Abstract:
A filter includes a first varactor with a first end electrically connected to a signal input end, a second varactor with a first end electrically connected to a second end of the first varactor, and a second end electrically connected to ground, and an inductor with a first end electrically connected to the second end of the first varactor, and a second end electrically connected to ground. The filter is capable of adjusting its frequency response by changing capacitance of the first varactor and/or the second varactor.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The present invention relates to a filter, and more particularly, to a filter capable of adjusting frequency response. 
         [0003]    2. Description of the Prior Art 
         [0004]    Please refer to  FIG. 1 .  FIG. 1  is a diagram showing a notch filter  100  of the prior art. As shown in  FIG. 1 , the notch filter  100  comprises a capacitor C, a varactor Cv, and an inductor L. A first end of the capacitor C is electrically connected to a signal input end IN. A first end of the varactor Cv is electrically connected to a second end of the capacitor C. A second end of the varactor Cv is electrically connected to ground. A first end of the inductor L is electrically connected to the second end of the capacitor C. A second end of the inductor L is electrically connected to ground. 
         [0005]    Please refer to  FIG. 2 .  FIG. 2  is a diagram showing a frequency response of impedance of the notch filter  100  of  FIG. 1 . A frequency fp located at a peak of a frequency response curve of the impedance of the notch filter  100  is a pole frequency. A frequency fz located at a bottom of the frequency response curve of the impedance of the notch filter  100  is a zero frequency. The notch filter  100  allows required signals with frequencies around the pole frequency fp to pass through, and filters out unwanted signals with frequencies around the zero frequency fz, such that the notch filter  100  is able to filter input signals from the signal input end IN. The pole frequency fp and the zero frequency fz of the notch filter can be determined according to the following equations: 
         [0000]    
       
         
           
             
               
                 
                   
                     f 
                     z 
                   
                   = 
                   
                     1 
                     
                       2 
                        
                       π 
                        
                       
                         
                           Lv 
                           · 
                           
                             ( 
                             
                               
                                 C 
                                  
                                 
                                     
                                 
                                  
                                 1 
                               
                               + 
                               
                                 C 
                                  
                                 
                                     
                                 
                                  
                                 2 
                               
                             
                             ) 
                           
                         
                       
                     
                   
                 
               
               
                 
                   equation 
                    
                   
                       
                   
                    
                   
                     ( 
                     1 
                     ) 
                   
                 
               
             
             
               
                 
                   
                     f 
                     p 
                   
                   = 
                   
                     1 
                     
                       2 
                        
                       π 
                        
                       
                         
                           
                             Lv 
                             · 
                             C 
                           
                            
                           
                               
                           
                            
                           2 
                         
                       
                     
                   
                 
               
               
                 
                   equation 
                    
                   
                       
                   
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                     ( 
                     2 
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         [0000]    wherein Lv is an inductance value of the inductor L, C 1  is a capacitance value of an upper side of the notch filter  100  (that is, a capacitance value of the capacitor C), and C 2  is a capacitance value of a lower side of the notch filter  100  (that is, a capacitance value of the varactor Cv). The pole frequency fp and the zero frequency fz can be adjusted by changing the capacitance of the varactor Cv in order to set positions of the pole frequency fp and the zero frequency fz. However, according to equation (1) and equation (2), both the pole frequency fp and the zero frequency fz change when the capacitance of the varactor Cv changes. The pole frequency fp or the zero frequency fz of the notch filter  100  of the prior art can not be changed independently in order to move the pole frequency fp and the zero frequency fz to required positions respectively. Therefore, the notch filter  100  of the prior art imposes limitations on design. 
       SUMMARY OF THE INVENTION 
       [0006]    The present invention provides a filter capable of adjusting frequency response. The filter comprises a first varactor with a first end electrically connected to a signal input end, a second varactor with a first end electrically connected to a second end of the first varactor and a second end electrically connected to ground, and an inductor with a first end electrically connected to the second end of the first varactor and a second end electrically connected to ground. 
         [0007]    The present invention further provides another filter capable of adjusting frequency response, which comprises a varactor, at least one capacitance adjusting unit, and an inductor. A first end of the varactor is electrically connected to a signal input end. The capacitance adjusting unit comprises a capacitor and a switch. A first end of the capacitor is electrically connected to a second end of the varactor. A first end of the switch is electrically connected to a second end of the capacitor, and a second end of the switch is electrically connected to ground. The switch is for electrically connecting the second end of the capacitor to ground when the switch is turned on. A first end of the inductor is electrically connected to the second end of the first varactor, and a second end of the inductor is electrically connected to ground. 
         [0008]    The present invention further provides another filter capable of adjusting frequency response, which comprises a first varactor, a second varactor, at least one first capacitance adjusting unit, at least one second capacitance adjusting unit, and an inductor. A first end of the first varactor is electrically connected to a signal input end. A first end of the second varactor is electrically connected to a second end of the first varactor, and a second end of the second varactor is electrically connected to ground. The first capacitance adjusting unit comprises a first capacitor and a first switch. A first end of the first capacitor is electrically connected to the signal input end. A first end of the first switch is electrically connected to a second end of the first capacitor, and a second end of the first switch is electrically connected to the second end of the first varactor. The first switch is for electrically connecting the second end of the first capacitor to the second end of the first varactor when the first switch is turned on. The second capacitance adjusting unit comprises a second capacitor and a second switch. A first end of the second capacitor is electrically connected to the first end of the second varactor. A first end of the second switch is electrically connected to a second end of the second capacitor, and a second end of the second switch is electrically connected to ground. The second switch is for electrically connecting the second end of the second capacitor to ground when the second switch is turned on. A first end of the inductor is electrically connected to the second end of the first varactor, and a second end of the inductor is electrically connected to ground. 
         [0009]    The present invention further provides another filter capable of adjusting frequency response, which comprises a first varactor, a second varactor, at least one capacitance adjusting unit, and an inductor. A first end of the first varactor is electrically connected to a signal input end. A first end of the second varactor is electrically connected to a second end of the first varactor, and a second end of the second varactor is electrically connected to ground. The capacitance adjusting unit comprises a capacitor and a switch. A first end of the capacitor is electrically connected to the signal input end. A first end of the switch is electrically connected to a second end of the capacitor, and a second end of the switch is electrically connected to the second end of the first varactor. The switch is for electrically connecting the second end of the capacitor to the second end of the first varactor when the switch is turned on. A first end of the inductor is electrically connected to the second end of the first varactor, and a second end of the inductor is electrically connected to ground. 
         [0010]    The present invention further provides another filter capable of adjusting frequency response, which comprises a plurality of capacitance adjusting units, a varactor, and an inductor. Each capacitance adjusting unit comprises a capacitor and a switch. A first end of the capacitor is electrically connected to a signal input end. A first end of the switch is electrically connected to a second end of the capacitor. The switch is for electrically connecting the second end of the capacitor to a second end of the switch when the switch is turned on. A first end of the varactor is electrically connected to the second end of the switch, and a second end of the varactor is electrically connected to ground. A first end of the inductor is electrically connected to the second end of the switch, and a second end of the inductor is electrically connected to ground. 
         [0011]    The present invention further provides another filter capable of adjusting frequency response, which comprises a plurality of first capacitance adjusting units, a plurality of second capacitance adjusting units, and an inductor. Each first capacitance adjusting unit comprises a first capacitor and a first switch. A first end of the first capacitor is electrically connected to a signal input end. A first end of the first switch is electrically connected to a second end of the first capacitor. The first switch is for electrically connecting the second end of the first capacitor to a second end of the first switch when the first switch is turned on. Each second capacitance adjusting unit comprises a second capacitor and a second switch. A first end of the second capacitor is electrically connected to the second end of the first switch. A first end of the second switch is electrically connected to a second end of the second capacitor, and a second end of the second switch is electrically connected to ground. The second switch is for electrically connecting the second end of the second capacitor to ground when the second switch is turned on. A first end of the inductor is electrically connected to the second end of the first switch, and a second end of the inductor is electrically connected to ground. 
         [0012]    In addition, the present invention further provides a method for adjusting frequency response of a filter. The filter comprises a first capacitor, a second capacitor, and an inductor. A first end of the first capacitor is electrically connected to a signal input end. A first end of the second capacitor is electrically connected to a second end of the first capacitor, and a second end of the second capacitor is electrically connected to ground. A first end of the inductor is electrically connected to the second end of the first capacitor, and a second end of the inductor is electrically connected to ground. The method comprises measuring a gain of the filter at a particular frequency, and adjusting a capacitance of the first capacitor according to the gain of the filter at the particular frequency. 
         [0013]    These and other objectives of the present invention will no doubt become obvious to those of ordinary skill in the art after reading the following detailed description of the preferred embodiment that is illustrated in the various figures and drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0014]      FIG. 1  is a diagram showing a notch filter of the prior art. 
           [0015]      FIG. 2  is a diagram showing frequency response of impedance of the notch filter of  FIG. 1 . 
           [0016]      FIG. 3  is a diagram showing a first embodiment of a filter of the present invention. 
           [0017]      FIG. 4  is a diagram showing a second embodiment of a filter of the present invention. 
           [0018]      FIG. 5  is a diagram showing a third embodiment of a filter of the present invention. 
           [0019]      FIG. 6  is a diagram showing a fourth embodiment of a filter of the present invention. 
           [0020]      FIG. 7  is a diagram showing a fifth embodiment of a filter of the present invention. 
           [0021]      FIG. 8  is a diagram showing a sixth embodiment of a filter of the present invention. 
           [0022]      FIG. 9  is a diagram showing a seventh embodiment of a filter of the present invention. 
           [0023]      FIG. 10  is a diagram showing the filter of the present invention adjusting a frequency response of a high frequency band of a low noise block. 
           [0024]      FIG. 11  is a diagram showing the filter of the present invention adjusting a frequency response of a low frequency band of the low noise block. 
       
    
    
     DETAILED DESCRIPTION 
       [0025]    Please refer to  FIG. 3 .  FIG. 3  is a diagram showing a first embodiment of a filter  300  of the present invention. As shown in  FIG. 3 , the filter  300  comprises a first varactor Cv 1 , a second varactor Cv 2 , and an inductor L. A first end of the first varactor Cv 1  is electrically connected to a signal input end IN. A first end of the second varactor Cv 2  is electrically connected to a second end of the first varactor Cv 1 , and a second end of the second varactor Cv 2  is electrically connected to ground. A first end of the inductor L is electrically connected to the second end of the first varactor Cv 1 , and a second end of the inductor L is electrically connected to ground. According to the above arrangement, because a capacitance of the first varactor Cv 1  is adjustable, the zero frequency fz can be changed independently by adjusting the capacitance of the first varactor Cv 1 . That is, the pole frequency fp and the zero frequency fz can be changed independently for moving the pole frequency fp and the zero frequency fz to required positions respectively. 
         [0026]    Please refer to  FIG. 4 .  FIG. 4  is a diagram showing a second embodiment of a filter  400  of the present invention. As shown in  FIG. 4 , the filter  400  further comprises a plurality of first capacitance adjusting units T 1 . The first capacitance adjusting unit T 1  comprises a first capacitor Ca and a first switch S 1 . A first end of the first capacitor Ca is electrically connected to a signal input end IN. A first end of the first switch S 1  is electrically connected to the second end of the first capacitor. A second end of the first switch S 1  is electrically connected to the second end of the first varactor Cv 1 . The first switch S 1  is for electrically connecting the second end of the first capacitor Ca to the second end of the first varactor Cv 1 . According to the above arrangement, the first varactor Cv 1  can be utilized to fine tune a capacitance of an upper side of the filter  400 , and the first capacitance adjusting unit T 1  can be utilized to coarse tune the capacitance of the upper side of the filter  400  by controlling on and off states of the first switch S 1  of the first capacitance adjusting unit T 1 . The second varactor Cv 2  can be utilized to fine tune a capacitance of a lower side of the filter  400 . 
         [0027]    Please refer to  FIG. 5 .  FIG. 5  is a diagram showing a third embodiment of a filter  500  of the present invention. As shown in  FIG. 5 , the filter  500  further comprises a plurality of second capacitance adjusting units T 2 . The second capacitance adjusting unit T 2  comprises a second capacitor Cb and a second switch S 2 . A first end of the second capacitor Cb is electrically connected to the second end of the first varactor Cv 1 . A first end of the second switch S 2  is electrically connected to a second end of the second capacitor Cb, and a second end of the second switch S 2  is electrically connected to ground. The second switch S 2  is for electrically connecting the second capacitor Cb to ground when the second switch S 2  is turned on. According to the above arrangement, the first varactor Cv 1  can be utilized to fine tune a capacitance of an upper side of the filter  500 . The second varactor Cv 2  can be utilized to fine tune a capacitance of a lower side of the filter  500 , and the second capacitance adjusting unit T 2  can be utilized to coarse tune the capacitance of the lower side of the filter  500  by controlling on and off states of the second switch S 2  of the second capacitance adjusting unit T 2 . 
         [0028]    Please refer to  FIG. 6 .  FIG. 6  is a diagram showing a fourth embodiment of a filter  600  of the present invention. As shown in  FIG. 6 , the filter  600  comprises a first varactor Cv 1 , a plurality of second capacitance adjusting units T 2 , and an inductor L. According to the above arrangement, the first varactor Cv 1  can be utilized to fine tune a capacitance of an upper side of the filter  600 , and the second capacitance adjusting unit T 2  can be utilized to coarse tune a capacitance of a lower side of the filter  600  by controlling on and off states of the second switch S 2  of the second capacitance adjusting unit T 2 . 
         [0029]    Please refer to  FIG. 7 .  FIG. 7  is a diagram showing a fifth embodiment of a filter  700  of the present invention. As shown in  FIG. 7 , the filter  700  comprises a first varactor Cv 1 , a second varactor Cv 2 , a plurality of first capacitance adjusting units T 1 , a plurality of second capacitance adjusting units T 2 , and an inductor L. According to the above arrangement, the first varactor Cv 1  can be utilized to fine tune a capacitance of an upper side of the filter  700 , and the first capacitance adjusting unit T 1  can be utilized to coarse tune the capacitance of the upper side of the filter  700  by controlling on and off states of the first switch S 1  of the first capacitance adjusting unit T 1 . The second varactor Cv 2  can be utilized to fine tune a capacitance of a lower side of the filter  700 , and the second capacitance adjusting unit T 2  can be utilized to coarse tune the capacitance of the lower side of the filter  700  by controlling on and off states of the second switch S 2  of the second capacitance adjusting unit T 2 . 
         [0030]    Please refer to  FIG. 8 .  FIG. 8  is a diagram showing a sixth embodiment of a filter  800  of the present invention. As shown in  FIG. 8 , the filter  800  comprises a plurality of first capacitance adjusting units T 1 , a second varactor Cv 2 , and an inductor L. According to the above arrangement, the first capacitance adjusting unit T 1  can be utilized to coarse tune a capacitance of an upper side of the filter  800  by controlling on and off states of the first switch S 1  of the first capacitance adjusting unit T 1 , and the second varactor Cv 2  can be utilized to fine tune a capacitance of a lower side of the filter  800 . 
         [0031]    Please refer to  FIG. 9 .  FIG. 9  is a diagram showing a seventh embodiment of a filter  900  of the present invention. As shown in  FIG. 9 , the filter  900  comprises a plurality of first capacitance adjusting units T 1 , a plurality of second capacitance adjusting units T 2 , and an inductor L. According to the above arrangement, the first capacitance adjusting unit T 1  can be utilized to coarse tune a capacitance of an upper side of the filter  900  by controlling on and off states of the first switch S 1  of the first capacitance adjusting unit T 1 , and the second capacitance adjusting unit T 2  can be utilized to coarse tune a capacitance of a lower side of the filter  900  by controlling on and off states of the second switch S 2  of the second capacitance adjusting unit T 2 . 
         [0032]    The above filters can be utilized in several applications. For example, the filter can be applied to a low noise amplifier (LNA), or a low noise block (LNB) for adjusting their frequency responses. In manufacturing processes of the low noise block, the frequency response of the filter of the prior art is difficult to adjust after packaging. However, an inductance of an inductor may have some deviation, which may cause the frequency response of the packaged filter to be unable to meet specifications due to the deviation of the inductance. The filter of the present invention can compensate for an offset of the frequency response caused by the deviation of the inductance of the inductor by adjusting the capacitance of the upper side of the filter (that is, independently adjusting the zero frequency fz). 
         [0033]    Please refer to  FIG. 10  and  FIG. 11 .  FIG. 10  is a diagram showing the filter of the present invention adjusting a frequency response of a high frequency band of a low noise block.  FIG. 11  is a diagram showing the filter of the present invention adjusting a frequency response of a low frequency band of a low noise block. In the high frequency band, a local oscillation frequency of the low noise block is 10.6 Hz. Image rejection values of the low noise block are the difference between power gain at frequency of 11.55 Hz and power gain at frequency of 9.65 Hz, and the difference between power gain at frequency of 12.75 Hz and power gain at frequency of 8.45 Hz. In the low frequency band, the local oscillation frequency of the low noise block is 9.75 Hz. The image rejection values of the low noise block are the difference between power gain at frequency of 10.7 Hz and power gain at frequency of 8.8 Hz, and the difference between power gain at frequency of 11.9 Hz and power gain at frequency of 7.6 Hz. The larger the image rejection value is, the better the capability of the low noise block to reject interference will be. However, as shown in  FIG. 10  and  FIG. 11 , the image rejection values of the low noise block deviate from design values when the inductance of the inductor has ±10% deviation. The image rejection values of the low noise block approach the design values after adjusting the capacitance of the upper side of the filter (such as adjusting a first varactor and/or a first capacitance adjusting unit). 
         [0034]    In contrast to the prior art, the filter of the present invention can independently adjust the pole frequency and the zero frequency respectively in order to set the pole frequency and the zero frequency to the required positions. In addition, when the filter of the present invention is applied to the low noise block, the filter of the present invention can compensate for the offset of the frequency response caused by the deviation of the inductance of the inductor. 
         [0035]    Those skilled in the art will readily observe that numerous modifications and alterations of the device and method may be made while retaining the teachings of the invention. Accordingly, the above disclosure should be construed as limited only by the metes and bounds of the appended claims.