Patent Publication Number: US-2011068882-A1

Title: Filter and amplifying circuit

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is based upon and claims the benefit of priority of the prior Japanese Patent Application No. 2009-217547, filed on Sep. 18, 2009, the entire contents of which are incorporated herein by reference. 
     FIELD 
     The embodiments discussed herein are related to a filter and an amplifying circuit for an electronic signal. 
     BACKGROUND 
     One of filters for attenuating high frequency signals is a stub coupled to an output terminal of an amplifier. A short stub has an electrical length λ/4for a fundamental wavelength λ of output signal of an amplifier. The short stub may suppress a wave of even multiple of the fundamental frequency and allows the waves of the fundamental frequency and odd multiple of the fundamental frequency to pass through. 
     An example of a filter configuration is illustrated in  FIG. 1  in which the filter may attenuate harmonics having integral multiple orders of the fundamental frequency. A resonator  53  is coupled to an amplifying circuit  50  so as to be parallel and apart by a certain distance d to a transmission line  52  which is coupled to an output portion of an amplifier  51  composed of a field effect transistor (FET). The resonator  53  has a length of λ nf /4 and is grounded at its one end, where λ nf  is a wavelength corresponding to a target harmonic nf. The resonator  53  resonates at the harmonic nf. Accordingly, the connection of the resonator  53  to the transmission line  52  causes the harmonic nf existing over the transmission line  52  to travel back to the amplifier  51  which is arranged at an input side of the transmission line  52 . A related invention is, for example, disclosed in Japanese Laid-open Patent Publication No. 08-139535. 
     In Japanese Laid-open Patent Publication No. 05-199047, there is proposed a microwave semiconductor amplifier which includes input and output impedance matching circuits and a semiconductor such as a field effect transistor. The output impedance matching circuit includes a harmonic suppression circuit having a plurality of open end lines. The lines are connected in parallel and have respective lengths of one-fourth of the wavelengths of corresponding harmonics. The harmonic suppression circuit is used as an impedance matching element in the amplifier. 
     Japanese Laid-open Patent Publication No. H5-191175 discloses a microwave power amplifier. The amplifier comprises a power amplifying means, and a first and a second coupling line means. The power amplifying means amplifies a power of a high frequency signal by an amplifying element to output the high frequency power to an output terminal. Both ends of the first coupling line means are a first connection point as the output terminal and a second connection point, respectively, at which a high frequency signal is output. An electrical length between the first and the second connection points is a first electrical length. The second coupling line means has an electrical length same to the first electrical length, electro-magnetically couples with the first coupling line means each other, and operates so as to ground an output of the amplifying element where an electrical length of including an parasitic line between the amplifying element and the output terminal is corresponding to an electrical length by which an output of the amplifying element is virtually grounded for a second-order harmonic of a fundamental frequency of the high frequency signal amplified by the power amplifying means. 
     SUMMARY 
     According to an aspect of the invention, a filter comprises a transmission line, a stub branched from the transmission line, the stub electrically being coupled with the transmission line; and a resonator configured to electromagnetically couple with the stub and to resonate at an odd harmonic frequency of a fundamental wave, the fundamental wave propagating through the transmission line. 
     The object and advantages of the invention will be realized and attained by means of the elements and combinations particularly pointed out in the claims. It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are not restrictive of the invention, as claimed. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a diagram illustrating a configuration of an existing amplifier; 
         FIG. 2  is a diagram illustrating a configuration of an amplifying circuit according to an embodiment; 
         FIG. 3  is a first example of a filter in the amplifying circuit illustrated in  FIG. 2 ; 
         FIG. 4  is a second example of the filter in the amplifying circuit illustrated in  FIG. 2 ; 
         FIG. 5  is a third example of the filter in the amplifying circuit illustrated in  FIG. 2 ; 
         FIG. 6  is a fourth example of the filter in the amplifying circuit illustrated in  FIG. 2 ; 
         FIG. 7  is a fifth example of the filter in the amplifying circuit illustrated in  FIG. 2 ; 
         FIG. 8  is a diagram illustrating a result obtained by a simulation on an attenuation characteristic of the filter illustrated in  FIG. 5 ; and 
         FIG. 9  is a diagram illustrating a result obtained by a simulation on an attenuation characteristic of the filter illustrated in  FIG. 6 . 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     The existing filter with the stub described in Background may not reduce the odd-order harmonics, while the existing filter illustrated  FIG. 1  may be configured to reduce the odd-order harmonics. However, the existing filter illustrated in  FIG. 1  needs a length of the transmission line  52  extended to some extent for sufficient electromagnetic coupling between the transmission line  52  and the resonator  53 . Accordingly, the extended length of the transmission line  52  may result in an increase in intensity of signal propagating in the transmission line  52 . 
     There will be described the embodiments according to the present invention with reference to figures.  FIG. 2  illustrates a configuration diagram of an embodiment applied to an amplifying circuit. An amplifying circuits  1  includes an amplifier  2  and a filter  3  connected to an output terminal of the amplifier  2 . The amplifier  2  is, for example, a field effect transistor (FET) of which source is grounded in the embodiment illustrated in  FIG. 2 . The drain common FET may be usable also. The amplifying circuit  1  may be a high frequency amplifying circuit such as a microwave amplifying circuit. 
       FIG. 3  illustrates a first example of a configuration of the filter  3  illustrated in  FIG. 2 . The filter  3  includes the transmission line, a stub  20  branched from the transmission line  10 , and a resonator  30  electromagnetically coupled with the stub  20 . 
     As described above, the resonator  30  is electromagnetically coupled with the stub and resonates at an odd harmonic of a fundamental frequency of the signal propagating though the transmission line  10 . The resonator  30  may be a coupling line with a grounded and an open ends configured with a distributed parameter circuit. The resonator  30  is disposed parallel to and coupled electromagnetically to the stub  20 . The electrical length L 21  of the resonator  30  may be λ o /4 where λ o  is the wavelength of the odd harmonic to be suppressed. When composing a filter for suppressing the third order harmonic of a fundamental wave having a wavelength λ, the electrical length L 21  of the resonator  30  is λ/12. The electrical length L 21  of the resonator  30  is λ/20 for the filter  3  suppressing the fifth order harmonics 
     The electromagnetic coupling of the resonator  30  to the stub  20  results in that a connecting point “a” therebetween is grounded with respect to the odd harmonic to be reduced. Accordingly, the odd harmonic may be reflected back to the amplifier  2 . Therefore, the odd harmonic in the output from the amplifying circuit  1  may be reduced. 
     In the present embodiment, the resonator  30  is electromagnetically coupled to not the transmission line  10  but the stub  20 . Thus extending the transmission line  10  is not necessary for electromagnetically coupling with the resonator  30 , unlike the existing filter illustrated in  FIG. 1 . The present embodiment provides a filter that suppresses the odd harmonics and includes a transmission line having a length shorter than that of the existing filter. Accordingly, the loss of signal intensity decreases in the present embodiment. 
     The stub  20  may be, for example, a short stub having an electrical length L 11  equivalent to λ/4 with respect to a fundamental wave of a wave length λ propagating as a signal in the transmission line  10 . The end T 1  of the stub  20  is connected to the connecting point “a” of the transmission line  10  and the other end T 2  of the stub  20  is grounded. Connecting the stub  20  to the transmission line  10  allows the connecting point “a” to be grounded with respect to an even harmonic of a fundamental wave and the even harmonic to be reflected back to the amplifier  2  from the connecting point  2 . Accordingly, the even harmonic in the signal output from the amplifying circuit  1  is suppressed, while the fundamental wave propagates though the portion of the connecting point “a”. 
     The filter  3  reduces the even harmonic and any odd harmonic even by using the short stub, as the stub  20 , having the electrical length λ/4 with respect to the fundamental wave having a wavelength λ. Further, the use of the short stub decreases also a harmful effect on the filter  3 . 
     The resonator  30  may be disposed at a portion near the end T 1  connected to the transmission line  10 . The intensity of the signal propagating through the stub  20  is stronger at the end TI than that at the other end T 2 . Thus disposing the resonator  30  near the end T 1  increases the effect of suppressing the odd harmonic desired to be reduced. 
     The second example of configuration of the filter  3  is explained with reference to  FIG. 4 . The filter  3  includes a stub  21  branched from the transmission line  10 , the resonator  30  electromagnetically coupled with the stub  21 . The resonator  30  may be provided in a same way as the resonator  30  described above with reference to  FIG. 3 . 
     The stub  21  may be an open stub having an electrical length L 12  of λ/8 with respect to a wavelength λ of a fundamental wave. The stub  21  is electrically coupled with the transmission line  10  through an end T 1  of the both ends of the stub  21  and the other end T 2  is opened. 
     The filter  3  of the second example may suppress also an even harmonic and any odd harmonic and decreases harmful effect on the fundamental wave. Further the stub  21  provided in a form of an open stub has an advantage of easy adjustment of its electrical length. In any embodiment described later, the stub branching from the transmission line  10  may be any one of the short stub or the open stub. 
     The other example of the filter  3  will be explained with reference to  FIG. 5  which illustrates the third example of the detail configuration of the filter  3  depicted in  FIG. 2 . Resonators  30 - 1  and  30 - 2  have each configuration same to the resonator  30  illustrated in  FIG. 3 . The filter  3  of the present example includes a stub  20  branched from the transmission line  10 , and the resonators  30 - 1  and  30 - 2  electromagnetically coupled with the stub  20 . 
     The resonators  30 - 1  and  30 - 2  are disposed on the both sides of the stub  20 , that is, the resonators  30 - 1  and  30 - 2  are arranged parallel to the stub  20  so as to allow the stub  20  to position between the resonators  30 - 1  and  30 - 2 . Electromagnetic coupling provided by the configuration of the resonators  30 - 1  and  30 - 2  and the stub  20  therebetween performs the suppression of the odd harmonic as a target more efficiently than that performed by electromagnetic coupling with a single resonator and a stub. 
     The resonators  30 - 1  and  30 - 2  may be disposed at a portion near to the end T 1 , of both ends of the stub  20 , through which the stub  20  and the transmission line  10  are connected each other. Further, the number of resonators provided at each side of the stub  20  is not limited to one. That is, a plurality of resonators may be disposed on the both sides of the stub  20 . 
       FIG. 6  illustrates the fourth embodiment of the filter  4  depicted in  FIG. 2 . The filter  3  of the present embodiment includes an open stub  21  branched from the transmission line  10  and resonators  30 - 1  and  30 - 2  each of which is disposed at each of the both sides of the open stub  21  respectively. Electromagnetic coupling provided by the configuration of the resonators  30 - 1  and  30 - 2  and the open stub  21  therebetween performs the suppression of the odd harmonic as a target more efficiently than that performed by electromagnetic coupling with a single resonator and a stub. 
     The other example of the filter  3  will be explained with reference to  FIG. 7  which illustrates the fifth example of the detail configuration of the filter  3  depicted in  FIG. 2 . Resonator  31  includes a configuration same to the resonator  30  illustrated in  FIG. 3 . The filter  3  of the present example includes a stub  20  branched from the transmission line  10 , and the resonators  30  and  31  electromagnetically coupled with the stub  20 . 
     The resonators  30  and  31  are formed to have respective different resonant frequencies, for example, the resonant frequencies of the resonators  30  and  31  may be the third harmonic and the fifth harmonic of the fundamental wave respectively. 
     As a detail example, the resonators  30  and  31  are coupling lines provided as coupling lines represented by respective distributed parameter circuits each having open and grounded ends. Supposing that the wavelengths of the third and fifth harmonics are λ th  and λ f , then the electrical length L 21  of the resonator  30  may be λ th /4 and the electrical length L 22  of the resonator  31  λ f /4. 
     The fifth example as the embodiment provides a filter that suppresses a plurality of even harmonics at a same time. 
     The fifth example may be configured so that the resonators  30  and  31  are disposed parallel to and on the both sides of the stub  20  so as to position the stub  20  between the resonators  30  and  31  as illustrated in  FIG. 7 . 
     Further the resonators  30  and  31  may be disposed at a portion near to the end T 1 , of both ends of the stub  20 , through which the stub  20  and the transmission line  10  are connected each other. Further, the number of resonant frequencies is not limited to two. That is, the filter  30  may include threes or more resonators which are electromagnetically coupled with the stub  20  to provide three or more resonant frequencies. Further, the filter  3  may be an open stub instead of the filter  30  as a short stub. 
       FIG. 8  is a diagram illustrating a graph obtained by a simulation to illustrate a result of an attenuation characteristic of the filter  3  illustrated in  FIG. 5 . The simulation was performed for a filter designed for attenuating or suppressing the second and the third harmonics of a RF or high frequency signal of 2.1 GHz band. The stub  20  was a short stub having an electrical length λ/4 of the fundamental wave having a wavelength λ. The abscissa axis represents the frequency of a signal input to the filter and the vertical axis represents the intensity of signal passing through the filter. 
       FIG. 9  is a diagram illustrating a graph obtained by a simulation to illustrate a result of an attenuation characteristic of the filter  3  illustrated in  FIG. 6 . The simulation was performed for a filter designed for attenuating or suppressing the second and the third harmonics of a RF or high frequency signal of 2.1 GHz band. The stub  21  was an open stub having an electrical length λ/8 of the fundamental wave having a wavelength λ. In  FIG. 9 , the abscissa axis represents the frequency of a signal input to the filter and the vertical axis represents the intensity of signal passing through the filter. 
       FIGS. 8 and 9  illustrate the results that the filters according to the embodiments increase the attenuation around at frequencies 4.280 GHz and 6.420 GHz which are corresponding to the second harmonic and the third harmonic respectively. Accordingly, depending on the embodiments, there is provided the filter that may suppress an even harmonic and any odd harmonic and includes a shorter transmission line. 
     All examples and conditional language recited herein are intended for pedagogical purposes to aid the reader in understanding the invention and the concepts contributed by the inventor to furthering the art, and are to be construed as being without limitation to such specifically recited examples and conditions, nor does the organization of such examples in the specification relate to a showing of the superiority and inferiority of the invention. Although the embodiments of the present inventions have been described in detail, it should be understood that the various changes, substitutions, and alterations could be made hereto without departing from the spirit and scope of the invention.