Abstract:
Two second-order bandpass filter are connected serially to obtain a dual bandpass filter. One is inductive and the other is capacitive. One has a low passband and the other has a high passband. Between them, a finite transmission zero is formed to suppress noise. The present invention achieves a passband as wide as 1 gigahertz so that it can be applied to a wireless local network, communication equipments and electronic devices.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates to a bandpass filter; more particularly, relates to serially connecting a second-order inductive/capacitive bandpass filter at low passband and a second-order capacitive/inductive bandpass filter at high passband to obtain a dual bandpass filter. 
       DESCRIPTION OF THE RELATED ART 
       [0002]    A prior art of a dual bandpass filter is proclaimed in Taiwan, having a first passband and a second passband corresponding to a first frequency and a second frequency. The dual bandpass filter comprises a first pair of resonators including a first resonator and a second resonator, the first resonator and the second resonator sharing a first grounding transmission line, the grounding transmission line grounding at an end, the first resonator and the second resonator coupling with each other by the grounding transmission line; a second pair of resonators including a third resonator and a fourth resonator, the third resonator and the fourth resonator sharing a second grounding transmission line, the second grounding transmission line grounding at an end, the third resonator and the fourth resonator coupling with each other by the second grounding transmission line; and a coupler between the first pair of resonators and the second pair of resonators to couple the first pair of resonators and the second pair of resonators to obtain the first passband and the second passband. 
         [0003]    Although the prior art uses pairs of resonators and obtains two mutually-coupled pairs of resonators to obtain dual response and a finite transmission zero is obtained between two passbands through a transmission line connecting the two pairs of resonators, the bandwidth at high passband in the dual bandpass filter is obviously not wide enough in this structure while having high insertion loss. As a result, signals passing this filter have small energy and the performance of the whole circuit is not good. Hence, the prior art does not fulfill users&#39; requests on actual use. 
       SUMMARY OF THE INVENTION 
       [0004]    The main purpose of the present invention is to use impedance series to obtain a finite transmission zero in order to suppress noise and to obtain a wide bandwidth at a high passband and a low insertion loss at the passband. 
         [0005]    To achieve the above purpose, the present invention is a dual bandpass filter having a serial configuration of coupled-line filters, comprising a second-order inductive bandpass filter at low passband and a second-order capacitive bandpass filter at high passband. 
         [0006]    Therein, the second-order inductive bandpass filter at low passband has a first input connected with a direct-current (DC) capacitor connecting to a first resonator; the first resonator is coupled with a second resonator in a way of a mutual inductive coupling; the second resonator is connected with a DC capacitor to connecting to a first output; an end of the first resonator and an end of the second resonator are respectively connected with a second input and a second output of the second-order capacitive bandpass filter at high passband. 
         [0007]    And, the second-order capacitive bandpass filter at high passband connects to the second input with an end of the first resonator; the second input is connected with a third resonator; the third resonator is coupled with a fourth resonator in a way of a mutual capacitive coupling to be connected with the second output; and, the third resonator and the fourth resonator of the second-order inductive bandpass filter at high passband are grounded. 
         [0008]    Accordingly, a novel dual bandpass filter having a serial configuration of coupled-line filters is obtained. 
     
     
       BRIEF DESCRIPTIONS OF THE DRAWINGS 
         [0009]    The present invention will be better understood from the following detailed descriptions of the preferred embodiments according to the present invention, taken in conjunction with the accompanying drawings, in which 
           [0010]      FIG. 1  is a structural view showing the circuit of the first preferred embodiment according to the present invention; 
           [0011]      FIG. 2  is a view showing the spectrum of the first preferred embodiment; 
           [0012]      FIG. 3  is a structural view showing the circuit of the second preferred embodiment; and 
           [0013]      FIG. 4  is a view showing the spectrum of the second preferred embodiment. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0014]    The following descriptions of the preferred embodiments are provided to understand the features and the structures of the present invention. 
         [0015]    Please refer to  FIG. 1 , which is a structural view showing the circuit of a first preferred embodiment according to the present invention. As shown in the figure, the present invention is a dual bandpass filter having a serial configuration of coupled-line filters, comprising a first second-order inductive bandpass filter  11  and a first second-order capacitive bandpass filter  12 . 
         [0016]    The first second-order inductive bandpass filter  11  has a first input  111  connected with a direct-current (DC) capacitor  112  connecting to a first resonator  113 ; the first resonator  113  is coupled to a second resonator  114  in a way of a mutual inductive coupling; and, the second resonator  114  is connected with a first DC capacitor  112  connecting to a first output  115 . Therein, an end of the first resonator  113  and an end of the second resonator  114  are respectively connected with a second input  121  and a second output  124  of the first second-order capacitive bandpass filter  12 . 
         [0017]    The first second-order capacitive bandpass filter  12  is connected to an end of the first resonator  113  with the second input  121 ; the second input  121  is connected to a third resonator  122 ; and, the third resonator  122  is coupled with a fourth resonator  123  in a way of a mutual capacitive coupling to be connected with the second output  124 . Therein, the third resonator  122  and the fourth resonator  123  of the first second-order capacitive bandpass filter  12  are grounded; and, the first resonator  113  and the second resonator  114  are respectively connected with the third resonator  122  and the fourth resonator  123 . 
         [0018]    The first second-order capacitive bandpass filter  12  has inverted coupled-lines to obtain a mutual capacitive coupling with a phase having a 180 degrees of difference to a phase of the first second-order inductive bandpass filter  11 . Thus, the first second-order inductive bandpass filter  11  is serially connected with the first second-order capacitive bandpass filter  12 ; and, by the impedance series of the first second-order inductive bandpass filter  11  and the first second-order capacitive bandpass filter  12 , a finite transmission zero is obtained in a passband zone between the two bandpass filters. Therein, the first second-order inductive bandpass filter  11  has a passband between 2 gigahertz (GHz) and 3 GHz, having a bandwidth of 100 megahertz (MHz); and, the first second-order capacitive bandpass filter  12  has a passband between 5 GHz and 6 GHz, having a bandwidth of 1 GHz. 
         [0019]    Please refer to  FIG. 2 , which is a view showing the spectrum of the first preferred embodiment. As shown in the figure, an electromagnetic (EM) simulation curve  21  and a curve of measurement  22  of the first preferred embodiment are obtained, where, in a circuit structure of the first preferred embodiment, a finite transmission zero in a passband zone between a second-order inductive bandpass filter and a second-order capacitive bandpass filter is obtained to suppress noise to less than −40 dB. 
         [0020]    Please refer to  FIG. 3 , which is a structural view showing the circuit of a se con d preferred embodiment. As shown in the figure, the present invention is a dual bandpass filter having a serial configuration of coupled-line filters, comprising a second second-order capacitive bandpass filter  13  and a second second-order inductive bandpass filter  14 . 
         [0021]    The second second-order capacitive bandpass filter  13  has a third input  131  connected with a second DC capacitor  132  connecting to a fifth resonator  133 ; the fifth resonator  133  is coupled with a sixth resonator  134  in a way of a mutual capacitive coupling; and, the sixth resonator  134  is connected with a second DC capacitor  132  connecting to a third output  135 . Therein, an end of the fifth resonator  133  and an end of the sixth resonator  134  are respectively connected with a fourth input  141  and a fourth output  144  of the second second-order inductive bandpass filter  14 . 
         [0022]    The second second-order inductive bandpass filter  14  connects to an end of the fifth resonator  133  with the fourth input  141  ; the fourth input  141  is connected with a seventh resonator  142 ; and, the seventh resonator  142  is coupled with an eighth resonator  143  in a way of a mutual inductive coupling to be connected with the fourth output  144  Therein, the seventh resonator  142  and the eighth resonator  143  of the second second-order inductive bandpass filter  14  are grounded; and, the fifth resonator  133  and the sixth resonator  134  are respectively connected with the seventh resonator  142  and the eighth resonator  143 . 
         [0023]    The second second-order capacitive bandpass filter  13  has inverted coupled-lines to obtain a mutual capacitive coupling with a phase having a 180 degrees of difference to a phase of the second second-order inductive bandpass filter  14 . Thus, the second second-order capacitive bandpass filter  13  is serially connected with the second second-order inductive bandpass filter  14 ; and, by the impedance series of the second second-order capacitive bandpass filter  13  and the second second-order inductive bandpass filter  14 , a finite transmission zero is obtained in a passband zone between the two bandpass filters. Therein, the second second-order capacitive bandpass filter  13  has a passband between 2 GHz and 3 GHz, having a bandwidth of 100 MHz; and, the second second-order inductive bandpass filter  14  has a passband between 5 GHz and 6 GHz, having a bandwidth of 1 GHz. 
         [0024]    Please refer to  FIG. 4 , which is a view showing the spectrum of the second preferred embodiment. As shown in the figure, a circuit simulation curve  31  and an EM simulation curve  32  of the second preferred embodiment are obtained, where, in a circuit structure of the second preferred embodiment, a finite transmission zero in a passband zone between a second-order inductive bandpass filter and a second-order capacitive bandpass filter is obtained to suppress noise to less than −40 dB. 
         [0025]    To sum up, the present invention is a dual bandpass filter having a serial configuration of coupled-line filters, where a finite transmission zero is obtained through an impedance series to suppress noise and to obtain a wide bandwidth at a high passband and a low insertion loss at the passband; and, thus, a ratio of signal energy to noise energy is improved. 
         [0026]    The preferred embodiments herein disclosed are not intended to unnecessarily limit the scope of the invention. Therefore, simple modifications or variations belonging to the equivalent of the scope of the claims and the instructions disclosed herein for a patent are all within the scope of the present invention.