Abstract:
The present invention discloses a switchable frequency response microwave filter, which uses voltage-controlled varactors to attain the separation or combination of the odd mode and even mode of signals in a dual-mode ring resonator to realize a bandpass or bandstop function and then controls the frequency response of the output filtered signals. Further, the present invention integrates different circuit architectures having bandpass and bandstop functions into a single circuit to reduce the complexity of the circuit.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a microwave filter, particularly to a switchable frequency response microwave filter. 
     2. Description of the Related Art 
     The filter plays an important role in wireless communication. When the frequency of a signal is at the bandpass region of the filter, the signal is allowed to pass. When the frequency of a signal is at the bandstop region of the filter, the signal is attenuated. In other words, the filter controls the response of a communication system around a certain frequency. 
     Generally, filters are classified into high pass filters, low pass filters, bandpass filters and bandstop filters, which respectively have different circuit architectures. Therefore, only via adjusting bandwidth or changing the center frequency can signal attenuation be achieved in a single circuit architecture. However, circuit designers sometimes cannot attain the desired filtered signal merely via adjusting bandwidth or changing the center frequency but have to use filters of other circuit architectures. For example, a bandpass filter allows medium-frequency signals to pass but intercepts high-frequency signals and low-frequency signals. It is impossible for a bandpass filter to intercept medium-frequency signals but allow high-frequency signals and low-frequency signals to pass because high-frequency signals and low-frequency signals have opposite frequency response in a bandpass filter. When two different frequency responses are needed, two independent filter structures are usually adopted, and a control circuit is used to shift the signal path from a filter structure to another filter structure. However, such a design has the disadvantages of a complicated circuit and an increased circuit area. 
     For overcoming the abovementioned conventional problems, the present invention proposes a switchable frequency response microwave filter, which can switch between a bandpass frequency response and a bandstop frequency response, wherein totally replacing the circuit architecture is unnecessary, and the complexity of the conventional circuit is reduced, and the circuit area is decreased. 
     SUMMARY OF THE INVENTION 
     The primary objective of the present invention is to provide a switchable frequency response microwave filter, which can switch between a bandpass frequency response and a bandstop frequency response without totally replacing the circuit architecture. 
     Another objective of the present invention is to provide a switchable frequency response microwave filter, which integrates both circuit architectures of a bandpass filter and a bandstop filter into a single circuit to reduce the complexity of the circuit. 
     Further objective of the present invention is to provide a switchable frequency response microwave filter, which can switch between a bandpass frequency response and a bandstop frequency response, wherein the two frequency responses have an identical center frequency. 
     To achieve the abovementioned objectives, the present invention proposes a switchable frequency response microwave filter, which comprises: a signal input electrode receiving an external signal, which is to be processed; an input voltage-controlled varactor coupled to the signal input electrode and a first voltage source; a dual-mode ring resonator coupled to the input voltage-controlled varactor, a grounding terminal and a second voltage source and receiving the signals via the input voltage-controlled varactor; a set of perturbing voltage-controlled varactors connected with the dual-mode ring resonator; an output voltage-controlled varactor coupled to the dual-mode ring resonator; and a signal output electrode coupled to the output voltage-controlled varactor and the grounding terminal. The output voltage-controlled varactor transfers the signal from the dual-mode ring resonator to the signal output electrode so as to output a filtered signal. The two voltage sources are used to modulate the perturbing voltage-controlled varactors, whereby the phase velocities of the even mode and odd mode of the signal are controlled in the dual-mode ring resonator. Thereby, the frequency response of the filtered signal is controlled. The center frequencies of the bandpass and bandstop responses are expressed by the following two equations:
 
 f   c,BP   =f   u {1−(1/π)tan −1 ( x   S /2)+(½π)[ x   F /(1+ x   F   2 )] Z   R   /Z   O }
 
 f   c,BS   =f   u {1+(½π)[ x   F /(1+ x   F   2 )] Z   R   /Z   O }.
 
In the present invention, the capacitances of the input voltage-controlled varactor and output voltage-controlled varactor can be used to influence the center frequencies of the bandpass and bandstop responses, and the frequency shift of the center frequencies of the two responses can be improved via careful calculation.
 
     Below, the preferred embodiments will be described in detail in cooperation with the drawings to make easily understood the characteristics and accomplishments of the present invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a diagram schematically showing the structure of a microwave filter according to the present invention; 
         FIG. 2(   a ) to  FIG. 2(   n ) are diagrams schematically showing the arrangements of the perturbing voltage-controlled varactors according to the present invention; and 
         FIG. 3(   a ) and  FIG. 3(   b ) are diagrams showing the simulation results and measurement results of the switchable frequency response microwave filter according to the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Refer to  FIG. 1  a diagram schematically showing the structure of a microwave filter according to the present invention. The microwave filter of the present invention comprises: a signal input electrode  10  receiving an external signal, which is to be processed; an input voltage-controlled varactor  12  coupled to the signal input electrode  10  and a first voltage source  14 ; a dual-mode ring resonator  16  coupled to the input voltage-controlled varactor  12 , a grounding terminal  18  and a second voltage source  24 ; two perturbing voltage-controlled varactors  20  respectively arranged in different positions of the dual-mode ring resonator  16 , wherein two ends of each perturbing voltage-controlled varactor  20  are connected with the dual-mode ring resonator  16 ; an output voltage-controlled varactor  26  coupled to the dual-mode ring resonator  16 ; and a signal output electrode  28  coupled to the output voltage-controlled varactor  26  and the grounding terminal  18 . The output voltage-controlled varactor  26  transfers the signal from the dual-mode ring resonator  16  to the signal output electrode  28  so as to output a filtered signal. The two voltage sources  14  and  24  are used to modulate the two perturbing voltage-controlled varactors  20 , the input voltage-controlled varactor  12  and the output voltage-controlled varactor  26 , whereby the phase velocities of the even mode and odd mode of the signal are controlled in the dual-mode ring resonator  16 . Thus, the frequency response of the filtered signal is controlled. In the present invention, the dual-mode ring resonator  16  is formed of a transmission line, and the transmission line may be a strip line, a microstrip line, two open conductive lines, a coaxial cable, a slotted line, a square waveguide, a round waveguide, or a coplanar waveguide. 
     Refer to  FIG. 2(   a ). The positions where the voltage-controlled varactors are arranged are related to the phases of signals. In this embodiment, the signal phases in the input voltage-controlled varactor  12  and output voltage-controlled varactor  26  have a phase difference of 90 degrees; the signal phase in a perturbing voltage-controlled varactor  30  respectively has a phase difference of 45 degrees with respect to the signal phases in the input voltage-controlled varactor  12  and the output voltage-controlled varactor  26 ; the signal phase in a perturbing voltage-controlled varactor  32  respectively has a phase difference of 135 degrees with respect to the signal phases of the input voltage-controlled varactor  12  and the output voltage-controlled varactor  26 , and the signal phases in the perturbing voltage-controlled varactors  30  and  32  have a phase difference of 180 degrees. Refer to  FIG. 2(   b ). In this embodiment, the signal phase in a perturbing voltage-controlled varactor  32  has a phase difference of 45 degrees with respect to the signal phase in the input voltage-controlled varactor  12 ; the signal phase in a perturbing voltage-controlled varactor  30  has a phase difference of 45 degrees with respect to the signal phase in the input voltage-controlled varactor  26 ; and the signal phases in the perturbing voltage-controlled varactors  30  and  32  have a phase difference of 180 degrees. In the embodiments shown in  FIG. 2(   a ) and  FIG. 2(   b ), the two perturbing voltage-controlled varactors  30  and  32  are in series. In other words, two ends of each of the perturbing voltage-controlled varactors  30  and  32  are connected to the dual-mode ring resonator  16 . Refer to  FIG. 2(   c ) and  FIG. 2(   d ), wherein the two perturbing voltage-controlled varactors  30  and  32  are in parallel, and wherein one end of each of the perturbing voltage-controlled varactors  30  and  32  is connected to the dual-mode ring resonator  16 , and the other end of each of the perturbing voltage-controlled varactors  30  and  32  is grounded. 
     Refer to  FIG. 2(   e ), wherein only a single perturbing voltage-controlled varactor is used. In this embodiment, the signal phases in the input voltage-controlled varactor  12  and output voltage-controlled varactor  26  have a phase difference of 90 degrees; and the signal phase in a perturbing voltage-controlled varactor  34  respectively has a phase difference of 45 degrees with respect to the signal phases in the input voltage-controlled varactor  12  and the output voltage-controlled varactor  26 . Refer to  FIG. 2(   f ), wherein only a single perturbing voltage-controlled varactor is used also. In this embodiment, the signal phase in a perturbing voltage-controlled varactor  34  respectively has a phase difference of 135 degrees with respect to the signal phases in the input voltage-controlled varactor  12  and the output voltage-controlled varactor  26 . In the embodiments shown in  FIG. 2(   e ) and  FIG. 2(   f ), the perturbing voltage-controlled varactors  34  is in series with the dual-mode ring resonator  16 . In other words, two ends of the perturbing voltage-controlled varactor  34  are connected to the dual-mode ring resonator  16 . Refer to  FIG. 2(   g ) and  FIG. 2(   h ), wherein the perturbing voltage-controlled varactors  34  is in parallel with the dual-mode ring resonator  16 , and wherein one end of the perturbing voltage-controlled varactor  34  is connected to the dual-mode ring resonator  16 , and the other end of the perturbing voltage-controlled varactor  34  is grounded. 
     Refer to  FIG. 2(   i ), wherein three perturbing voltage-controlled varactors are used. In this embodiment, the signal phases in the input voltage-controlled varactor  12  and output voltage-controlled varactor  26  have a phase difference of 90 degrees; the signal phase in a perturbing voltage-controlled varactor  38  respectively has a phase difference of 45 degrees with respect to the signal phases in the input voltage-controlled varactor  12  and the output voltage-controlled varactor  26 ; the signal phase in a perturbing voltage-controlled varactor  40  has a phase difference of 45 degrees with respect to the signal phase in the input voltage-controlled varactor  12 ; the signal phase in the perturbing voltage-controlled varactor  36  has a phase difference of 45 degrees with respect to the signal phase in the output voltage-controlled varactor  26 ; and the signal phases in the perturbing voltage-controlled varactors  36  and  40  have a phase difference of 180 degrees. Refer to  FIG. 20) , wherein three perturbing voltage-controlled varactors are used also. In this embodiment, the signal phase in a perturbing voltage-controlled varactor  38  respectively has a phase difference of 135 degrees with respect to the signal phases of the input voltage-controlled varactor  12  and the output voltage-controlled varactor  26 ; the signal phase in a perturbing voltage-controlled varactor  40  has a phase difference of 45 degrees with respect to the signal phase in the input voltage-controlled varactor  12 ; the signal phase in a perturbing voltage-controlled varactor  36  has a phase difference of 45 degrees with respect to the signal phase in the output voltage-controlled varactor  26 ; and the signal phases in the perturbing voltage-controlled varactors  36  and  40  have a phase difference of 180 degrees. In the embodiments shown in  FIG. 2(   i ) and  FIG. 2(   j ), the perturbing voltage-controlled varactors are in series. In other words, two ends of each of the perturbing voltage-controlled varactors  36 ,  38  and  40  are connected to the dual-mode ring resonator  16 . Refer to  FIG. 2(   k ) and  FIG. 2(   l ), wherein the three perturbing voltage-controlled varactors  36 ,  38  and  40  are in parallel, and wherein one end of each of the perturbing voltage-controlled varactors  36 ,  38  and  40  is connected to the dual-mode ring resonator  16 , and the other end of each of the perturbing voltage-controlled varactors  36 ,  38  and  40  is grounded. 
     Refer to  FIG. 2(   m ), wherein four perturbing voltage-controlled varactors are used. In this embodiment, the signal phases in the input voltage-controlled varactor  12  and output voltage-controlled varactor  26  have a phase difference of 90 degrees; the signal phase in a perturbing voltage-controlled varactor  48  respectively has a phase difference of 45 degrees with respect to the signal phases in the input voltage-controlled varactor  12  and the output voltage-controlled varactor  26 ; the signal phases in a perturbing voltage-controlled varactor  44  and the perturbing voltage-controlled varactor  48  have a phase difference of 180 degrees; the signal phase in a perturbing voltage-controlled varactor  42  has a phase difference of 45 degrees with respect to the signal phase in the input voltage-controlled varactor  12 ; the signal phase in the perturbing voltage-controlled varactor  46  has a phase difference of 45 degrees with respect to the signal phase in the output voltage-controlled varactor  26 ; and the signal phases in the perturbing voltage-controlled varactors  42  and  46  have a phase difference of 180 degrees. In the embodiments shown in  FIG. 2(   m ), the perturbing voltage-controlled varactors are in series. In other words, two ends of each of the perturbing voltage-controlled varactors  42 ,  44 ,  46  and  48  are connected to the dual-mode ring resonator  16 . Refer to  FIG. 2(   n ), wherein the four perturbing voltage-controlled varactors  42 ,  44 ,  46  and  48  are in parallel, and wherein one end of each of the perturbing voltage-controlled varactors  42 ,  44 ,  46  and  48  is connected to the dual-mode ring resonator  16 , and the other end of each of the perturbing voltage-controlled varactors  42 ,  44 ,  46  and  48  is grounded. 
     Refer to  FIG. 1  again. External signals are received by the signal input electrode  10  and processed by the input voltage-controlled varactor  12 , the dual-mode ring resonator  16  and the output voltage-controlled varactor  26  and then output from the signal output electrode  28  as filtered signals. With all the varactors controlled by the two voltage sources  14  and  24 , the two perturbing voltage-controlled varactors  20  are modulated to separate or combine the odd mode and even mode of the signals in the dual-mode ring resonator  16 . When none perturbation effect exists, i.e. when the dual-mode ring resonator  16  resonates and the two perturbing voltage-controlled varactors  20  resonate also, the phase velocities of the odd mode and even mode of a signal are identical, and the phases thereof are counterbalanced in the signal output electrode  28 , and the bandstop response is thus formed. When there is a capacitive perturbation, i.e. when the dual-mode ring resonator  16  resonates and the two perturbing voltage-controlled varactors  20  become capacitive, the phase velocities of the odd mode and even mode of a signal are different, and the phases thereof are out of phase in the signal output electrode  28 ; thus, the bandpass response is formed, and two zero-transmission points are created beside the bandpass. 
     When two voltage sources  14  and  24  control two perturbing voltage-controlled varactors  20  to form two different responses, the center frequencies of the two responses are not identical. The center frequencies of the two responses are expressed by the following two equations:
 
 f   c,BP   =f   u {1−(1/π)tan −1 ( x   S /2)+(½π)[ x   F /(1+ x   F   2 )] Z   R   /Z   O }
 
 f   c,BS   =f   u {1+(½π)[ x   F /(1+ x   F   2 )] Z   R   /Z   O }
 
wherein f u  is the resonance frequency of the unperturbed ring resonator, x S  the normalized reactance of the perturbing varactor, x F  the normalized reactance of the feeding varactor, Z R  the ring characteristic impedance, and Z O  the port impedance. The problem of frequency shift can be improved via modulating the input voltage-controlled varactor  12  and the output voltage-controlled varactor  26  according to the following equation:
 
     
       
         
           
             
               
                 
                   
                     x 
                     
                       F 
                       , 
                       BS 
                     
                   
                   = 
                   
                     
                       2 
                       ⁡ 
                       
                         [ 
                         
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                         ] 
                       
                     
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     Refer to  FIG. 3(   a ) and  FIG. 3(   b ) diagrams showing the simulation results and measurement results of the switchable frequency response microwave filter of the present invention, wherein S 11  denotes the return loss, and S 21  denotes the insertion loss. In the diagram showing the simulation results and measurement results of the switchable frequency response microwave filter in the bandpass state, the insertion loss is very small at the center frequency, and the return loss is very great at the center frequency, which means the power of the microwave having the center frequency can propagate. In the diagram showing the simulation results and measurement results of the switchable frequency response microwave filter in the bandstop state, the insertion loss is very great at the center frequency, and the return loss is very small at the center frequency, which means the power of the microwave having the center frequency cannot propagate. 
     In conclusion, the present invention can switch between a bandpass frequency response and a bandstop frequency response without totally replacing the circuit architecture. Further, the present invention integrates both circuit architectures of a bandpass filter and a bandstop filter into a single circuit to decrease circuit complexity and reduce circuit area. Besides, the present invention also proposes a detailed solution for center frequency shift. Therefore, the present invention will be of great usefulness. 
     The preferred embodiments described above are only to exemplify the present invention but not to limit the scope of the present invention. Therefore, any equivalent modification or variation according to the shapes, structures, characteristics and spirit disclosed in the present invention is to be also included within the scope of the present invention.