Patent Publication Number: US-2009231058-A1

Title: Band-pass filter

Description:
TECHNICAL FIELD 
     The present invention relates to a band-pass filter which includes a plurality of connected unit circuits, in each of which a series circuit of a coil and a capacitor is connected with a parallel circuit of a coil and a capacitor, and which is used for high-frequency communication or the like. 
     BACKGROUND ART 
     A high-frequency filter circuit obtained by connecting a plurality of unit circuits each including a series circuit of a coil and a capacitor and a parallel circuit of a coil and a capacitor has a steep frequency characteristic. However, a pass phase change of a high-frequency signal is significant, so it is difficult to use the high-frequency filter circuit for communication devices. On the other hand, a band elimination frequency characteristic is steep and a reflection phase change is small. 
     The high-frequency circuit obtained by connecting the plurality of unit circuits each including the series circuit of the coil and the capacitor and the parallel circuit of the coil and the capacitor as described above is called a right and left mixing circuit. It has been known that when element values for the coils and the capacitors are suitably selected, a circuit having a desirable elimination frequency is obtained (see, for example, Non-patent Document 1). 
     There is also a micro-mechanical high-frequency device which is called an RF micro electro-mechanical systems (MEMS) device and manufactured by microfabrication. The micro-mechanical high-frequency device receives widespread attention as a low-loss and low-distortion device, and a micro-mechanical variable capacitor using a MEMS technology has been under development (see, for example, Non-patent Document 2). 
     Non-patent Document 1: C. Caloz, and T Itoh, “Novel Microwave Devices and Structures Based on the Transmission Line Approach of Meta-Materials”, International Microwave Symposium 2003, pp. 195-198 
     Non-patent Document 2: J. J. Yao, S. Park and J. DeNatale, “HIGH TUNING-RATIO MEMS-BASED TUNABLE CAPACITORS FOR RF COMMUNICATIONS APPLICATIONS”, Solid State Sensor and Actuator Workshop, Hilton Head Island, S.C. June 1998 pp. 124-127 
     DISCLOSURE OF THE INVENTION 
     Problems to be Solved by the Invention 
     As described above, a high-frequency circuit obtained by connecting a plurality of unit circuits, in each of which a series circuit of a coil and a capacitor is connected with a parallel circuit of a coil and a capacitor, has advantages in that the band elimination frequency characteristic is steep and the reflection phase change is small. On the other hand, a pass phase change of a high-frequency signal is significant, so it is difficult to use the high-frequency filter circuit for communication devices. 
     The present invention has been made to solve the above-mentioned problem, and an object thereof is to obtain a band-pass filter having a small phase change in a pass band and a steep frequency characteristic. 
     Means for Solving the Problems 
     A band-pass filter according to the present invention includes: a four-terminal pair 90-degree hybrid circuit for distributing power into two directions with a phase difference of 90 degrees; and a band elimination filter circuit including a plurality of unit circuits connected to each other, each of the plurality of unit circuits including a series circuit of a coil and a capacitor and a parallel circuit of a coil and a capacitor, which are connected with each other, input terminals of the plurality of unit circuits being connected with coupling terminals of the hybrid circuit, and output terminals of the plurality of unit circuits being terminated with resistors having impedance values equal to an output impedance. 
     EFFECTS OF THE INVENTION 
     According to the present invention, the four-terminal pair 90-degree hybrid circuit for distributing power into two directions with the phase difference of 90 degrees is used such that reflection waves at the elimination frequency of the band elimination filter circuits are passed therethrough and reflection waves at a pass frequency of the band elimination filter circuit are consumed by the termination resistors. Therefore, a band-pass filter having a small phase change in a pass band and. a steep frequency characteristic can be obtained. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS  
         FIG. 1  is a structural diagram showing a unit circuit included in a band elimination filter of a band-pass filter according to Embodiment 1 of the present invention. 
         FIG. 2  is a graph showing a pass amplitude characteristic of the band elimination filter circuit in which 16 unit circuits, each of which is shown in  FIG. 1 , are connected. 
         FIG. 3  is a graph showing a pass phase characteristic of a band elimination filter circuit equal to that shown in  FIG. 2 . 
         FIG. 4  is a structural diagram showing the band-pass filter according to Embodiment 1 of the present invention. 
         FIG. 5  is a graph showing a pass amplitude characteristic from an input terminal to an output terminal of the band-pass filter according to Embodiment 1 of the present invention. 
         FIG. 6  is a graph showing a pass phase characteristic from the input terminal to the output terminal of the band-pass filter according to Embodiment 1 of the present invention. 
         FIG. 7  is a graph showing a pass amplitude characteristic of the band-pass filter according to Embodiment 1 of the present invention in a case where a series capacitor value of the unit circuit is adjusted. 
         FIG. 8  is a graph showing a pass amplitude characteristic of the band-pass filter according to Embodiment 1 of the present invention in a case where a parallel capacitor value of the unit circuit is adjusted. 
     
    
    
     BEST MODE FOR CARRYING OUT THE INVENTION 
     Embodiment 1 
       FIG. 1  is a structural diagram showing a unit circuit included in a band elimination filter circuit of a band-pass filter according to Embodiment 1 of the present invention. As shown in  FIG. 1 , the unit circuit includes a series circuit of a capacitor  1  and a coil  2  and a parallel circuit of a capacitor  3  and a coil  47  which are connected with each other, and further includes an input terminal  5  and an output terminal  6 . 
       FIGS. 2 and 3  show, for example, a pass amplitude-frequency characteristic and a pass phase-frequency characteristic of a band elimination filter circuit including  16  connected unit circuits, in each of which a value of the capacitor  1  is 1 pF, a value of the coil  2  is 1 nH, a value of the capacitor  3  is 3 pF, and a value of the coil  4  is 2 nH. 
       FIG. 4  is a structural diagram showing the band-pass filter according to Embodiment 1 of the present invention. As shown in  FIG. 4 , the band-pass filter according to Embodiment 1 of the present invention includes. a four-terminal pair 90-degree hybrid circuit  10  for distributing power into two directions with a phase difference of 90 degrees, and band elimination filter circuits  15  and  16 , in each of which a plurality of unit circuits as shown in  FIG. 1  are connected, and in which input terminals are connected with coupling terminals  13  and  14  of the 90-degree hybrid circuit  10  and output terminals are terminated with resistors  17  and  18  having impedance values equal to an output impedance. 
     The 90-degree hybrid circuit  10  includes an input terminal  11 , an output terminal  12 , and the coupling terminals  13  and  14 . When a pass phase from the input terminal  11  to the coupling terminal  13  is used as reference, a pass phase from the input terminal  11  to the coupling terminal  14  is delayed by 90 degrees. When a pass phase from the coupling terminal  14  to the output terminal  12  is used as reference, a pass phase from the coupling terminal  13  to the output terminal  12  is delayed by 90 degrees. The coupling terminals  13  and  14  are connected with the band elimination filter circuits in each of which sixteen unit circuits  15  or  16  as shown in  FIG. 1  are arranged. Final terminals of the band elimination filter circuits are terminated with the resistors  17  and  18  in order to prevent reflection. 
       FIG. 5  shows a frequency characteristic of a pass amplitude from the input terminal  11  to the output terminal  12  of the circuit shown in  FIG. 4 . In this case, a quality factor of the capacitor and the coil is set to 30.  FIG. 6  shows a frequency characteristic of a pass phase from the input terminal  11  to the output terminal  12  of the circuit shown in  FIG. 4 . 
     Next, an operation will be described. For example, a high-frequency signal whose frequency is 4 GHz is inputted from the input terminal  11 , passes through the hybrid circuit  10 , and enters, with an equal amplitude, a 16-stage band elimination filter circuit in which the unit circuit  15  is provided in a first stage (right and left mixing circuit) and a 16-stage band elimination filter circuit in which the unit circuit  16  is provided in a first stage (right and left mixing circuit). Because 4 GHz is an elimination frequency, the signal is reflected at a portion close to the first stage. A signal reflected at the coupling terminal  13  is transferred with an equal amplitude to the input terminal  11  and the output terminal  12 . 
     On the other hand, a signal reflected at the coupling terminal  14  is also transferred with an equal amplitude to the input terminal  11  and the output terminal  12 . At this time, as compared with a signal transferred in the order of the input terminal  11 , the coupling terminal  13 , and the input terminal  11 , a signal transferred in the order of the input terminal  11 , the coupling terminal  14 , and the input terminal  11  is delayed in phase by 180 degrees. Therefore, the signals are cancelled with each other, so the signals are not transferred to the input terminal  11 . 
     In contrast to this, a signal transferred in the order of the input terminal  11 , the coupling terminal  13 , and the output terminal  12 , and a signal transferred in the order of the input terminal  11 , the coupling terminal  14 , and the output terminal  12  are delayed by 90 degrees. Therefore, the signals are strengthened with each other at the output terminal  12 . 
     Therefore, the signal inputted to the input terminal  11  is transferred to the output terminal  12 , except for a loss occurring at the time of reflection at the 16-stage right and left mixing circuit in which the unit circuit  15  is provided in the first stage, a loss occurring at the time of reflection at the 16-stage right and left mixing circuit in which the unit circuit  16  is provided in the first stage, and a loss occurring at the time of passing through the 90-degree hybrid circuit  10 . 
     Next, for example, a high-frequency signal whose frequency is 1.8 GHz is inputted from the input terminal  11 , passes through the hybrid circuit  10 , and enters, with an equal amplitude, the 16-stage right and left mixing circuit in which the unit circuit  15  is provided in the first stage and the 16-stage right and left mixing circuit in which the unit circuit  16  is provided in the first stage. The right and left mixing circuits extend to the resistors  17  and  18  in order to pass the signal of 1.8 GHz through the circuits. Because the termination resistors  17  and  18  are made equal in impedance to the right and left mixing circuits in order to prevent reflection, the entire signals are consumed by the resistors. Therefore, the signal inputted to the input terminal  11  is not reflected at the input terminal  11  and not outputted to the output terminal  12 . 
     When the values of the capacitors of the unit circuits  15  and  16  are adjusted, pass frequencies and elimination frequencies of the right and left mixing circuits change, thereby changing a pass band of the band-pass filter combined with the 90-degree hybrid circuit. 
       FIG. 7  shows a frequency characteristic of a pass amplitude from the input terminal  11  to the output terminal  12  of the circuit shown in  FIG. 4  in the case where a value of the capacitor  1  of the unit circuit shown in  FIG. 1  is adjusted to  1  pF, 1.5 pF, and 3 pF.  FIG. 8  shows a frequency characteristic of a pass amplitude from the input terminal  11  to the output terminal  12  of the circuit shown in  FIG. 4  in the case where a value of the capacitor  3  of the unit circuit shown in  FIG. 1  is adjusted to 1 pF, 1.5 pF, and 3 pF. 
     In this way, when the values of the capacitors of the unit circuits  15  and  16  are adjusted, the pass band can be varied. 
     In this embodiment, a variable capacitor whose capacitance value can be adjusted to change the pass band can be used as the capacitor included in the unit circuit. When a micro-mechanical variable capacitor whose capacitance value can be adjusted by a micro machine manufactured by microfabrication is used as the variable capacitor, a high quality factor can be realized. Therefore, a reflection loss becomes smaller, with the result that a loss of a variable band-pass filter combined with the 90-degree hybrid circuit can be reduced. 
     Alternatively, it is possible to provide any one or both of a stub line which is used instead of the coil included in the unit circuit and which has the same inductance value as the coil, and a stub line which is used instead of the capacitor included in the unit circuit and which has the same capacitance value as the capacitor That is, it is possible to embody a structure using the stub line serving as a distributed constant element instead of the capacitor or coil serving as a distributed constant element. 
     Further, when the stub line which has the same capacitance value as the capacitor is used instead of the capacitor included in the unit circuit, for example, a line length of the stub line is made adjustable. Therefore, the capacitance value can be adjusted to change the pass band. 
     As described above, according to the present invention, the four-terminal pair 90-degree hybrid circuit for distributing power into two directions with the phase difference of 90 degrees is used such that reflection waves at the elimination frequency of the band elimination filter circuits are passed therethrough and the reflection waves at the pass frequency of the band elimination filter circuit are consumed by the termination resistors. Therefore, a band-pass filter having a small phase change in a pass band and a steep frequency characteristic can be obtained. The pass waves are absorbed by the termination resistor at the pass frequency of the band elimination filter circuits, so there is obtained an effect of influencing a circuit provided in a preceding stage of each of the filter circuits. 
     It is also possible to obtain a band-pass filter in which the values of the capacitors of the band elimination filter circuits are adjusted to change a pass frequency band. 
     Further, when the stub line is used instead of the capacitor or the coil, a small capacitance value and a small inductance value which are required at a high frequency can be easily realized. 
     INDUSTRIAL APPLICABILITY 
     According to the band-pass filter of the present invention, a phase change in a pass band is small and a steep frequency characteristic can be obtained. In a communication system using a portable terminal, the band-pass filter can be applied to a cognitive radio type portable terminal which automatically detects an available frequency which is not used in the communication system by itself and then starts communication.