter for a wireless communication system; and, more particularly, to a HTS low-pass filter for suppressing broadband harmonics.  
         DESCRIPTION OF THE PRIOR ART  
         [0002]    Recently, as various wireless communication systems and services are developed intensively, the considerable performance improvement such as small insertion loss, high selectivity, high sensitivity and small size are needed in development of communication components for a cellular phone and a personal communication system. In order to satisfy these demands, the development of materials, design (circuits) and fabrication (processes) technologies are essential for the communication devices.  
           [0003]    Since low-pass filter (LPF) is a frequency selective and passive device with low levels of attenuation, LPF is widely used to reject harmonics or spurious signals in a integrated mixer, a voltage controlled oscillator (VCO) and so on. But an open-stub type low-pass filter and a step-impedance type low pass filter have a narrow stopband (about 3 times of cutoff frequency in case of a conventional LPF).  
           [0004]    [0004]FIGS. 1A and 1B show an equivalent circuit diagram and a schematic diagram of a conventional microstrip low-pass filter.  
           [0005]    [0005]FIG. 1A shows the equivalent circuit diagram of the lumped-element low-pass filter designed through the transformation of impedance level and frequency scale from the prototype low-pass filter (not shown). The lumped-element low-pass filter (or π-type low-pass filter) includes an inductance L 2  corresponded to the microstrip transmission line, a first shunt capacitance C 1  and a second shunt capacitance C 2  corresponded to the two parallel microstrip open-stubs (in this case: C 1 =C 2 ).  
           [0006]    Referring to FIG. 1B, the conventional microstrip low-pass filter includes a crystalline substrate  180  (hereinafter, referred to as “an MgO substrate”), a signal transmission input port  150  and a signal transmission output port  160 , two parallel stripe lines  170  of a microstrip open-stub type, a microstrip line  140  and a ground plane  190 .  
           [0007]    The signal transmission input port  150  and the signal transmission output port  160  are fabricated on both edges of the top face of the MgO substrate  180 . Two parallel microstrip open-stubs  170  between the signal transmission input port  150  and the signal transmission output port  160  are perpendicular to a signal propagation direction. Therefore, the microstrip line  140  is perpendicular to two parallel microstrip open-subs  170 . The groundplane (e.g., Au or Ag film)  190  is coated at the bottom (backside) of the MgO substrate  180 .  
           [0008]    In general, there are some problems in the conventional low-pass filter as described above. Since the conventional low-pass filter has a narrow stopband range in frequency domain, an interference occurred by the adjacent wireless communication systems and a noise generated by the communication system itself are quite serious.  
         SUMMARY OF THE INVENTION  
         [0009]    It is, therefore, an object of the present invention to provide a low-pass filter having a high-efficiency broad stopband characteristics, in which attenuation poles and a frequency range of the stopband can be controlled easily.  
           [0010]    In accordance with an aspect of the present invention, there is provided a low-pass filter comprising: a circuit pattern having at least one or more units, wherein the circuit pattern includes a coupled line section having a pair of parallel stripe lines and a transmission line section having a pair of parallel straight lines whose two ports of one side are opened and whose two ports of the other side are connected to each other, each port of one side of the pair of the parallel straight lines being connected with each port of one side of the coupled line section. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0011]    Other objects and aspects of the invention will become apparent from the following description of the embodiments with reference to the accompanying drawings, in which:  
         [0012]    [0012]FIGS. 1A and 1B show an equivalent circuit diagram and a schematic diagram of a conventional microstrip low-pass filter, respectively;  
         [0013]    [0013]FIGS. 2A to  2 C illustrate a schematic diagram, a basic circuit diagram and an equivalent circuit diagram of a high-temperature superconductor (HTS) coupled line low-pass filter in accordance with the present invention, respectively;  
         [0014]    [0014]FIGS. 3A to  3 C illustrate a schematic diagram, a basic circuit diagram and an equivalent circuit diagram of a seventh-order coupled line low-pass filter in accordance with the present invention, respectively;  
         [0015]    [0015]FIGS. 4A and 4B are graphs illustrating simulated results of the seventh-order coupled line low-pass filter shown in FIG. 3A;  
         [0016]    [0016]FIGS. 5A to  5 F are cross-sectional views illustrating sequential steps associated with a method for fabricating the seventh-order coupled line low-pass filter; and  
         [0017]    [0017]FIG. 6 shows comparison of the simulated and measured results of the seventh-order HTS coupled line low-pass filter. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0018]    [0018]FIG. 2A shows a microstrip circuit of a high-temperature superconductor (HTS) low-pass filter (LPF) in accordance with an embodiment of the present invention. Referring to FIG. 2A, the HTS low-pass filter includes a transmission line section  241  and a coupled line section  242 . The transmission line section  241  includes a microstripe line  243  and the coupled line section  242  includes a pair of parallel stripe lines  244  and  245 .  
         [0019]    The pair of the parallel stripe lines  244  and  245  are stacked on a HTS epitaxial thin film (not shown). A first lead line  246  is extended from the first parallel stripe line  244  to a signal transmission input port. A second lead line  247  is extended from the second parallel stripe line  245  to a signal transmission output port. The microstripe line  243  connects the first and the second parallel stripe lines  244  and  245 . The microstripe line  243  is more slender and longer than the first and the second lead lines  246  and  247 .  
         [0020]    At this time, an electrical length ratio of the coupled line section to the transmission line section is approximately 1:2, and a distance from the first parallel stripe line  244  to the second parallel wire  245  is less than 10 μm. A width of the microstripe line  243  is less than that of the first and the second lead lines  246  and  247 .  
         [0021]    [0021]FIG. 2B shows an equivalent circuit of the high-temperature superconductor low-pass filter in FIG. 2A.  
         [0022]    As shown in FIG. 2B, the HTS high-temperature superconductor low-pass filter includes a first π type equivalent circuit portion  235  corresponding to the transmission line section  241  and a second π type equivalent circuit portion  234  corresponding to the coupled line section  242 .  
         [0023]    Compared with the conventional low-pass filter shown in FIG. 1B, the high-temperature superconductor low-pass filter in accordance with the present invention further includes a third capacitor C R . That is, an inductor L R  is disposed between the signal transmission input port and the signal transmission output port. A first capacitor C P1  is connected between the signal transmission input port and a ground, and a second capacitor C P2  is connected between the signal transmission output port and the ground. The third capacitor C R  is connected in parallel with the inductor LR between the first and the second capacitors C P1  and CP 2 . The first and the second capacitors C P1  and C P2  are constituted with capacitors C C1  and C C2  which are physically isolated, respectively.  
         [0024]    [0024]FIG. 2C shows an equivalent circuit of the high-temperature superconductor low-pass filter shown in FIG. 2B. As shown in FIG. 2C, the equivalent circuit diagram includes an inductor L 1  disposed between the signal transmission input port and the signal transmission output port, a first capacitor C 1  connected between the signal transmission input port and the ground, and a second capacitor C 2  connected between the signal transmission output port and the ground.  
         [0025]    Such a low-pass filter has three attenuation poles due to the electrical length φ of the transmission line section and the coupled line section. Two attenuation poles are positioned at two points where a susceptance of a serial element becomes zero and one attenuation pole is positioned at a point where a susceptance of parallel elements becomes infinite.  
         [0026]    [0026]FIGS. 3A to  3 C illustrate a schematic diagram, a basic circuit diagram and an equivalent circuit diagram of a seventh-order low-pass filter in accordance with the present invention, respectively.  
         [0027]    Referring to FIG. 3A, the seventh-order low-pass filter includes a transmission line section  360  having three stripe lines and a coupled line section  370  having three pair of parallel stripe lines. Each stripe line is connected to each pair of the parallel stripe lines.  
         [0028]    Compared with the high-temperature superconductor low-pass filter shown in FIG. 2A, three circuit patterns are serially connected between the signal transmission input port and the signal transmission output port.  
         [0029]    [0029]FIG. 3B shows an equivalent circuit of the seventh-order low-pass filter in FIG. 3A. As shown, the seventh-order low-pass filter includes a first π type equivalent circuit portion  340  corresponding to the transmission line section  360  and a second π type equivalent circuit portion  350  corresponding to the coupled line section  370 . Three circuit patterns  310 ,  320  and  330  are serially connected between the signal transmission input port and the signal transmission output port.  
         [0030]    [0030]FIG. 3C shows an equivalent circuit of the seventh-order low-pass filter in FIG. 3B. Compared with the low-pass filter shown in FIG. 2C, the seventh-order low-pass filter includes three circuit patterns which are connected in series. Each circuit pattern includes an inductor L 1  disposed between the signal transmission input port and the signal transmission output port, a first capacitor C 1  connected between the signal transmission input port and the ground, and a second capacitor C 2  connected between the signal transmission output port and the ground.  
         [0031]    According to a filter design of the present invention, respective parameters of the π type equivalent circuit are expressed as follows: 
           jω   0   C   1   =jω   0   C   c   +jω   0   C   p   (Eq. 1) 
         [0032]    [0032]                 jω   0          C   1       =         jω   0          C   c       +       jω   0          C   p                 (     Eq   .              1     )                   jω   0          L   2       =     1         jω   0          C   r       +     1       jω   0          L   r                     (     Eq   .              2     )                                 
         [0033]    where, jω o C r =j (Y oo −Y oe )/2*tanφ, jω o L r =jZ o sin 2 φ. Here, ω 0  denotes a cutoff frequency of the proposed low-pass filter, C capacitance of low-pass filter, L inductance of low-pass filter, Y 00  an odd mode admittance of a coupled line, Y oe  an even mode admittance of the coupled line, Y o  a characteristic admittance and φ an electrical length of a coupled line.  
         [0034]    Using a transmission line and coupled line theory together with the equations 1 and 2, a susceptance (an imaginary number portion of an admittance in relation to a conductivity) is expressed as follows:  
               1       jω   0          L   n         =       j            Y   00     -     Y     o                 e         2        tan                 φ     -     j                   Y   0        c                 s                 c2                 φ               (     Eq   .              3     )                   jω   0          C   n       =       j                   Y     o                 e          tan                 φ     +     j                   Y   0        tan        φ   2                 (     Eq   .              4     )                               
 
         [0035]    The low-pass filter expressed as these physical parameters has three attenuation poles due to the electrical length φ of the transmission line section and the coupled line section. Two attenuation poles are positioned at two points where the susceptance of serial elements in the equation 3 becomes zero and one attenuation pole is positioned at a point where a susceptance of parallel elements in the equation 4 becomes infinite.  
         [0036]    Since the attenuation poles are dispersedly positioned at the stopband of the low-pass filter, the frequency range of the stopband is expanded up to ten times of the cutoff frequency. Also, a device size can be scaled down remarkably. That is, the positions and the number of the attenuation poles are controlled adjusting the electrical length of the transmission line section and the coupled line section, so that it is possible to implement the low-pass filter having a broad stopband.  
         [0037]    [0037]FIG. 4A is a graph illustrating simulation results of the seventh-order low-pass filter which is designed to have five attenuation poles. A cutoff frequency of the seventh-order low-pass filter is 900 MHz with a ripple level of 0.01 dB. FIG. 4B is a graph illustrating simulation results obtained using an EM simulator in order to design actually the low-pass filter based on the simulation results.  
         [0038]    As shown, the seventh-order low-pass filter in accordance with the present invention has a symmetrically elliptic low-pass characteristic at the center of 4 GHz. The attenuation poles are positioned at 1.5 GHz, 2.4 GHz, 3.8 GHz, 4.4 GHz and 6.1 GHz. The seventh-order low-pass filter exhibits an improved characteristic stopband in the range from 1 to 7 GHz at the cutoff frequency of 1 GHz.  
         [0039]    [0039]FIGS. 5A to  5 F are cross-sectional views illustrating sequential steps associated with a method for fabricating the seventh-order low-pass filter.  
         [0040]    Referring to FIG. 5A, a high-temperature superconductor (HTS) YBa 2 Cu 3 O 7-x  (YBCO) epitaxial thin film  520  is grown on an MgO substrate  510  in a C-axis direction. Then, an Au/Cr film  530  is formed on the HTS YBCO epitaxial thin film  520 .  
         [0041]    Referring to FIG. 5B, a photoresist  540  is formed on an entire structure using a spin coating method.  
         [0042]    Referring to FIG. 5C, a predetermined portion of the photoresist  540  is removed through an exposure of an ultraviolet (UV) source to thereby form a photoresist pattern  550  and mask aligner to form a photoresist pattern  550 .  
         [0043]    Referring to FIG. 5D, the HTS YBCO epitaxial thin film  520  with metal films  530  and photoresist pattern  550  is formed through the standard photolithographic and ion-milling etching processes.  
         [0044]    Referring to FIG. 5E, after the photoresist pattern  550  is removed by acetone, an Au/Cr pad  530  is formed by using a lift-off method to good contact with a K-connector.  
         [0045]    Referring to FIG. 5F, the groundplane  560  is fabricated by sputtering of the metal film (Cr/Ag film).  
         [0046]    [0046]FIG. 6 shows comparison of the simulated and measured results of the seventh-order HTS coupled line low-pass filter. The measured results are identical to the EM simulations.  
         [0047]    The HTS coupled line low-pass is fabricated using the HTS YBCO thin film grown on MgO substrate through surface treatment (polishing). Even if the HTS coupled line low-pass filters are fabricated as microstrip type, the microwave losses can be reduced considerably due to a very low surface resistance of HTS epitaxial films.  
         [0048]    The planar type HTS coupled line low-pass filter for suppression of harmonics and spurious signals can be applied to the various wireless communication systems for the remarkable improvement of a skirt characteristic as well as a broadband harmonics rejection characteristic, and reduction of interferences and noises.  
         [0049]    Although the preferred embodiments of the invention have been disclosed for illustrative purposes, those skilled in the art will appreciate that various modifications, additions and substitutions are possible, without departing from the scope and spirit of the invention as disclosed in the accompanying claims.