Patent Publication Number: US-2006001482-A1

Title: Filter device

Description:
TECHNICAL FIELD  
      The present invention relates to a filter device for use particularly in a reception IQ demodulator circuit of a mobile phone, which is installed in a semiconductor apparatus and whose variations can be corrected.  
     BACKGROUND ART  
      In recent years, there has been a strong demand for a reduction in the size of mobile phones, and thus a configuration has become common in which a filter device is installed in a semiconductor apparatus. Meanwhile, in order to allow a call waiting state to be maintained for as long as possible, it has been requested that current consumption be reduced drastically particularly in parts constituting a reception circuit.  
      Before the conventional technique is discussed, a method of correcting variations of a filter device installed in a semiconductor apparatus will be described in the following.  
      Variations mentioned herein refer to variations in a so-called CR product, namely, a product of variations in absolute value that occur in a resistance and a capacitor, which constitute a filter device, uniquely to a semiconductor apparatus.  
       FIG. 3  is a circuit diagram showing an example of a basic configuration of a filter circuit that is used in a filter device. In  FIG. 3 , reference characters  1  and  2  denote a signal input terminal and a signal output terminal, respectively. Further, reference characters  3  and  4  denote a first resistance having a resistance value R and a second resistance having the same resistance value R as that of the first resistance  3 , respectively. Reference characters  5 ,  6  and  7  denote a first capacitor having a capacity value C 1 , a second capacitor having a capacity value C 2 , and an OP amplifier for constituting an active filter, respectively.  
      With respect to a filter circuit having the above-described configuration, a method of correcting variations will be described in the following.  
      In the example of the basic configuration of a filter shown in  FIG. 3 , a cut-off frequency fc of this filter based on a transfer function between the signal input terminal  1  and the signal output terminal  2  is expressed by Expression (1) as follows.  
                   fc   =     1   /     (     2   ⁢           ⁢   π   ×       (     R   ×   C   ⁢           ⁢   1     )       1   /   2       ×       (     R   ×   C   ⁢           ⁢   2     )       1   /   2         )                   =     1   /     (     2   ⁢           ⁢   π   ×   R   ×       (     C   ⁢           ⁢   1   ×   C   ⁢           ⁢   2     )       1   /   2         )                     (   1   )             
 
      Where when values of a resistance and a capacitor that constitute the filter vary, variations in the CR product of these values are defined as v, and a cut-off frequency obtained in this case is defined as fcv, the cut-off frequency fcv is expressed by Expression (2) as follows.  
                   fcv   =     1   /     (     2   ⁢           ⁢   π   ×       (     v   ×   R   ×   C   ⁢           ⁢   1     )       1   /   2       ×       (     v   ×   R   ×   C   ⁢           ⁢   2     )       1   /   2         )                   =     1   /     (     2   ⁢           ⁢   π   ×   v   ×   R   ×       (     C   ⁢           ⁢   1   ×   C   ⁢           ⁢   2     )       1   /   2         )                     (   2   )             
 
      In the case where the CR product varies between, for example, −30% and +30% (v=0.7 to 1.3), it follows that the cut-off frequency fcv varies between +43% and −23%.  
      Herein, it is assumed that the resistance value R of the first resistance  3  and the second resistance  4  can be changed in some way to a value that is an inverse multiple of a value of the variations (1/v times). For example, when there are variations of +30% (v=1.3), the resistance value R is multiplied by 1/1.3 =0.769. As a result, an apparent product of resistance values and capacity values satisfies Expression (1), giving the cut-off frequency fc, thereby allowing a desired value to be obtained.  
      In the above-described manner, variations of a filter device installed in a semiconductor apparatus can be corrected.  
      The description is directed next to a conventional filter device whose variations can be corrected.  
       FIG. 4  is a circuit block diagram showing an example of a configuration of a conventional filter device. In  FIG. 4 , reference characters  11  and  12  denote a reference signal generating source and a reference filter, respectively. Further, reference character  13  denotes a phase difference detector that is formed of a multiplier circuit. Reference characters  14  and  15  denote a control voltage generator and a main filter that performs fundamental signal processing, respectively. Among these constituent elements, the reference filter  12  and the main filter  15  are formed of the above-described filter circuit, and the resistance value R of the first resistance  3  and the second resistance  4  that are shown in  FIG. 2  can be changed using a control voltage outputted from the control voltage generator  14 . As a method of changing the resistance value, for example, the resistances are formed of a MOS transistor, and a conductance of the MOS transistor is changed.  
      The description is directed next to the operation of a filter device with the above-described configuration.  
      Where a reference signal vs(t) generated by the reference signal generating source  11  has a frequency fs, the reference signal vs(t) can be expressed by Expression (3) as follows. 
 
 vs ( t ) =A× cos (2π ×fs×t )  (3) 
 
 In Expression (3) above, A and t denote an amplitude of vs(t) and a time, respectively. 
 
      An output signal vLPF(t) obtained by inputting the reference signal vs(t) to the reference filter  12  is expressed by Expression (4) as follows. 
 
 vLPF ( t ) =B× cos (2π ×fs×t+θ )  (4) 
 
 In Expression (4) above, B and θ denote an amplitude of vLPF(t) and a phase difference caused by the reference filter  12 , respectively. 
 
      In the case where the resistance value R of the first resistance  3  and the second resistance  4 , the capacity value C 1  of the first capacitor  5  and the capacity value C 2  of the second capacitor  6  are set so that a cut-off frequency of the reference filter  12  equals the frequency fs of the reference signal vs(t), the phase difference θ has a value of −90 degrees when there are no variations and a value of −110.6 degrees when there are variations of +30%.  
      When the reference signal vs(t) and the output signal vLPF(t) of the reference filter  12  are inputted to the phase difference detector  13 , an output signal vPD(t) obtained by multiplying these input signals is expressed as follows.  
                     vPD   ⁡     (   t   )       =       vs   ⁡     (   t   )       ×     vLPF   ⁡     (   t   )                     =     A   ×   B   ×       (       cos   ⁢           ⁢   θ     +     cos   ⁢           ⁢     (       4   ⁢           ⁢   π   ×   fs   ×   t     +   θ     )         )     /   2                     (   5   )             
 
 In the output signal vPD (t), cos θ is a direct current component corresponding to the phase difference θ and does not depend on the time t. The output signal vPD(t) is inputted to the control voltage generator  14 , so that a control voltage is outputted from the control voltage generator  14  so as to obtain a resistance value that can cancel variations of the reference filter  12  according to cos θ. By applying this control voltage to the reference filter  12 , a feedback loop is established, and thus the variations are corrected, thereby allowing a cut-off frequency of the reference filter  12  always to be equal to the frequency fs of the reference signal vs(t). 
 
      By establishing a connection so that even a resistance value of the main filter  15  is changed using a control voltage generated by the control voltage generator  14 , variations of the main filter  15  also can be corrected.  
      With the above-described configuration, a filter device that avoids variations, which unavoidably occur in the production process of semiconductor apparatuses, can be installed in a semiconductor apparatus (see, for example, JP 2002-76842 A, pages 4 to 7 and  FIG. 1 ).  
      However, in the above-described conventional configuration, correcting variations requires that a feedback loop always be established, and thus constituent elements irrelevant to a fundamental signal processing system such as a reference filter, a phase difference detector and a control voltage generator have to be operated all the time. Because of this, reduction in the current consumption of a filter device hardly can be achieved, which has been disadvantageous.  
     DISCLOSURE OF THE INVENTION  
      With the foregoing in mind, it is an object of the present invention to provide a filter device that easily can achieve reduction in current consumption and whose variations can be corrected.  
      In order to achieve the above-described object, a filter device according to the present invention includes as well as a main filter, a reference filter and a phase difference detecting circuit as in the conventional configuration, and a register that holds a control signal corresponding to a phase difference generated in the reference filter due to variations.  
      With this configuration, in order to correct variations, it is no longer required that the reference filter and the phase difference detecting circuit be operated all the time. Thus, in a filter device, reduction in current consumption easily can be achieved. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       FIG. 1  is a circuit block diagram showing an example of a configuration of a filter device according to an embodiment of the present invention.  
       FIG. 2  is a diagram showing a timing relationship between an output signal vPD(t) of a phase difference detector shown in  FIG. 1  and a reference signal vref(t).  
       FIG. 3  is a circuit diagram showing a basic configuration of a second-order filter circuit used in a conventional filter device whose variations can be corrected.  
       FIG. 4  is a circuit block diagram showing an example of a configuration of the conventional filter device whose variations can be corrected. 
    
    
     BEST MODE FOR CARRYING OUT THE INVENTION  
      Hereinafter, a preferred embodiment of the present invention will be described by referring to the appended drawings.  
       FIG. 1  is a circuit block diagram showing an example of a configuration of a filter device according to an embodiment of the present invention.  
      In  FIG. 1 , reference characters  11  and  21  denote a reference signal generating source and a frequency divider, respectively. Further, reference character  12  denotes a reference filter that is formed of a second-order filter such as shown in  FIG. 3 . Reference character  13  denotes a phase difference detector that is composed of a multiplier circuit  22  and a comparator  23 . Reference characters  24 ,  25  and  26  denote a counter, decoder that generates a control signal, and a register, respectively. Reference characters  27 ( 1 ),  27 ( 2 ), . . . , and  27 (n) denote control signal lines. Reference character  15  denotes a main filter that is composed of n-first resistances  31 ( 1 ),  31 ( 2 ), . . . , and  31 (n) that have resistance values R( 1 ), R( 2 ), . . . , and R(n), respectively, n-second resistances  32 ( 1 ),  32 ( 2 ), . . . , and  32 (n) that have the same resistance values as those of the first resistances  31 ( 1 ),  31 ( 2 ), . . . , and  31 (n), respectively, afirst capacitor  33  having a capacity value C 1 , a second capacitor  34  having a capacity value C 2 , and an OP amplifier  35  for constituting an active filter.  
      A selection is made among the first resistances  31 ( 1 ),  31 ( 2 ), . . . , and  31 (n) and among the second resistances  32 ( 1 ),  32 ( 2 ), . . . , and  32 (n) by being activated by a control signal of a corresponding one of the control signal lines  27 . In the following description, the n-first resistances  31 ( 1 ),  31 ( 2 ), . . . , and  31 (n) are referred to collectively also as first resistances  31 , and the n-second resistances  32 ( 1 ),  32 ( 2 ), . . . , and  32 (n) are referred to collectively also as second resistances  32 .  
      The description is directed next to an operation of the filter device according to this embodiment that has the above-described configuration.  
      A reference signal vref(t) generated by the reference signal generating source  11  is inputted to the frequency divider  21  having a frequency division ratio div, and based on an output of the frequency divider  21 , a divided signal vs(t) is obtained. When the divided signal vs(t) is inputted to the reference filter  12 , in an output signal vLPF(t) of the reference filter  12 , a phase difference θ with respect to the divided signal vs(t) is generated by the reference filter  12 .  
      Similarly to the conventional case, in the case where the resistance values R of the first resistances  31  and the second resistances  32 , the capacity value C 1  of the first capacitor  33  and the capacity value C 2  of the second capacitor  34  are set so that a cut-off frequency of the reference filter  12  equals a frequency fs of a reference signal vs(t), the phase difference θ has a value of −90 degrees when there are no variations and a value of −110.6 degrees when there are variations of +30%.  
      As in the conventional case, the output signal vLPF(t) of the reference filter  12  is inputted to the phase difference detector  13  and multiplied by the divided signal vs(t) in the multiplier circuit  22  thereby to obtain a signal vPD(t), though in this case, one of input signals is not a reference signal but a divided signal unlike the conventional case.  
      The output signal vPD(t) of the multiplier circuit  22  is inputted to the comparator  23  and compared with a predetermined reference level, and thus a rectangular wave signal VPD(t) is obtained as an output signal of the phase difference detector  13 . The rectangular wave signal VPD(t) has a frequency double the frequency of the divided signal vs(t), and a duty ratio D of the rectangular wave signal VPD(t) varies according to the phase difference θ generated by the reference filter  12 . Herein, the duty ratio D refers to a ratio of a segment of time in which the rectangular wave signal VPD(t) is at a high level to a period of the rectangular wave signal VPD(t). Using the phase difference θ, the duty ratio D can be expressed by Expression (6) as follows. 
 
 D= (180−θ)/180  (6) 
 
 For example, the duty ratio D has a value of 0.39 when there are variations of +30% in the reference filter  12  and a value of 0.5 when there are no variations in the reference filter  12 . 
 
      The rectangular wave signal VPD(t) outputted from the phase difference detector  13  is inputted to the counter  24  and counted based on the reference signal vref(t) as a dock.  FIG. 2  is a diagram showing a timing relationship between the rectangular wave signal VPD(t) that is inputted to the counter  24  and the reference signal vref(t) where the frequency division ratio div of the frequency divider  21  is  32 . In  FIG. 2 , reference character S 1  denotes the reference signal vref(t). Further, reference character S 2  denotes the rectangular wave signal VPD(t) obtained when there are no variations in CR product, and reference character S 3  denotes the rectangular wave signal VPD(t) obtained when there are variations in CR product of +30%.  
      The counter  24  counts the number of the reference signals vref(t) in a segment of time in which the rectangular wave signal VPD(t) is at a high level. In  FIG. 2 , a number Pent of counts is  8  when there are no variations and 6.17, or 6 with a fractional portion discarded, when there are variations of 30%. Using the frequency division ratio div. a relationship between the duty ratio D and the number Pcnt of counts can be expressed by Expression (7) as follows. 
 
 Pcnt=div×D/ 2  (7) 
 
 Therefore, based on Expressions (6) and (7) above, a relationship between the phase difference θ generated by the reference filter  12  and the number Pcnt of counts is expressed by Expression (8) as follows. 
 
 Pcnt=div× (180−θ)/360  (8) 
 
 However, with respect to each value given by Expressions (7) and (8), digits to the right of a decimal point of the value are discarded. When there are variations of −30% to +30%, where the frequency division ratio div of the frequency divider  21  is  32 , the number Pent of counts given by Expression (8) has a value among five values between 6 to 10 according to an amount of variations in CR product. The number Pent of counts is inputted to the decoder  25 , and a control signal corresponding to the number Pent of counts is outputted and held in the register  26 . 
 
      In this case, the control signal held by the register  26  activates any one of the five control signal lines  27 ( 1 ),  27 ( 2 ), . . . , and  27 ( 5 ) according to an amount of variations thereby to select a corresponding one of the first resistances  31 ( 1 ),  31 ( 2 ), . . . , and  31 ( 5 ) and a corresponding one of the second resistances  32 ( 1 ),  32 ( 2 ), . . . , and  32 ( 5 ).  
      The main filter  15  can have a cut-off frequency that is determined by these selected ones of the first resistances  31  and the second resistances  32 , the first capacitor  33  and the second capacitor  34 .  
      For example, it is assumed that when there are variations of +30%, the number Pcnt ofcountsby the counter  24  is  6 , and the first resistance  31 ( 1 ) and the second resistance  32 ( 1 ) that have the resistance value R( 1 ) are selected by the control signal line  27 ( 1 ) in this case. The resistance value R( 1 ) is corrected so as to be 0.816 times a value as the resistance value R that allows the filter to have a desired cut-off frequency when there are no variations in CR product, and thus an apparent amount of variations of +6.1% is obtained. In this case, a cut-off frequency of −5.7% with respect to the desired value is obtained, and thus a substantial improvement can be achieved with respect to the case of not performing correction, which results in an amount of variations of −23% from the desired cut-off frequency.  
      Table 1 shows examples of the phase difference θ of the reference filter  12  with respect to an amount of variations in CR product, the number Pcnt of counts by the counter  24 , a resistance correction value, and an apparent amount of variations in CR product after correction. However, in Table 1, consideration is given only to variations in product of a resistance value and a capacity value and not to other factors, i.e. amounts of variations and errors in terms of, for example, variations in temperature properties of elements constituting the filter, variations in detection accuracy of the phase difference detector  13 , the accuracy of the counts by the counter  24  and the like.  
                               TABLE 1                                       Apparent       Variations in   Phase difference   Number   Resistance   variations       CR product   θ   Pcnt of   correction   in CR product by       (%)   (degree)   counts   value   correction (%)                                                                30   15.1   −110.6   −101.3   6   0.816   6.1   −6.1       15   0.1   −101.2   −90.1   7   0.930   6.9   −6.9       0   −13   −90.0   −78.8   8   1.070   7.0   −7.0       −13.1   −25   −78.7   −67.6   9   1.235   7.4   −7.4       −25   −30   −67.5   −62.7   10   1.380   3.4   −3.4                  
 
      As described above, according to this embodiment, a control signal for correcting variations that can be obtained as a result of detecting a phase difference of a reference filter is held in a resistor, and thus once variations are detected, the reference filter and a phase difference detector that are used for detection of variations become no longer necessary and their operations can be halted.  
      This embodiment described as an example the reference filter and the main filter that basically had a circuit configuration of the second-order filter shown in  FIG. 3 . However, the present invention is not limited thereto, and a filter with any circuit configuration can be used as long as a reference filter is formed of a second-order filter, a main filter is formed of a filter of an arbitrary order, and a resistance and a capacitor that are installed in a semiconductor apparatus are used.  
      Furthermore, this embodiment described as an example the case where a resistance value was corrected. However, the present invention is not limited thereto, and it also is possible to correct a capacity value of a capacitor. Further, a resistance value and a capacity value also can be corrected so as to obtain a corrected product of these values.  
      Moreover, a phase difference detector with any circuit configuration can be used as long as the phase difference detector outputs a rectangular wave signal that has a duty ratio according to a phase difference generated by a reference filter.  
      As described in the foregoing discussion, the present invention provides a particular effect that an excellent filter device can be provided in which a value of a control signal for selecting a cut-off frequency of a main filter is held in a register, and thus operations of a reference filter and a phase difference detector can be halted, thereby easily achieving reduction in current consumption, and whose variations can be corrected.