Abstract:
A band-pass filter includes an input portion inputting an electromagnetic signal, an output portion outputting the electromagnetic signal, a plurality of transmission portions electrically connecting the input portion and the output portion to transmit the electromagnetic signal therebetween, and a pair of coupling members each shaping the frequency of the band-pass filter. Each of the coupling members includes a first coupling portion electrically connecting two of the transmission portions and a second coupling portion electrically connecting the first coupling portion. The first coupling portion includes a pair of parallel coupling microstrip lines of the same size. The second coupling portion includes a pair of transmission lines of different sizes.

Description:
BACKGROUND 
     1. Technical Field 
     The present disclosure generally relates to filters, and more particularly to a band-pass filter. 
     2. Description of Related Art 
     Conventionally, when a wireless network device operates at high power, harmonic components of high frequency are generated due to the nonlinear properties of the active components of the device, causing electromagnetic interference (EMI). 
     To address this, a filter is often used to suppress the harmonic components. Some manufacturers use a waveguide element, such as a microstrip, formed on a printed circuit board of the device. 
     Features of an ideal filter are signal attenuation of zero within a pass band, becoming infinite within a stop band, and transition as sharp as possible from the pass band to the stop band, providing the shortest possible distance between a transmission zero point and the stop band. In addition, increased transmission zero points improve performance of the filter in suppression of harmonic noise. However, most filters have only one transmission zero point and are thus unable to achieve or approach these ideals. 
     Therefore, a need exists in the industry to overcome the described limitations. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic diagram of a band-pass filter of an exemplary embodiment of the disclosure; 
         FIG. 2  is a schematic diagram illustrating dimensions of the band-pass filter of  FIG. 1 ; 
         FIG. 3  is a schematic diagram of an equivalent circuit of the band-pass filter of  FIG. 1 ; and 
         FIG. 4  is a diagram showing a relationship between amplitudes of insertion loss and frequency of electromagnetic signals through the band-pass filter of  FIG. 1 . 
     
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
       FIG. 1  is a schematic diagram of a band-pass filter  10  of an exemplary embodiment of the present disclosure. The band-pass filter  10  is a microstrip filter printed on a printed circuit board (PCB)  20 . 
     The band-pass filter  10  is rhomboid and includes an input portion  100 , an output portion  120  aligned with the input portion  100 , four transmission portions  140 ,  142 ,  144 , and  146 , a first coupling member  160 , and a second coupling member  180 . The four transmission portions  140 ,  142 ,  144 , and  146  are the four borders of the rhombus. The transmission portion  140  is parallel to the transmission portion  144 , and the transmission portion  142  is parallel to the transmission portion  146 . The input portion  100  and the output portion  120  are disposed at the outer opposite angles of the rhombus, and the first coupling member  160  and the second coupling member  180  are asymmetrically disposed at the inner opposite angles of the rhombus. Alternatively, the band-pass filter  10  may be rectangular. 
     In this embodiment, an angle between the transmission portion  140  and the transmission portion  142  is 90 degrees (°), as is an angle between the transmission portion  144  and the transmission portion  146 . 
     The input portion  100  inputs electromagnetic signals. The output portion  120  outputs the electromagnetic signals. The input portion  100  and the output portion  120  each have impedance values of approximately 50 ohms (Ω). 
     The transmission portions  140 ,  142 ,  144 , and  146  electrically connect the input portion  100  to the output portion  120 , transmitting the electromagnetic signals therebetween. 
     The first coupling member  160  shapes the frequency of the band-pass filter  10 , and comprises a first coupling portion  162  electrically connecting the transmission portions  140  and  146 , and a second coupling portion  164  electrically connecting the first coupling portion  162 . The first coupling portion  162  comprises a first transmission line  1620  and a second transmission line  1622  parallel to the first transmission line  1620 . The first transmission line  1620  and the second transmission line  1622  are formed of parallel coupling microstrip lines. An angle between the first transmission line  1620  and the transmission portion  146  is 45°, and an angle between the second transmission line  1622  and the transmission portion  140  is 45°. 
     The second coupling portion  164  comprises a third transmission line  1640  electrically connecting to the first transmission line  1620  and a fourth transmission line  1642  electrically connecting to the second transmission line  1622 . The fourth transmission line  1642  generally roughly shapes the frequency of the band-pass filter  10  to the 1.5 GHz range, and the third transmission line  1640  precisely shapes the frequency of the band-pass filter  10  to 1575.42 MHz. The central line of the first transmission line  1620  is the same as that of the third transmission line  1640 . The central line of the second transmission line  1622  is the same as that of the fourth transmission line  1642 . 
     The second coupling member  180  shapes the frequency of the band-pass filter  10  and comprises a third coupling portion  182  electrically connecting the transmission portions  142  and  144 , and a fourth coupling portion  184  electrically connecting third coupling portion  182 . The third coupling portion  182  comprises a fifth transmission line  1820  and a sixth transmission line  1822  parallel to the fifth transmission line  1820 . The fifth transmission line  1820  and the sixth transmission line  1822  are formed parallel coupling microstrip lines. An angle between the fifth transmission line  1820  and the transmission portion  142  is 45°, and an angle between the sixth transmission line  1822  and the transmission portion  144  is 45°. 
     The fourth coupling portion  184  comprises a seventh transmission line  1840  electrically connecting the fifth transmission line  1820  and a eighth transmission line  1842  electrically connecting the sixth transmission line  1822 . The eighth transmission line  1842  roughly shapes the frequency of the band-pass filter  10  to the 1.5 GHz range, and the seventh transmission line  1840  precisely shapes the frequency of the band-pass filter  10  to 1575.42 MHz. The central line of the fifth transmission line  1820  is the same as that of the seventh transmission line  1840 . The central line of the sixth transmission line  1822  is the same as that of the eighth transmission line  1842 . In this embodiment, the eighth transmission line  1842  is opposite to the third transmission line  1640 , and the seventh transmission line  1840  is opposite to the fourth transmission line  1642 , namely, the second coupling portion  164  and the fourth coupling portion  184  are asymmetric. 
     The width of the third transmission line  1640  exceeds that of the first transmission line  1620 , and the length of the third transmission line  1640  is smaller than that of the first transmission line  1620 , that is, the length and width of the first transmission line  1620  are different from those of the third transmission line  1640 . The width of the second transmission line  1622  exceeds that of the fourth transmission line  1642 , and the length of the fourth transmission line  1642  is smaller than that of the second transmission line  1622 , that is, the length and width of the second transmission line  1622  are different from those of the fourth transmission line  1642 . That is, the sizes of the transmission lines  1620 ,  1622  of the first coupling portion  162  are different from those of the transmission lines  1640 ,  1642  of the second coupling portion  164 . 
     The width of the third transmission line  1640  exceeds that of the fourth transmission line  1642 , and the length of the fourth transmission line  1642  exceeds that of the third transmission line  1640 , that is, the second coupling portion  164  comprises two transmission lines  1640 ,  1642  of different sizes. The length and width of the first transmission line  1620  are equal to those of the second transmission line  1622 , that is, the first coupling portion  160  comprises two transmission lines  1620 ,  1622  of the same size. 
     The width of the seventh transmission line  1840  exceeds that of the fifth transmission line  1820 , and the seventh transmission line  1840  is shorter than the fifth transmission line  1820 , that is, the length and width of the fifth transmission line  1820  are different from those of the seventh transmission line  1840 . The width of the eighth transmission line  1842  is less than that of the sixth transmission line  1822 , and the eighth transmission line  1842  is shorter than the sixth transmission line  1822 , that is, the length and width of the sixth transmission line  1822  are different from those of the eighth transmission line  1842 . That is, the sizes of the transmission lines  1820 ,  1822  of the third coupling portion  182  are different from those of the transmission lines  1840 ,  1842  of the fourth coupling portion  184 . 
     The width of the seventh transmission line  1840  exceeds that of the eighth transmission line  1842 , and the length of the eighth transmission line  1842  exceeds that of the seventh transmission line  1840 , that is, the fourth coupling portion  184  comprises two transmission lines  1840 ,  1842  of different sizes. The length and width of the fifth transmission line  1820  are equal to those of the sixth transmission line  1822 , that is, the third coupling portion  182  comprises two transmission lines  1820 ,  1822  of the same size. 
       FIG. 2  is a schematic diagram illustrating dimensions of the band-pass filter  10  of  FIG. 1 . In this embodiment, the length B of the diagonal between the input portion  100  and the output portion  120  is generally 18.5 mm, and the length A of the diagonal between the first coupling member  160  and the second coupling member  180  is generally 16.9 mm. The length C of the fifth transmission line  1820  is 4.9 mm, and the width C′ of the fifth transmission line  1820  is 1.0 mm. The lengths and widths of the first transmission line  1620 , the second transmission line  1622 , and the sixth transmission line  1822  are each equal to the length and width of the fifth transmission line  1820 . The length D of the eighth transmission line  1842  is 2.7 mm, and the width D′ of the eighth transmission line  1842  is 0.9 mm. The length and width of the fourth transmission line  1642  are equal to those of the eighth transmission line  1842 . The length E of the third transmission line  1640  is 0.4 mm, and the width E′ of the third transmission line  1640  is 1.1 mm. The length and width of the seventh transmission line  1840  are equal to those of the third transmission line  1640 . The lengths F of the transmission portions  140 ,  142 ,  144 , and  146  are each 12 mm, the widths of the transmission portions  140 ,  142 ,  144 , and  146  are each 0.1 mm. 
       FIG. 3  is a schematic diagram of an equivalent circuit of the band-pass filter  10 . As shown, the four transmission portions  140 ,  142 ,  144 , and  146  are equivalent to an inductor L 1 , an inductor L 2 , an inductor L 3 , and an inductor L 4 , respectively. Capacitors C 1 , C 2 , C 3 , and C 4  are respectively formed between the four transmission portions  140 ,  142 ,  144 , and  146  and the ground of the PCB  20 . The first coupling member  160  is equivalent to the T-shaped filter between the inductor L 1  and the inductor L 4 . The second coupling member  180  is equivalent to the T-shaped filter between the inductor L 2  and the inductor L 3 . 
       FIG. 4  is a diagram showing a relationship between amplitudes of insertion and frequency of an electromagnetic signal through the band-pass filter  10 . The horizontal axis represents the frequency in gigahertz (GHz) of the electromagnetic signal traveling through the band-pass filter  10 , and the vertical axis represents amplitudes of the insertion in decibels (dB) of the band-pass filter  10 . 
     The curve S 21  indicates a relationship between input power and output power of electromagnetic signals traveling through the filter  10 , represented by the formula:
 
 S 21=10*Log [(Output Power)/(Input Power)].
 
     For a filter, when the output power of the electromagnetic signal in a pass band frequency range approaches the input power of the electromagnetic signal, distortion of the electromagnetic signal is low and performance of the band-pass filter increased. As shown by curve S 21  of  FIG. 4 , the absolute value of the insertion loss of the electromagnetic signal in the pass band frequency range is close to 0, indicating that band-pass filter  10  performs well. 
     As shown in  FIG. 4 , two transmission zero points (e.g., rejections) are generated because the width of the first low impedance transmission portion  162  is different from that of the second low impedance transmission portion  164 , so that the band-pass filter  10  can effectively suppress harmonic noise. Therefore, filtering by the band-pass filter  10  is improved. 
     While an embodiment of the present disclosure has been described, it should be understood that it has been presented by way of example only and not by way of limitation. Thus the breadth and scope of the present disclosure should not be limited by the above-described exemplary embodiments, but should be defined only in accordance with the following claims and their equivalents.