Patent Publication Number: US-11038462-B2

Title: Semiconductor device and wireless communication apparatus

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a U.S. National Phase of International Patent Application No. PCT/JP2018/032801 filed on Sep. 5, 2018, which claims priority benefit of Japanese Patent Application No. JP 2017-236992 filed in the Japan Patent Office on Dec. 11, 2017. Each of the above-referenced applications is hereby incorporated herein by reference in its entirety. 
     TECHNICAL FIELD 
     The present disclosure relates to a semiconductor device and a wireless communication apparatus. 
     BACKGROUND ART 
     A radio receiver is able to obtain a sound signal by amplifying a high-frequency radio signal with an RF (Radio Frequency) amplifier, converting the amplified radio signal into an IF (Intermediate Frequency) signal having a low frequency with a mixer circuit, and performing demodulation with a demodulator by using the converted IF signal. It is possible to perform frequency conversion for converting a high-frequency radio signal into an IF signal by multiplying a local oscillation signal by the radio signal in the above-described mixer circuit. The local oscillation signal has a frequency corresponding to the sum of the input frequency of the radio signal and the frequency of the IF signal. 
     Accordingly, to accurately and stably control the frequency of the local oscillation signal, the radio receiver is sometimes provided with a PLL (Phase Locked Loop) synthesizer that controls the frequency of the local oscillation signal. The PLL synthesizer is able to accurately and stably obtain a local oscillation signal having a desired frequency by providing feedback for matching with the phase of a signal from a crystal oscillator that oscillates a signal having a fixed frequency. In addition, the use of the PLL synthesizer allows the radio receiver to not only obtain a local oscillation signal having a desired frequency accurately and stably, but also obtain a plurality of types of local oscillation signals having different frequencies with one crystal oscillator alone. For example, a circuit configuration including the PLL synthesizer includes PTL 1 below. 
     However, in a case where the radio receiver is provided with a PLL synthesizer as described above, various circuit blocks and parts such as a phase comparator are necessary. This increases the manufacturing cost, and further leads to increase in the power consumption. Accordingly, there is a radio receiver provided with a variable condenser instead of the PLL synthesizer as described above to suppress increase in the manufacturing cost and the power consumption. The variable condenser is a mechanical part whose electrostatic capacity value is manually variable by a user. Providing the variable condenser to an oscillator that generates a local oscillation signal in the radio receiver thus makes it possible to linearly and continuously vary the frequency of the local oscillation signal in accordance with variations in electrostatic capacity. The radio receiver including such a variable condenser has less accuracy of controlling the frequency of a local oscillation signal than that of the radio receiver including a PLL synthesizer, but makes it possible to suppress increase in the manufacturing cost and the power consumption. 
     CITATION LIST 
     Patent Literature 
     PTL 1: Japanese Patent No. 5668082 
     SUMMARY OF THE INVENTION 
     Problems to be Solved by the Invention 
     In recent years, however, less variable condensers have been manufactured, and the price of a variable condenser is increasing as a result. It is thus difficult to suppress the manufacturing cost and the power consumption while maintaining the operability of a radio receiver to allow a user to manually select a radio broadcasting station. A semiconductor device provided with a novel oscillation circuit (oscillator) and mounted on a radio receiver has been requested in such a situation to suppress the manufacturing cost and the power consumption while maintaining the manual operability. 
     Accordingly, the present disclosure proposes a novel and improved semiconductor device and wireless communication apparatus each of which makes it possible to suppress the manufacturing cost and the power consumption while maintaining the manual operability. 
     Means for Solving the Problems 
     According to the present disclosure, there is provided a semiconductor device including: an oscillation circuit including a plurality of capacitors provided on a semiconductor substrate; a conversion circuit that converts an analog signal into a digital signal; and a switch circuit that switches the capacitors on the basis of the digital signal. An oscillation frequency linearly varies with respect to a variation in the analog signal. 
     In addition, according to the present disclosure, there is provided a wireless communication apparatus including a semiconductor device including an oscillation circuit including a plurality of capacitors provided on a semiconductor substrate, a conversion circuit that converts an analog signal into a digital signal, and a switch circuit that switches the capacitors on the basis of the digital signal. An oscillation frequency linearly varies with respect to a variation in the analog signal. 
     Effects of the Invention 
     According to the present disclosure as described above, it is possible to provide a semiconductor device and a wireless communication apparatus each of which makes it possible to suppress the manufacturing cost and the power consumption while maintaining the manual operability. 
     It is to be noted that the above-described effects are not necessarily limitative. Any of the effects indicated in this description or other effects that may be understood from this description may be exerted in addition to the above-described effects or in place of the above-described effects. 
    
    
     
       BRIEF DESCRIPTION OF DRAWING 
         FIG. 1  is a circuit diagram schematically illustrating a configuration of an oscillator  10  according to a first embodiment of the present disclosure. 
         FIG. 2  is a circuit diagram schematically illustrating a configuration of an oscillator  10   a  according to a modification example of the embodiment. 
         FIG. 3  is a circuit diagram schematically illustrating a configuration of a capacity bank  100  according to the embodiment. 
         FIG. 4  is a graph illustrating an example of a variation in an oscillation frequency of an oscillator  10  according to the embodiment. 
         FIG. 5  is an explanatory diagram (first) for describing a method of controlling a frequency according to a second embodiment of the present disclosure. 
         FIG. 6  is an explanatory diagram (second) for describing the method of controlling the frequency according to the embodiment. 
         FIG. 7  is an explanatory diagram (third) for describing the method of controlling the frequency according to the embodiment. 
         FIG. 8  is an explanatory diagram (first) describing an example of a variation in a capacity value in the method of controlling the frequency according to the embodiment. 
         FIG. 9  is an explanatory diagram (first) describing an example of a variation in an oscillation frequency in the method of controlling the frequency according to the embodiment. 
         FIG. 10  is an explanatory diagram (fourth) for describing the method of controlling the frequency according to the embodiment. 
         FIG. 11  is an explanatory diagram (second) describing an example of a variation in the capacity value in the method of controlling the frequency according to the embodiment. 
         FIG. 12  is an explanatory diagram (second) describing an example of a variation in the oscillation frequency in the method of controlling the frequency according to the embodiment. 
         FIG. 13  is a circuit diagram schematically illustrating a configuration of a capacity bank  100   a  according to a third embodiment of the present disclosure. 
         FIG. 14  is an explanatory diagram for describing a method of controlling a frequency according to the embodiment. 
         FIG. 15  is a layout diagram schematically illustrating a configuration example of a capacitor  102  according to a fourth embodiment of the present disclosure. 
         FIG. 16  is a layout diagram schematically illustrating a configuration example of the oscillator  10  according to the embodiment. 
         FIG. 17  is a circuit diagram schematically illustrating a configuration of a radio receiver  60  according to a fifth embodiment of the present disclosure. 
         FIG. 18  is a circuit diagram schematically illustrating a configuration of a local oscillation signal generator  70  according to a sixth embodiment of the present disclosure. 
         FIG. 19  is an explanatory diagram for describing frequency division by the local oscillation signal generator  70  according to the embodiment. 
         FIG. 20  is a graph (first) illustrating an example of a variation in a frequency of the local oscillation signal generator  70  according to the embodiment. 
         FIG. 21  is a graph (second) illustrating an example of a variation in the frequency of the local oscillation signal generator  70  according to the embodiment. 
         FIG. 22  is a circuit diagram schematically illustrating a configuration of a radio receiver  60   a  according to a comparative example. 
         FIG. 23  is a circuit diagram schematically illustrating a configuration of a radio receiver  60   b  according to a comparative example. 
         FIG. 24  is a graph illustrating an example of a variation in a capacity value of a variable condenser  602  relative to a rotational angle. 
         FIG. 25  is an explanatory diagram for describing a method of controlling a frequency according to a comparative example. 
     
    
    
     MODES FOR CARRYING OUT THE INVENTION 
     The following describes a preferred embodiment of the present disclosure in detail with reference to the accompanying drawings. It is to be noted that, in this description and the accompanying drawings, components that have substantially the same functional configuration are indicated by the same reference signs, and thus redundant description thereof is omitted. 
     In addition, in this description and the drawings, a plurality of components that has substantially the same or similar functional configuration is sometimes distinguished from each other by attaching different numerals after the same reference numerals. However, in a case where there is no need in particular to distinguish a plurality of components that has substantially the same or similar functional configuration, the same reference signs alone are attached. In addition, similar components of different embodiments are sometimes distinguished by attaching different alphabets to the same reference numerals. However, in a case where there is no need in particular to distinguish similar components from each other, the same reference signs alone are attached. 
     Further, when describing a circuit configuration below, unless otherwise specified, “coupling” refers to electrical coupling between a plurality of elements. Further, “coupling” in the following description includes not only a case of coupling a plurality of elements directly and electrically, but also a case of coupling a plurality of elements indirectly and electrically via another element. In addition, in the following description, “coupling in series” means coupling a plurality of elements in a line, and “coupling in parallel” means coupling two respective terminals of elements to each other. For example, in a case where there are two elements each including an A terminal and a B terminal, coupling the B terminal of one of the elements and the A terminal of the other element is referred to as coupled in series, and coupling the respective A terminals of the two elements to each other and coupling the respective B terminals to each other are referred to as coupled in parallel. 
     In addition, the layout diagrams to be referred to in the following description are intended to describe an embodiment of the present disclosure and promote an understanding thereof, and in some cases, shapes, dimensions, ratios, etc. illustrated in the drawings are different from reality for better understanding. Further, it is possible to change the design of the shape of each element, etc. illustrated in the drawings as appropriate upon consideration of the following description and the known art. 
     It is to be noted that description is given in the following order.
     1. Background to the creation of an embodiment of the present disclosure by the inventors of the present application   2. First embodiment   3. Second embodiment   3.1 Comparative example   3.2 Present embodiment 1   3.3 Present embodiment 2   4. Third embodiment   4.1 Configuration of capacity bank  100   a      4.2 Method of controlling frequency   5. Fourth embodiment   6. Fifth embodiment   6.1 Configuration of radio receiver  60     6.2 Configuration of local oscillation signal generator  70     7. Conclusion   8. Supplement   

     1. Background to the Creation of an Embodiment of the Present Disclosure by the Inventors of the Present Application 
     An embodiment of the present disclosure described below relates to an oscillator usable for a radio receiver, a local oscillation signal generator including the oscillator, and a radio receiver including these. The oscillator according to the embodiment of the present disclosure described below is not, however, limited to use for a radio receiver, but is usable for various wireless communication apparatuses. 
     The background to the creation of the embodiment of the present disclosure by the inventors of the present application is first described with reference to  FIGS. 22 to 24  before the embodiment of the present disclosure is described in detail.  FIGS. 22 and 23  are circuit diagrams schematically illustrating configuration examples of radio receivers  60   a  and  60   b  according to a comparative example. In addition,  FIG. 24  is a graph illustrating an example of variations in the capacity value of a variable condenser  602  relative to a rotational angle. It is to be noted that the comparative example here means radio receivers, variable condensers, and the like studied before the inventors of the present application create the embodiment of the present disclosure. 
     The radio receiver  60   a  amplifies a high-frequency radio signal that has been subjected to modulation such as AM (Amplitude Moduration) or FM (Frequency Moduration) with an RF amplifier  702 , and converts the amplified radio signal into an IF signal having a low frequency with a mixer circuit  704 , for example, as illustrated in  FIG. 22 . Further, the radio receiver  60   a  removes an unnecessary signal from the converted IF signal with a band-pass filter  718 , and amplifies the converted IF signal with an IF amplifier  720 . Thereafter, the radio receiver  60   a  demodulates the converted IF signals with a demodulator  722 . The radio receiver  60   a  is hereby able to obtain a sound signal. As described above, it is possible to perform frequency conversion for converting a high-frequency radio signal into an IF signal by multiplying a local oscillation signal by the radio signal in the above-described mixer circuit  704 . The local oscillation signal has a frequency corresponding to the sum of the input frequency of the radio signal and the frequency of the IF signal. 
     A local oscillation signal is requested to have a broadband frequency to cover the entire band of a radio signal. This requests an oscillator  90   a  to considerably vary the frequency of an oscillation signal. The oscillator  90   a  generates a local oscillation signal. Examples of the oscillator  90   a  like this include a configuration in which an inductor  300 , a variable capacitor  502 , and a negative resistance circuit  40  are coupled in parallel as illustrated in  FIG. 23 . In a case where the inductance value of the inductor  300  is represented as L and the capacity value of the variable capacitor  502  is represented as C in the oscillator  90   a , it is possible to express a frequency F of an oscillation signal generated by the oscillator  90   a  as Expression (1) below. 
     
       
         
           
             
               
                 
                   [ 
                   
                     Expression 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     1 
                   
                   ] 
                 
               
               
                 
                     
                 
               
             
             
               
                 
                   F 
                   = 
                   
                     1 
                     
                       2 
                       ⁢ 
                       π 
                       ⁢ 
                       
                         LC 
                       
                     
                   
                 
               
               
                 
                   ( 
                   1 
                   ) 
                 
               
             
           
         
       
     
     Varying the capacity value of the variable capacitor  502  then allows the oscillator  90   a  to considerably vary the frequency of an oscillation signal. This allows a local oscillation signal to have a broadband frequency that covers the entire band of a radio signal. 
     If described in detail, the radio receiver  60   a  is able to select a broadcast of a desired radio station by varying the frequency of an oscillation signal of the oscillator  90   a  and tuning the frequency to the frequency of a radio signal for a desired radio station as described above. Without highly accurately tuning the radio receiver  60   a  to the frequency of a radio signal for a desired radio station at this time, it is not possible to correctly select a station. The radio receiver  60   a  is thus requested to accurately vary the frequency of a local oscillation signal. 
     More specifically, the radio receiver  60   a  according to the comparative example includes the voltage controlled oscillator  90   a  including the variable capacitor  502  whose capacity value varies in accordance with an input voltage as illustrated in  FIG. 22 . Further, the radio receiver  60   a  includes a PLL synthesizer  716  that uses the accuracy of the frequency of a reference signal generated by a crystal oscillator  706  to highly accurately control the frequency of an oscillation signal. The PLL synthesizer  716  includes a phase comparator  708 , a frequency divider  710 , a charge pump  712 , a loop filter  714 , and the like in addition to the oscillator  90   a . The PLL synthesizer  716  inputs a signal from the crystal oscillator  706  that oscillates a signal having a fixed frequency and a signal obtained by dividing the frequency of an oscillation signal from the oscillator  90   a  to the phase comparator  708 . The PLL synthesizer  716  compares the phases of the respective signals and provides feedback to the oscillator  90   a  to make the phases match. This allows the radio receiver  60   a  to accurately and stably obtain an oscillation signal having a desired frequency. In addition, varying the number of frequencies divided by the above-described frequency divider  710  allows the radio receiver  60   a  to vary the frequency of an oscillation signal and allows the oscillation signal to have a broadband frequency that covers the entire band of a radio signal. The frequency of an oscillation signal generated by the oscillator  90   a  is thus set to be an integer multiple of the frequency of a reference signal transmitted by the crystal oscillator  706 . 
     However, in a case where the radio receiver  60   a  is provided with the PLL synthesizer  716  as illustrated in  FIG. 22 , the crystal oscillator  706  that generates a reference signal, the phase comparator  708 , the frequency divider  710 , the charge pump  712 , the loop filter  714 , and the like are necessary in addition to the voltage controlled oscillator  90   a  whose frequency varies in accordance with an input voltage. Further, a microcomputer (not illustrated) for control, a liquid crystal display (not illustrated), and the like are also necessary in the radio receiver  60   a . The radio receiver  60   a  according to the comparative example illustrated in  FIG. 22  is therefore able to accurately and stably obtain an oscillation signal having a desired frequency, but requests various circuit blocks. This increases the manufacturing cost, and further leads to increase in the power consumption. 
     The inexpensive and low-power consuming radio receiver  60   b  that has to operate for a long time with a dry battery or the like such as an emergency radio receiver as used at the time of a disaster then include a radio receiver that does not include the PLL synthesizer  716  as described above. If described in detail, as illustrated in  FIG. 23 , the radio receiver  60   b  does not include the PLL synthesizer  716 , but includes a variable condenser  602  in the oscillator  90   b  instead. A user is able to manually and mechanically vary the electrostatic capacity value of the variable condenser  602 . 
     For example, it is possible to vary the electrostatic capacity value of the variable condenser  602  by rotating one of the electrodes of an electrode pair to vary the facing area of the electrode pair. Specifically, as illustrated in  FIG. 24 , rotating one of electrodes of an electrode pair of the variable condenser  602  makes it possible to continuously vary the electrostatic capacity value of the variable condenser  602 . It is assumed that the variable condenser  602  has a curved relationship between a rotational angle and an electrostatic capacity as illustrated in  FIG. 24 . This makes it possible to linearly vary the frequency of an oscillation signal generated by the oscillator  90   a  with respect to an angle of manual rotation by a user. It is thus possible in the radio receiver  60   b  to linearly and continuously the frequency of an oscillation signal within a minute variation range in accordance with a manual operation of a user. The radio receiver  60   b  thus has less accuracy of controlling the frequency of an oscillation signal than that of the radio receiver including the PLL synthesizer  716 , but is able to be manually operated by a user directly when selecting a radio broadcasting station. This makes it possible to suppress increase in the manufacturing cost and the power consumption. 
     In recent years, however, the less variable condensers  602  have been manufactured as described above, and the price of the variable condenser  602  is increasing as a result. The radio receiver  60   b  requested to be inexpensive and consume less power has been thus requested to replace the variable condenser  602  with another part. Various parts are conceivable as the other part with which the variable condenser  602  is replaced, but a part is more preferable that is manufacturable on a semiconductor chip (semiconductor device) as with another circuit block of the radio receiver  60   b  to further suppress increase in the manufacturing cost. 
     Accordingly, in view of the situations as described above, the inventors of the present application have created the embodiment of the present disclosure that makes it possible to linearly and continuously vary the frequency of an oscillation signal within a minute variation range in accordance with a manual operation of a user as with the radio receiver  60   b  including the variable condenser  602 , and suppress the manufacturing cost and the power consumption while maintaining the manual operability. 
     If described in detail, the embodiment of the present disclosure created by the inventors of the present application is an oscillator including a capacitor provided on a semiconductor chip instead of a mechanical part provided to the outside of the semiconductor chip like the variable condenser  602 . In the present embodiment, the electrostatic capacity value of the capacitor is switched by converting an input voltage for oscillation frequency control into a digital signal, and using the converted digital signal. The input voltage for oscillation frequency control is an analog signal inputted from a user. Switching the electrostatic capacity value of the capacitor included in the oscillator in this way makes it possible to obtain an oscillation signal having a frequency that linearly and continuously varies with respect to the input voltage within a minute variation range. The oscillator according to the present embodiment thus makes it possible to suppress the manufacturing cost and the power consumption while maintaining the manual operability. The following sequentially describes the respective embodiments of the present disclosure created by the inventors of the present application in detail. 
     2. First Embodiment 
     First, a first embodiment of the present disclosure is described in detail with reference to  FIGS. 1 and 2 .  FIG. 1  is a circuit diagram schematically illustrating a configuration of an oscillator  10  according to a first embodiment of the present disclosure, and  FIG. 2  is a circuit diagram schematically illustrating a configuration of an oscillator  10   a  according to a modification example of the present embodiment. 
     As illustrated in  FIG. 1 , the oscillator  10  according to the present embodiment includes an LC oscillator (oscillation circuit)  20  including a capacity bank (capacitor)  100  and the inductor  300 , an AD converter (Analog Digital Converter) (conversion circuit) (that is referred to as “ADC” in the diagram)  30 , a negative resistance circuit  40 , and a constant current source  50 . The capacity bank (capacitor)  100  includes a plurality of capacitors (small capacitors)  102  (see  FIG. 3 ). The oscillator  10  is an oscillator that oscillates an oscillation signal having a predetermined frequency, and the frequency of the oscillation signal linearly and continuously varies within a minute variation range in accordance with an analog signal inputted to the AD converter  30 . It is to be noted that it is possible to form the oscillator  10  on one semiconductor substrate as described below. 
     If described in detail, in the LC oscillator  20 , the capacity bank  100  and the inductor  300  are coupled in parallel. In such a case, in a case where the inductance value of the inductor of the LC oscillator  20  is represented as L and the electrostatic capacity value of the capacity bank  100  is represented as C, the frequency F of a signal to be oscillated is expressed as Expression (2) below. In the present embodiment, varying the total capacity value of the capacity bank  100  makes it possible to vary the frequency of an oscillation signal. It is to be noted that the LC oscillator  20  according to the present embodiment may be provided with the plurality of capacity banks  100  coupled in parallel, and the respective total capacity values (sum of the electrostatic capacity values of the plurality of capacitors  102  included in the capacity bank  100 ) of the plurality of capacity banks  100  may be the same or different from each other. In addition, the capacity bank  100  according to the present embodiment is described in detail below. 
     
       
         
           
             
               
                 
                   [ 
                   
                     Expression 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     2 
                   
                   ] 
                 
               
               
                 
                     
                 
               
             
             
               
                 
                   F 
                   = 
                   
                     1 
                     
                       2 
                       ⁢ 
                       π 
                       ⁢ 
                       
                         LC 
                       
                     
                   
                 
               
               
                 
                   ( 
                   2 
                   ) 
                 
               
             
           
         
       
     
     The negative resistance circuit  40  is coupled to the LC oscillator  20  in parallel. In the example illustrated in  FIG. 1 , the negative resistance circuit  40  includes an NMOS cross couple obtained by cross-coupling two n-type MOS (Metal Oxide Semiconductor) transistors  400 . It is to be noted that the configuration of the negative resistance circuit  40  is not limited in particular, but may include a CMOS (Complementary Metal Oxide Semiconductor) circuit as illustrated in  FIG. 2 . 
     The constant current source  50  is coupled to the negative resistance circuit. The constant current source  50  may include various known constant current source circuits, and is replaceable with a resistor. 
     The AD converter  30  is a conversion circuit that is coupled to a switch  202  (see  FIG. 3 ) for switching the electrostatic capacity value of the capacity bank  100 , and converts an inputted analog signal into a digital signal. The digital signal converted by the AD converter  30  thus allow the plurality of switches (switch circuits)  202  coupled to the respective capacitors  102  of the capacity bank  100  to be switched on/off. Such switching makes it possible to vary the electrostatic capacity value of the capacity bank  100 . Further, to secure operability similar to that of the radio receiver  60   b  including the above-described variable condenser  602 , an analog signal to be inputted to the AD converter  30  is able to be a direct-current voltage obtained by a variable resistor  80  whose resistance value is variable by an input from a user. 
     Next, a configuration of the capacity bank  100  according to the present embodiment is described with reference to  FIGS. 3 and 4 .  FIG. 3  is a circuit diagram schematically illustrating a configuration of the capacity bank  100  according to the present embodiment.  FIG. 4  is a graph illustrating an example of variations in the oscillation frequency of the oscillator  10  according to the present embodiment. 
     As illustrated in  FIG. 3 , the capacity bank  100  according to the present embodiment includes the capacitor (if described in detail, MIM (Metal Insulator Metal) capacitor, MOM (Metal Oxide Metal) capacitor, or the like)  102  having a fixed electrostatic capacity value, and the switch  202  including an MOS transistor. If described in detail, the one switch  202 , and the two capacitors  102  coupled in series on both sides of the switch  202  are included in one unit. The capacity bank  100  is configured by coupling a plurality of such units in parallel. The switch  202  of each unit is turned on/off by switching the value of a voltage applied to the gate of the MOS transistor that is the switch  202  in accordance with the above-described digital signal, and this makes it possible to discretely switch the total capacity value of the capacity bank  100 . Thus, in the LC oscillator  20  including the capacity bank  100 , the total capacity value of the capacity bank  100  is discretely varied by a digital signal, thereby discretely varying the frequency of an oscillation signal. 
     The oscillator  10  according to the present embodiment is then able to generate an oscillation signal whose oscillation frequency linearly and continuously varies with respect to an inputted digital signal as illustrated in  FIG. 4  by appropriately selecting the capacity values of the respective capacitors  102  included in the capacity bank  100  and the number of capacitors  102  included in the capacity bank  100  (e.g., selecting the electrostatic capacity values of the capacitors  102  of each unit to cause the switch  202  to discretely vary the total capacity value of the capacity bank  100  within a minute variation range). 
     In addition, in the present embodiment, as described above, a digital signal is obtained by converting an analog signal with the AD converter  30 , and the analog signal is a direct-current voltage obtained by the variable resistor  80  whose resistance value is variable by an input from a user. This allows the oscillator  10  according to the present embodiment to linearly and continuously vary the frequency of an oscillation signal in accordance with a manual operation of a user. Accordingly, it is possible to secure operability similar to that of the above-described radio receiver  60   b  including the variable condenser  602  according to the comparative example. 
     3. Second Embodiment 
     As described above, in the above-described first embodiment of the present disclosure, appropriately selecting the capacity values of the respective capacitors  102  included in the capacity bank  100  and the number of capacitors  102  included in the capacity bank  100  makes it possible to generate an oscillation signal whose oscillation frequency linearly and continuously varies with respect to an inputted digital signal. In the first embodiment, however, the frequency of an oscillation signal is varied by discretely switching the total capacity value of the capacity bank  100  with a digital signal. In fact, the oscillator  10  thus has to switch the total capacity value of the capacity bank  100  within a minute variation range to generate an oscillation signal whose oscillation frequency linearly and continuously varies with respect to a digital signal as illustrated in  FIG. 4 . 
     In a semiconductor manufacturing process, however, the minimum capacity value or capacity value of the manufacturable capacitor  102  is limited by the semiconductor manufacturing process in which the capacitor  102  is manufactured. It is sometimes difficult to obtain the plurality of capacitors  102  each having a desired capacity value that makes it possible to switch the total capacity value of the capacity bank  100  within a minute variation range. Accordingly, a method of controlling a frequency in the oscillator  10  is proposed in a second embodiment of the present disclosure described below. The method of controlling the frequency in the oscillator  10  makes it possible to generate an oscillation signal whose oscillation frequency linearly and continuously varies with respect to a digital signal within a minute variation range. 
     3.1 Comparative Example 
     If described in detail, in a case where the frequency of an oscillation signal is linearly and continuously varied within a minute variation range in accordance with a manual operation of a user as with the radio receiver  60   b  including the variable condenser  602  according to the comparative example, it is requested to switch the total capacity value of the capacity bank  100  within a minute variation range. The following describes a method of controlling a frequency according to the comparative example with reference to  FIG. 25 .  FIG. 25  is an explanatory diagram for describing the method of controlling the frequency according to the comparative example. It is to be noted that the comparative example here means an oscillator and a method of controlling a frequency in the oscillator studied before the inventors of the present application create the embodiment of the present disclosure. 
     It is assumed that the oscillator according to the comparative example is provided with a fixed capacitor (electrostatic capacity value C 0 )  104  whose electrostatic capacity value is not variable, and a capacity bank  900  including the plurality of capacitors  102  (electrostatic capacity values Cu) coupled in parallel, for example, as illustrated in the upper portion of  FIG. 25 . It is possible to switch the capacity value of the capacity bank  900  with the switch  202 . It is to be noted that it is assumed that the fixed capacitor  104  and the capacity bank  900  are coupled in parallel in the oscillator according to the comparative example. In addition, it is assumed that each capacitor  102  of the above-described capacity bank  900  has a minimum capacity value (Cu) manufacturable in a semiconductor manufacturing process to be used. 
     Here, it is considered to vary the frequency of an oscillation signal in such an oscillator according to the comparative example. For example, it is assumed that the oscillator according to the comparative example uses the fixed capacitor  104  and the N capacitors  102  in the capacity bank  900  to generate an oscillation signal before a frequency adjustment as illustrated in the left portion of  FIG. 25 . In such a case, the total capacity value of the oscillator according to the comparative example is C 0 +NCu. An oscillation frequency F 0  for the oscillator is thus expressed as Expression (3) below. 
     
       
         
           
             
               
                 
                   [ 
                   
                     Expression 
                     ⁢ 
                     
                         
                     
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                     3 
                   
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                     F 
                     0 
                   
                   = 
                   
                     1 
                     
                       2 
                       ⁢ 
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                       ⁢ 
                       
                         
                           L 
                           ⁡ 
                           
                             ( 
                             
                               
                                 C 
                                 0 
                               
                               + 
                               
                                 NC 
                                 u 
                               
                             
                             ) 
                           
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   3 
                   ) 
                 
               
             
           
         
       
     
     Next, the oscillator according to the comparative example turns on the switch  202  of the one capacitor  102  in the capacity bank  900 , and the oscillator uses the fixed capacitor  104  and the N+1 capacitors  102  in the capacity bank  900  to generate an oscillation signal after a frequency adjustment as illustrated in the right portion of  FIG. 25 . In such a case, the total capacity value of the oscillator according to the comparative example is C 0 +(N+1)Cu. An oscillation frequency F 1  for the oscillator is thus expressed as Expression (4) below. 
     
       
         
           
             
               
                 
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                     Expression 
                     ⁢ 
                     
                         
                     
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                     F 
                     1 
                   
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                           ⁡ 
                           
                             ( 
                             
                               
                                 C 
                                 0 
                               
                               + 
                               
                                 
                                   ( 
                                   
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                                     + 
                                     1 
                                   
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                                   u 
                                 
                               
                             
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                   ( 
                   4 
                   ) 
                 
               
             
           
         
       
     
     Thus, in the oscillator according to the comparative example, a frequency difference between the oscillation frequency F 0  and the oscillation frequency F 1  serves as the minimum difference between oscillation frequencies adjustable by the oscillator. However, the capacity value Cu of each capacitor of the capacity bank  900  corresponds to the minimum capacity value manufacturable in a semiconductor manufacturing process to be used, and the value is limited by the semiconductor manufacturing process to be used. In the oscillator according to the comparative example, a frequency variation range is thus greater than a desired frequency variation range, generating an oscillation signal whose oscillation frequency discretely varies with respect to a digital signal in a rough step. In other words, it is difficult in the oscillator according to the comparative example to generate an oscillation signal whose oscillation frequency linearly and continuously varies with respect to a digital signal within a minute variation range. 
     3.2 Present Embodiment 1 
     Accordingly, the method of controlling the frequency of the oscillator  10  is proposed in the second embodiment of the present disclosure. The method of controlling the frequency of the oscillator  10  makes it possible to generate an oscillation signal whose oscillation frequency linearly and continuously varies with respect to a digital signal within a minute variation range even if the manufacturable minimum capacity value is limited by the semiconductor manufacturing process to be used. The following describes the present embodiment with reference to  FIGS. 5 to 9 . Each of  FIGS. 5 to 7  is an explanatory diagram for describing a method of controlling a frequency according to a second embodiment of the present disclosure. In addition,  FIG. 8  is an explanatory diagram describing an example of variations in a capacity value in the method of controlling the frequency according to the present embodiment.  FIG. 9  is an explanatory diagram describing an example of variations in an oscillation frequency in the method of controlling the frequency according to the present embodiment. It is to be noted that  FIGS. 5 to 7  each illustrate that the capacity bank  100  according to the present embodiment includes the plurality of capacitors  102  coupled in parallel, but the present embodiment is not limited thereto. The respective capacitors  102  illustrated in the diagrams may include the two capacitors  102  coupled in series. 
     As illustrated in the upper portion of  FIG. 5 , the oscillator  10  according to the present embodiment is provided with the fixed capacitor (electrostatic capacity value C 0 )  104 , a capacity bank  100 - 1 , and a capacity bank  100 - 2 . It is not possible to vary the electrostatic capacity value of the fixed capacitor (electrostatic capacity value C 0 )  104 . The capacity bank  100 - 1  includes the plurality of capacitors  102  (electrostatic capacity values Cu) (first small capacitors) coupled in parallel, and it is possible to switch the capacity value of the capacity bank  100 - 1  with the switch  202 . The capacity bank  100 - 2  includes the plurality of capacitors  102  (electrostatic capacity values C 1 , C 2 , . . . , Cn) coupled in parallel, and it is possible to switch the capacity value of the capacity bank  100 - 2  with the switch  202 . The plurality of capacitors  102  (electrostatic capacity values C 1 , C 2 , . . . , Cn) have electrostatic capacity values that are different from each other. Further, in the oscillator  10  according to the present embodiment, the fixed capacitor  104 , the capacity bank  100 - 1 , and the capacity bank  100 - 2  are coupled in parallel. It is to be noted that the fixed capacitor  104  is provided to the oscillator  10  to generate an oscillation signal having a frequency close to a desired frequency. 
     If described in more detail, the capacity bank  100 - 1  includes the N capacitors  102 , and it is assumed that the capacity Cu of each capacitor  102  has the minimum capacity value (Cu) manufacturable in the semiconductor manufacturing process to be used as with the capacity bank  900  according to the comparative example. Further, the capacity bank  100 - 2  includes the N capacitors  102 , and it is assumed that the capacity varies between the respective capacitors  102 , and the capacity of each capacitor  102  has a capacity value that is a value obtained by adding a minute capacity value (minute capacity) (ΔC) multiplied by an integer to the minimum capacity value (minimum unit capacity) (Cu) manufacturable in the semiconductor manufacturing process to be used. It is to be noted that it is assumed that the minute capacity value ΔC is smaller than the minimum capacity value Cu. Thus, a capacity value Cn of each capacitor  102  included in the capacity bank  100 - 2  is expressed as Expression (5) below. Specifically, the capacity value C 1  of the first capacitor (second small capacitor) is Cu+ΔC. The capacity value C 2  of the second capacitor (third small capacitor) is Cu+2ΔC.
 
[Expression 5]
 
 C   n   =Cu+nΔC   (5)
 
     It is to be noted that, although depending on the semiconductor manufacturing process as described above, for example, the minimum capacity value Cu is about several tens of fF and the minute capacity value ΔC is about several fF. In addition, the one oscillator  10  may include the two or more capacity banks  100  coupled in parallel. The respective capacity banks  100  are further allowed to have total capacity values that are different from each other. In this case, the above-described minimum capacity value and minute amount value may vary between the respective capacity banks  100 , and the above-described minimum capacity value is not limited to the minimum capacity value manufacturable in the semiconductor manufacturing process. More specifically, it is possible to provide the oscillator  10  according to the present embodiment with about the about 10 to 20 capacity banks  100 , and provide each capacity bank  100  with the about several capacitors  102 . 
     The method of controlling a frequency for the oscillator  10  to generate an oscillation signal whose oscillation frequency linearly and continuously varies with respect to a digital signal within a minute variation range in the oscillator  10  according to the present embodiment like this is as follows. 
     First, as illustrated in the upper portion of  FIG. 5 , all the N capacitors  102  of the capacity bank  100 - 1  of the oscillator  10  are turned on, and all the N capacitors  102  of the capacity bank  100 - 2  are turned off before a frequency adjustment. In this case, the total capacity value of the oscillator  10  is C 0 +NCu, and the oscillation frequency F 0  is thus expressed as Expression (6) below. 
     
       
         
           
             
               
                 
                   [ 
                   
                     Expression 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     6 
                   
                   ] 
                 
               
               
                 
                     
                 
               
             
             
               
                 
                   
                     F 
                     0 
                   
                   = 
                   
                     1 
                     
                       2 
                       ⁢ 
                       π 
                       ⁢ 
                       
                         
                           L 
                           ⁡ 
                           
                             ( 
                             
                               
                                 C 
                                 0 
                               
                               + 
                               
                                 NC 
                                 u 
                               
                             
                             ) 
                           
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   6 
                   ) 
                 
               
             
           
         
       
     
     Next, as illustrated in the upper portion of  FIG. 6 , only the first capacitor  102  (C 1 =Cu+ΔC) of the capacity bank  100 - 2  of the oscillator  10  is turned on during a frequency adjustment. In this case, the total capacity value of the oscillator  10  is C 0 +NCu+Cu+ΔC, and the oscillation frequency F 1 ′ is thus expressed as Expression (7) below. 
     
       
         
           
             
               
                 
                   [ 
                   
                     Expression 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     7 
                   
                   ] 
                 
               
               
                 
                     
                 
               
             
             
               
                 
                   
                     F 
                     1 
                   
                   = 
                   
                     1 
                     
                       2 
                       ⁢ 
                       π 
                       ⁢ 
                       
                         
                           L 
                           ⁡ 
                           
                             ( 
                             
                               
                                 C 
                                 0 
                               
                               + 
                               
                                 
                                   ( 
                                   
                                     N 
                                     + 
                                     1 
                                   
                                   ) 
                                 
                                 ⁢ 
                                 
                                   C 
                                   u 
                                 
                               
                               + 
                               
                                 Δ 
                                 ⁢ 
                                 
                                     
                                 
                                 ⁢ 
                                 C 
                               
                             
                             ) 
                           
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   7 
                   ) 
                 
               
             
           
         
       
     
     Further, as illustrated in the upper portion of  FIG. 7 , the one capacitor  102  of the capacity bank  100 - 1  of the oscillator  10  is turned off after a frequency adjustment. The total capacity value of the oscillator  10  is C 0 +(N−1)Cu+(Cu+ΔC), that is, C 0 +NCu+ΔC, and the oscillation frequency F 1  is thus expressed as Expression (8) below. 
     
       
         
           
             
               
                 
                   [ 
                   
                     Expression 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     8 
                   
                   ] 
                 
               
               
                 
                     
                 
               
             
             
               
                 
                   
                     F 
                     1 
                   
                   = 
                   
                     1 
                     
                       2 
                       ⁢ 
                       π 
                       ⁢ 
                       
                         
                           L 
                           ⁡ 
                           
                             ( 
                             
                               
                                 C 
                                 0 
                               
                               + 
                               
                                 NC 
                                 u 
                               
                               + 
                               
                                 Δ 
                                 ⁢ 
                                 
                                     
                                 
                                 ⁢ 
                                 C 
                               
                             
                             ) 
                           
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   8 
                   ) 
                 
               
             
           
         
       
     
     Next, a comparison between the total capacity values and the oscillation frequencies of the oscillator  10  according to the present embodiment before the above-described frequency adjustment and after the above-described frequency adjustment is described with reference to  FIGS. 8 and 9 .  FIGS. 8 and 9  illustrate digital signals before a frequency adjustment and after a frequency adjustment as D and D+1. 
     As illustrated in  FIG. 8 , as the total capacity values of the oscillator  10  according to the present embodiment before a frequency adjustment and after a frequency adjustment, C 0 +NCu varies to C 0 +NCu+ΔC. This demonstrates that a variation is made within the variation range (step range) of the minute capacity value ΔC. In contrast, in the comparative example described earlier, as the total capacity values before a frequency adjustment and after a frequency adjustment, C 0 +NCu varies to C 0 +(N+1)Cu. A variation is thus made within the variation range of the minimum capacity value Cu. 
     In addition, the variations in the total capacity value as described above bring about the variations in the oscillation frequency as illustrated in  FIG. 9 . If described in detail, as illustrated in  FIG. 9 , as the oscillation frequencies of the oscillator  10  according to the present embodiment before a frequency adjustment and after a frequency adjustment, F 0  varies to F 1 . If a difference between the oscillation frequency F 0  and the oscillation frequency F 1  is represented as ΔF, the minute frequency variation range ΔF corresponds to the minute capacity value ΔC. In contrast, in the comparative example described earlier, as the oscillation frequencies before a frequency adjustment and after a frequency adjustment, F 0  varies to F 0 -Fu, and the oscillation frequency varies within a variation range Fu corresponding to the minimum capacity value Cu. It is to be noted that the minute capacity value ΔC is smaller than the minimum capacity value Cu, and the minute frequency variation range ΔF according to the present embodiment is thus smaller than the variation range Fu according to the comparative example. 
     Thus, even in a case where a semiconductor manufacturing process to be used limits the manufacturable minimum capacity value, the method of controlling a frequency according to the present embodiment allows for the presence of the minute frequency variation range ΔF. As a result, according to the present embodiment, it is possible to generate an oscillation signal whose oscillation frequency linearly and continuously varies with respect to a digital signal within a minute variation range. 
     It is to be noted that the method of controlling a frequency is not limited in the present embodiment to  FIGS. 5 to 7 , but, for example, the oscillator  10  may be controlled to directly enter the state of  FIG. 7  from  FIG. 5 . Alternatively, in the present embodiment, for example, after the one capacitor  102  of the capacity bank  100 - 1  of the oscillator  10  is turned off, the first capacitor  102  (C 1 =Cu+ΔC) of the capacity bank  100 - 2  of the oscillator  10  may be turned on. 
     3.3 Present Embodiment 2 
     Further, the above-described method of controlling a frequency makes it possible to generate an oscillation signal having an oscillation frequency that varies within the minute frequency variation range corresponding to the minute capacity value ΔC. The method of controlling a frequency for the oscillator  10  in such a case is described with reference to  FIGS. 10 to 12 .  FIG. 10  is an explanatory diagram for describing the method of controlling the frequency according to the present embodiment. In addition,  FIG. 11  is an explanatory diagram describing an example of variations in a capacity value in the method of controlling the frequency according to the present embodiment.  FIG. 12  is an explanatory diagram describing an example of variations in an oscillation frequency in the method of controlling the frequency according to the present embodiment. 
     If described in detail, in the present embodiment, the oscillator  10  is then brought from the above-described state of  FIG. 7  into the state illustrated in the third row of  FIG. 10 . If described in detail, in the present embodiment, the first capacitor  102  (C 1 =Cu+ΔC) of the capacity bank  100 - 2  of the oscillator  10  is turned off, and the second capacitor  102  (C 2 =Cu+2ΔC) of the capacity bank  100 - 2  is turned on. In this case, the total capacity value of the oscillator  10  is C 0 +NCu+2ΔC, and the oscillation frequency F 2  is thus expressed as Expression (9) below. 
     
       
         
           
             
               
                 
                   [ 
                   
                     Expression 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     9 
                   
                   ] 
                 
               
               
                 
                     
                 
               
             
             
               
                 
                   
                     F 
                     2 
                   
                   = 
                   
                     1 
                     
                       2 
                       ⁢ 
                       π 
                       ⁢ 
                       
                         
                           L 
                           ⁡ 
                           
                             ( 
                             
                               
                                 C 
                                 0 
                               
                               + 
                               
                                 NC 
                                 u 
                               
                               + 
                               
                                 2 
                                 ⁢ 
                                 Δ 
                                 ⁢ 
                                 
                                     
                                 
                                 ⁢ 
                                 C 
                               
                             
                             ) 
                           
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   9 
                   ) 
                 
               
             
           
         
       
     
     Further, in the present embodiment, the oscillator  10  is then brought from the above-described state illustrated in the third row of  FIG. 10  into the state illustrated in the fourth row of  FIG. 10 . If described in detail, in the present embodiment, the first capacitor  102  (C 1 =Cu+ΔC) of the capacity bank  100 - 2  of the oscillator  10  is turned on. In this case, the total capacity value of the oscillator  10  is C 0 +NCu+3ΔC, and the oscillation frequency F 3  is thus expressed as Expression (10) below. 
     
       
         
           
             
               
                 
                   [ 
                   
                     Expression 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     10 
                   
                   ] 
                 
               
               
                 
                     
                 
               
             
             
               
                 
                   
                     F 
                     3 
                   
                   = 
                   
                     1 
                     
                       2 
                       ⁢ 
                       π 
                       ⁢ 
                       
                         
                           L 
                           ⁡ 
                           
                             ( 
                             
                               
                                 C 
                                 0 
                               
                               + 
                               
                                 NC 
                                 u 
                               
                               + 
                               
                                 3 
                                 ⁢ 
                                 Δ 
                                 ⁢ 
                                 
                                     
                                 
                                 ⁢ 
                                 C 
                               
                             
                             ) 
                           
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   10 
                   ) 
                 
               
             
           
         
       
     
     Next, a comparison between the total capacity values and the oscillation frequencies of the oscillator  10  according to the present embodiment before the above-described frequency adjustment and after the above-described frequency adjustment is described with reference to  FIGS. 11 and 12 .  FIGS. 11 and 12  each illustrate variations in a digital signal in the above-described method of controlling a frequency as D, D+1, D+2, and D+3. 
     As illustrated in  FIG. 11 , as the oscillator  10  according to the present embodiment varies a digital signal like D, D+1, D+2, and D+3, the total capacity value varies within the variation range (step range) of the minute capacity value ΔC. 
     In addition, the variations in the total capacity value as described above bring about the variations in the oscillation frequency as illustrated in  FIG. 12 . If described in detail, as illustrated in  FIG. 12 , as the oscillator  10  according to the present embodiment varies a digital signal like D, D+1, D+2, and D+3, the oscillation frequency varies like F 0 , F 1 , F 2 , and F 3 . Thus, in the present embodiment, appropriately selecting the minute capacity value ΔC of the capacitor  102  included in the capacity bank  100  and the total capacity value of each capacity bank  100  makes it possible to linearly and continuously vary the oscillation frequency with respect to an inputted digital signal within a minute variation range. 
     For example, it is to be noted that the oscillator  10  may include the plurality of capacity banks  100  having total capacity values different from each other to linearly and continuously vary an oscillation frequency with respect to an inputted digital signal within a minute variation range in the present embodiment. Further, it is preferable to set the plurality of capacity banks  100  to cause the plurality of capacity banks  100  to have total capacity values (Cn) that sequentially and non-linearly increase step by step, thereby more linearly increasing the oscillation frequency F of the oscillation frequency in Expression (11) below. In addition, the plurality of capacitors  102  included in each capacity bank  100  preferably has electrostatic capacity values that are different from each other. Specifically, the plurality of capacitors  102  included in each capacity bank  100  each preferably has the capacity value of the value obtained by adding the minute capacity value (ΔC) multiplied by an integer to a predetermined capacity value. This causes the plurality of capacitors  102  to have electrostatic capacity values that sequentially and linearly increase step by step in the same capacity bank  100 . 
     
       
         
           
             
               
                 
                   [ 
                   
                     Expression 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     11 
                   
                   ] 
                 
               
               
                 
                     
                 
               
             
             
               
                 
                   F 
                   = 
                   
                     1 
                     
                       2 
                       ⁢ 
                       π 
                       ⁢ 
                       
                         
                           LC 
                           n 
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   11 
                   ) 
                 
               
             
           
         
       
     
     It is to be noted that it is not limitative that the plurality of capacitors  102  has electrostatic capacity values which sequentially and linearly increase step by step in the same capacity bank  100  in the present embodiment as described above, but the plurality of capacitors  102  may have the same electrostatic capacity value. However, in a case where the plurality of capacitors  102  has electrostatic capacity values that sequentially and linearly increase step by step, it is possible to vary an oscillation frequency over a broadband within a minute variation range while suppressing increase in the area occupied by the plurality of capacitors  102  on the semiconductor substrate. 
     It is to be noted that the total capacity value of the oscillator  10  is switched by using a binary code (binary system) in the above-described method of controlling a frequency, but the binary code is not limitative in the present embodiment. A thermometer code may be used that includes a string of continuous “1&#39;s” and a string of continuous “0&#39;s”. In addition, in the present embodiment, a binary code may be switched to a thermometer code or vice versa for use in accordance with the total capacity value of the oscillator  10 . 
     4. Third Embodiment 
     It has been described that the oscillator  10  is provided in the above-described second embodiment with the capacity bank  100  in which the plurality of capacitors  102  is coupled in parallel. However, the plurality of capacitors  102  in the capacity bank  100  according to the embodiment of the present disclosure does not have to be coupled in parallel, but may also be coupled in series. Accordingly, as a third embodiment of the present disclosure, a capacity bank  100   a  in which the plurality of capacitors  102  is coupled in series, and a method of controlling an oscillation frequency in the oscillator  10  including the capacity bank  100   a  are described with reference to  FIGS. 13 and 14 .  FIG. 13  is a circuit diagram schematically illustrating a configuration of the capacity bank  100   a  according to the third embodiment of the present disclosure, and  FIG. 14  is an explanatory diagram for describing the method of controlling a frequency according to the present embodiment. 
     4.1 Configuration of Capacity Bank  100   a    
     First, a configuration of the capacity bank  100   a  according to the present embodiment is described with reference to  FIG. 13 . As illustrated in  FIG. 13 , in the capacity bank  100   a  according to the present embodiment, the plurality of capacitors  102  is coupled in series. In addition, it is assumed that the capacity value Cu of each capacitor  102  corresponds to the minimum capacity value manufacturable in the semiconductor manufacturing process to be used. Further, these capacitors  102  are coupled to each other via the switch  202  including a MOS transistor. Additionally, the capacity bank  100   a  includes a coupling line  204  extending along the line of the plurality of these capacitors  102  coupled in series. The coupling line  204  and each node  206  are coupled via the switch  202 . Each node  206  is located between the two capacitors  102 . 
     4.2 Method of Controlling Frequency 
     Next, the method of controlling an oscillation frequency in the oscillator  10  including the capacity bank  100   a  according to the present embodiment as illustrated in  FIG. 13  is described with reference to  FIG. 14 . It is assumed that the oscillator  10  according to the present embodiment is provided with a fixed capacitor (electrostatic capacity value C 0 )  104  whose electrostatic capacity value is not variable, and the capacity bank  100   a  including the plurality of capacitors  102  (electrostatic capacity values Cu) coupled in series as illustrated in the lower portion of  FIG. 14 . It is possible to switch the capacity value of the capacity bank  100   a  with the switch  202 . Further, in the oscillator  10  according to the present embodiment, the fixed capacitor  104  and the capacity bank  100   a  are coupled in parallel. 
     The method of controlling a frequency for the oscillator  10  to generate an oscillation signal whose oscillation frequency linearly and continuously varies with respect to a digital signal within a minute variation range in the oscillator  10  according to the present embodiment like this is as follows. 
     First, as illustrated in the left portion of  FIG. 14 , before a frequency adjustment, the switches  202  are turned on to couple all the M capacitors  102  of the capacity bank  100   a  in the oscillator  10  in series. In this case, the oscillator  10  uses the fixed capacitor  104  and the M capacitors  102  coupled in series to generate an oscillation signal. The total capacity value of the oscillator  10  is expressed as Expression (12) below. It is to be noted that it is assumed that ΔC of the following expression is equal to Cu/M. 
     
       
         
           
             
               
                 
                   [ 
                   
                     Expression 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     12 
                   
                   ] 
                 
               
               
                 
                     
                 
               
             
             
               
                 
                   
                     
                       
                         C 
                         = 
                           
                         ⁢ 
                         
                           
                             C 
                             0 
                           
                           + 
                           
                             Cu 
                             M 
                           
                         
                       
                     
                   
                   
                     
                       
                         = 
                           
                         ⁢ 
                         
                           
                             C 
                             0 
                           
                           + 
                           
                             Δ 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             C 
                           
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   12 
                   ) 
                 
               
             
           
         
       
     
     Next, as illustrated in the right portion of  FIG. 14 , after a frequency adjustment, the switches  202  are turned off, and half the M capacitors  102  of the capacity bank  100   a  are uncoupled from the series coupling. In this case, the oscillator  10  uses the fixed capacitor  104  and the M/2 capacitors  102  coupled in series to generate an oscillation signal. The total capacity value of the oscillator  10  is expressed as Expression (13) below. 
     
       
         
           
             
               
                 
                   [ 
                   
                     Expression 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     13 
                   
                   ] 
                 
               
               
                 
                     
                 
               
             
             
               
                 
                   
                     
                       
                         C 
                         = 
                           
                         ⁢ 
                         
                           
                             C 
                             0 
                           
                           + 
                           
                             Cu 
                             
                               M 
                               / 
                               2 
                             
                           
                         
                       
                     
                   
                   
                     
                       
                         = 
                           
                         ⁢ 
                         
                           
                             C 
                             0 
                           
                           + 
                           
                             2 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             Δ 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             C 
                           
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   13 
                   ) 
                 
               
             
           
         
       
     
     That is, in the present embodiment, even in a case where the capacity bank  100   a  including the plurality of capacitors  102  coupled in series is used, switching the number of capacitors  102  coupled in series makes it possible to vary the total capacity value of the oscillator  10  within a minute variation range. 
     However, in the present embodiment, in a case where there is a great difference between the minimum capacity value Cu and the variation range of a desired total capacity value, the number M of the capacitor  102  coupled in series in the capacity bank  100   a  increases, which also increases the number of switches  202  that control the number M of capacitors  102  coupled in series. In such a case, this may lead to increase in the area of the semiconductor chip in which the oscillator  10  is provided, and increase the manufacturing cost. Further, it is possible to set nothing but an integer as the number M of capacitors  102  coupled in series, and this thus reduces the degree of freedom for variability in the total capacity value of the oscillator  10 . Thus, from such a viewpoint, it is preferable to use the capacity bank  100  in which the plurality of capacitors  102  is coupled in parallel like the above-described second embodiment. 
     5. Fourth Embodiment 
     In the above-described embodiment of the present disclosure, appropriately selecting the minute capacity value ΔC of the capacitor  102  included in the capacity bank  100  and the total capacity value of each capacity bank  100  makes it possible to linearly and continuously vary the oscillation frequency with respect to an inputted digital signal within a minute variation range. Further, the capacity bank  100  like this is provided on the semiconductor substrate. As a fourth embodiment of the present disclosure, the capacity bank  100  provided on the semiconductor substrate is then described with reference to  FIGS. 15 and 16 .  FIG. 15  is a layout diagram schematically illustrating a configuration example of the capacitor  102  according to the fourth embodiment of the present disclosure, and  FIG. 16  is a layout diagram schematically illustrating a configuration example of the oscillator  10  according to the present embodiment. It is to be noted that these layout diagrams each omit a wiring line, an element, or the like, but illustrate only a main portion according to the present embodiment for the sake of simplicity. 
     If described in detail, as illustrated in  FIG. 15 , the one capacity bank  100  according to the present embodiment includes the plurality of capacitors  102  and the plurality of switches  202  each including a MOS transistor. The plurality of capacitors  102  and the plurality of switches  202  are arranged on a semiconductor substrate  800 . The electrostatic capacity values of the plurality of capacitors  102  sequentially and linearly vary step by step in the vertical direction of  FIG. 15 . Thus, the layout areas occupied by the respective capacitors  102  also sequentially and linearly vary step by step. In addition, in the example illustrated in  FIG. 15 , the seven rows of capacitor pairs  106  are provided in the vertical direction of the diagram. The two capacitors  102  of each capacitor pair  106  having the same electrostatic capacity value are disposed across the switch  202  to have line symmetry. Further, the two capacitors  102  of each capacitor pair  106  are coupled to the switch  202  by coupling lines (wiring lines)  204 . 
     Further, in the present embodiment, the plurality of capacity banks  100  each having the configuration as illustrated in  FIG. 15  is formed on the semiconductor substrate  800 . For example, as illustrated in  FIG. 16 , the oscillator  10  according to the present embodiment has a capacity bank formation region  804  on the semiconductor substrate  800 . In the capacity bank formation region  804 , the plurality of capacity banks  100  is formed. Further, the oscillator  10  according to the present embodiment may have an inductor formation region  802 , a negative resistance circuit formation region  806 , a frequency divider formation region  808 , and the like on the semiconductor substrate  800 . In the inductor formation region  802 , the inductor  300  is formed. In the negative resistance circuit formation region  806 , the negative resistance circuit  40  and the like are formed. In the frequency divider formation region  808 , the frequency divider  710  described below is formed. 
     If described in more detail, the plurality of capacity banks  100  may have total capacity values that are different from each other. Specifically, the plurality of capacity banks  100  has total capacity values that sequentially and non-linearly increase step by step. Thus, the layout areas occupied by the respective capacity banks  100  also sequentially and non-linearly vary step by step. In addition, the capacity bank formation region  804  may be provided with the fixed capacitor  104 , and may be provided with a dummy capacitor (not illustrated) unrelated to the generation of an oscillation signal to more accurately form each capacitor  102 . 
     Further, the inductor  300  formed in the inductor formation region  802  may be an inductor element and may be formed by a wiring line. The inductor  300  is not limited in particular. 
     It is to be noted that each of the layout diagrams illustrated in  FIGS. 15 and 16  is merely an example. The capacity bank  100  and oscillator  10  according to the present embodiment do not have to have the forms illustrated in  FIGS. 15 and 16 . 
     6. Fifth Embodiment 
     6.1 Configuration of Radio Receiver  60   
     The oscillator  10  according to the embodiment of the present disclosure described above is applicable, for example, to the radio receiver  60 . As a fifth embodiment of the present disclosure, an example of the radio receiver  60  to which the oscillator  10  according to the present embodiment is applied is then described with reference to  FIG. 17 .  FIG. 17  is a circuit diagram schematically illustrating a configuration of the radio receiver  60  according to the fifth embodiment of the present disclosure. 
     The oscillation frequency of the oscillator  10  is controlled with a direct-current input voltage in the radio receiver  60  according to the present embodiment to make it possible to select a radio broadcasting station. The direct-current input voltage is obtained by mechanically varying the resistance value of the variable resistor  80  with a manual operation of a user. 
     The radio receiver  60  illustrated in  FIG. 17  converts the above-described input voltage that is an analog signal into a digital signal, and uses the digital signal resulting from the conversion to switch the total capacity value of the oscillator  10 . This makes it possible to obtain an oscillation signal having a desired frequency from the oscillator  10 . This allows the radio receiver  60  according to the present embodiment to supply the oscillation signal obtained in this way from a local oscillation signal generator  70  including the above-described oscillator  10  to the mixer circuit (frequency conversion circuit)  704  as a local signal to convert a radio signal into an IF signal. 
     If described in detail, as illustrated in  FIG. 17 , an inputted radio signal of the radio receiver  60  is amplified by the RF amplifier  702  and converted by the two mixer circuits  704  into two (I signal and Q signal) IF signals. Local oscillation signals whose phases are orthogonal to each other are inputted to the two mixer circuits  704 . Further, the band-pass filter  718  removes image interfering waves from the converted IF signals, and the demodulator (demodulation circuit)  722  then performs AM/FM demodulation to offer a sound signal. 
     It is to be noted that the radio receiver  60  according to the present embodiment may adopt a superheterodyne method in which a signal is temporarily converted into an IF signal as described above, or may adopt a direct conversion method in which a signal is directly converted into a baseband signal. It is to be noted that each block such as the mixer circuits  704  or the demodulator  722  of the above-described radio receiver  60  is preferably provided on one semiconductor chip along with the oscillator  10  or the local oscillation signal generator  70  including the oscillator  10 . 
     6.2 Configuration of Local Oscillation Signal Generator  70   
     Incidentally, in a case where the one radio receiver  60  receives an FM/AM radio broadcast, the frequency of a local oscillation signal (local signal) used for the reception depends on a broadcast band. Specifically, the frequency of a local oscillation signal is 76 MHz to 108 MHz, for example, for receiving an FM broadcast in a case where the frequency of an IF signal is 150 kHz. In addition, the frequency of a local oscillation signal is 575.6 kHz to 1766.1 kHz, for example, for receiving an AM broadcast in a case where the frequency of an IF signal is 55 kHz. 
     Accordingly, the radio receiver  60  according to the present embodiment is provided with the local oscillation signal generator  70  to support both AM/FM radio broadcasts. The local oscillation signal generator  70  makes it possible to switch the number of frequencies divided by the frequency divider  710  between FM reception and AM reception when the frequency of an oscillation signal generated by the oscillator  10  is divided. 
     The local oscillation signal generator  70  according to the present embodiment that makes it possible to switch the number of divided frequencies as described above is then described with reference to  FIGS. 18 to 21 .  FIG. 18  is a circuit diagram schematically illustrating a configuration of the local oscillation signal generator  70  according to the present embodiment.  FIG. 19  is an explanatory diagram for describing frequency division by the local oscillation signal generator  70  according to the present embodiment. Further, each of  FIGS. 20 and 21  is a graph illustrating an example of variations in the frequency of the local oscillation signal generator  70  according to the present embodiment. 
     Incidentally, the frequency of a local oscillation signal for FM reception ranges from 76 MHz to 108 MHz, offering a frequency variation rate of 1.42 times or more. In addition, the frequency of a local oscillation signal for AM reception ranges from 575.6 kHz to 1766.1 kHz, offering a frequency variation rate of about 3.068 times. The local oscillation frequency is thus requested of frequency variation rates that are greatly different between FM reception and AM reception. 
     However, in a case where a local oscillation frequency of 575.6 kHz to 1766.1 kHz for AM reception is equally divided into three with the same width, each range has a frequency variation rate of about 1.453 times. This is substantially the same as the frequency variation rate for FM reception. Accordingly, in the present embodiment, the band of the AM signal is equally divided into three by securing about 76 MHz to 110 MHz as the range of the local oscillation frequency for FM reception, and further selecting the number of divided frequencies. This makes it possible to support both AM/FM broadcasts even in a case where the one oscillator  10  is used. 
     If described in detail, as illustrated in  FIG. 18 , the local oscillation signal generator  70  that generates a local oscillation signal to be inputted to the mixer circuit  704  includes a frequency divider  710   a  that divides the frequency of an oscillation signal from the oscillator  10  into four in addition to the above-described oscillator  10 . Further, the local oscillation signal generator  70  includes a frequency divider  710   b  and a frequency divider  710   c . The frequency divider  710   b  divides the frequency of a signal from the frequency divider  710   a  into N. The frequency divider  710   c  divides the frequency of a signal from the frequency divider  710   a  or the frequency divider  710   b  into four. Additionally, the local oscillation signal generator  70  includes a switch  208  and a controller (control circuit section)  210 . The switch  208  switches a path of a signal from the frequency divider  710   a . The controller (control circuit section)  210  controls the frequency divider  710   b  and the switch  208 . Further, it is assumed that the frequency divider  710   b  is able to switch the number of divided frequencies between three types: 131 divided frequencies; 90 divided frequencies; and 62 divided frequencies. It is to be noted that the above-described local oscillation signal generator  70  is preferably provided on one semiconductor chip along with each block such as the mixer circuit  704  or the demodulator  722  of the radio receiver  60 . 
     Next, the operation of the local oscillation signal generator  70  illustrated in  FIG. 18  is described with reference to  FIG. 19 . First, an oscillation signal generated by the oscillator  10  has a frequency of 1206.4 to 1752 MHz, and has a path led by the switch  208  through the frequency divider  710   a  and the frequency divider  710   c  for FM reception as illustrated in the uppermost row of  FIG. 19 . Thus, the frequency of the above-described oscillation signal is divided into four by each of the frequency dividers  710   a  and  710   c . That is, the frequency of the above-described oscillation signal is divided into sixteen. As a result, the local oscillation signal generator  70  is able to obtain a local oscillation signal having a frequency within a range of 76 MHz to 108 MHz. 
     Further, the switch  208  is switched for AM reception to lead an oscillation signal from the frequency divider  710   a  to the frequency divider  710   b . The AM band is divided into three bands MW 1  (575. KHz to 835 KHz), MW 2  (837.8 kHz to 1216.7 KHz), and MW 3  (1216.1 KHz to 1766.1 KHz) for AM reception, and the number of frequencies divided by the frequency divider  710   b  is switched in accordance with the bands. 
     First, for receiving an AM signal having the frequency of MW 1  (575. KHz to 835 KHz), as illustrated in the second row of  FIG. 19 , the frequency of an oscillation signal (1206.4 to 1752 MHz) generated by the oscillator  10  is divided into four by the frequency divider  710   a , and further divided into 131 by the frequency divider  710   b . Further, the frequency of a signal whose frequency has been divided by the frequency divider  710   b  is divided into four by the frequency divider  710   c . That is, the frequency of the above-described oscillation signal is divided into 2096. This allows the local oscillation signal generator  70  to obtain a local oscillation signal having the frequency within a range of 575. KHz to 835 KHz. 
     Next, for receiving an AM signal having the frequency of MW 2  (837.8 kHz to 1216.7 KHz), as illustrated in the third row of  FIG. 19 , the frequency of an oscillation signal (1206.4 to 1752 MHz) generated by the oscillator  10  is divided into four by the frequency divider  710   a , and further divided into 90 by the frequency divider  710   b . Further, the frequency of a signal whose frequency has been divided by the frequency divider  710   b  is divided into four by the frequency divider  710   c . That is, the frequency of the above-described oscillation signal is divided into 1440. This allows the local oscillation signal generator  70  to obtain a local oscillation signal having the frequency within a range of 837.8 kHz to 1216.7 KHz. 
     Further, for receiving an AM signal having the frequency of MW 3  (1216.1 kHz to 1766.1 KHz), as illustrated in the fourth row of  FIG. 19 , the frequency of an oscillation signal (1206.4 to 1752 MHz) generated by the oscillator  10  is divided into four by the frequency divider  710   a , and further divided into 62 by the frequency divider  710   b . Further, the frequency of a signal whose frequency has been divided by the frequency divider  710   b  is divided into four by the frequency divider  710   c . That is, the frequency of the above-described oscillation signal is divided into 992. This allows the local oscillation signal generator  70  to obtain a local oscillation signal having the frequency within a range of 1216.1 KHz to 1766.1 KHz. 
     In this way, in the present embodiment, when the frequency of an oscillation signal generated by the oscillator  10  is divided, the number of divided frequencies is switched between FM reception and AM reception. Further, the number of divided frequencies is switched for each of bands obtained by equally dividing the band of an AM signal into three. As a result, in the present embodiment, even the radio receiver  60  provided with the local oscillation signal generator  70  including the one oscillator  10  is able to support both AM/FM broadcasts. 
     As can be seen from  FIG. 20 , the local oscillation signal generator  70  like this makes it possible to obtain a local oscillation signal having a frequency that linearly and continuously varies with respect to an inputted digital signal within a minute variation range for receiving an FM signal. In addition, as can be seen from  FIG. 21 , the local oscillation signal generator  70  also makes it possible to obtain, for each band, a local oscillation signal having a frequency that linearly and continuously varies with respect to an inputted digital signal within a minute variation range for receiving an AM signal. 
     It is to be noted that the number of frequencies divided by each frequency divider  710  or the frequency of an oscillation signal from the oscillator  10  is not limited to the above in the present embodiment, but is selectable as appropriate. Further, the configuration of the local oscillation signal generator  70  is not also limited to the configuration illustrated in  FIG. 18  in the present embodiment, but is variable as appropriate. 
     7. Conclusion 
     As described above, according to the embodiment of the present disclosure, it is possible to obtain the oscillator  10  that is able to generate an oscillation signal whose oscillation frequency linearly and continuously varies within a minute variation range in accordance with a manual operation of a user. The oscillator  10  according to the present embodiment thus makes it is possible to secure operability similar to that of the radio receiver  60   b  including the variable condenser  602  according to the comparative example. Additionally, according to the present embodiment, the variable condenser  602  or the PLL synthesizer  716  is not used, thereby making it possible to suppress the manufacturing cost and the power consumption. 
     In addition, it has been described that the above-described oscillator  10  according to the embodiment of the present disclosure is applied to the radio receiver  60 . The oscillator  10  according to the embodiment of the present disclosure is not, however, limited to the application to the radio receiver  60 , but is applicable to various wireless communication apparatuses each of which is requested to suppress the power consumption and be smaller, and mounted on a smartwatch, a wearable terminal, or the like. 
     8. Supplement 
     A preferred embodiment(s) of the present disclosure has/have been described above in detail with reference to the accompanying drawings, but the technical scope of the present disclosure is not limited to such an embodiment(s). It is apparent that a person having ordinary skill in the art of the present disclosure may arrive at various alterations and modifications within the scope of the technical idea described in the appended claims, and it is understood that such alterations and modifications naturally fall within the technical scope of the present disclosure. 
     In addition, the effects described herein are merely illustrative and exemplary, and not limitative. That is, the technology according to the present disclosure may exert other effects that are apparent to those skilled in the art from the description herein, in addition to the above-described effects or in place of the above-described effects. 
     It is to be noted that the following configurations also fall within the technical scope of the present disclosure. 
     (1) 
     A semiconductor device including: 
     an oscillation circuit including a plurality of capacitors provided on a semiconductor substrate; 
     a conversion circuit that converts an analog signal into a digital signal; and 
     a switch circuit that switches the capacitors on the basis of the digital signal, 
     an oscillation frequency linearly varying with respect to a variation in the analog signal. 
     (2) 
     The semiconductor device according to (1), in which the switch circuit switches the plurality of capacitors on the basis of the digital signal, the plurality of capacitors having respective capacity values selected to linearly vary the oscillation frequency. 
     (3) 
     The semiconductor device according to (1) or (2), further including a variable resistor that generates the analog signal. 
     (4) 
     The semiconductor device according to (2), in which the plurality of capacitors is provided to sequentially and non-linearly increase the capacity values. 
     (5) 
     The semiconductor device according to (4), in which the plurality of capacitors is coupled in parallel. 
     (6) 
     The semiconductor device according to (1), in which each of the capacitors includes a plurality of small capacitors. 
     (7) 
     The semiconductor device according to (6), in which the plurality of small capacitors is switched by the switch circuit. 
     (8) 
     The semiconductor device according to (7), in which the plurality of small capacitors is coupled in series. 
     (9) 
     The semiconductor device according to (8), in which each of the small capacitors has a capacity value of minimum unit capacity determined in a semiconductor manufacturing process. 
     (10) 
     The semiconductor device according to (7), in which the plurality of small capacitors is coupled in parallel. 
     (11) 
     The semiconductor device according to (10), in which 
     the plurality of small capacitors includes a first small capacitor and a second small capacitor, the first small capacitor having a capacity value of minimum unit capacity, the second small capacitor having a capacity value obtained by adding minute capacity to the minimum unit capacity, and 
     the minute capacity is smaller than the minimum unit capacity. 
     (12) 
     The semiconductor device according to (11), in which the plurality of small capacitors of each of the capacitors further includes a third small capacitor having a capacity value obtained by adding a capacity value twice as large as the capacity value of the minute capacity to the minimum unit capacity. 
     (13) 
     The semiconductor device according to (11) or (12), in which the minimum unit capacity or the minute capacity varies between the capacitors. 
     (14) 
     The semiconductor device according to any one of (1) to (13), in which the oscillation circuit includes an inductor. 
     (15) 
     The semiconductor device according to any one of (1) to (14), further including: 
     a frequency conversion circuit that uses an oscillation signal from the oscillation circuit as a local signal; and 
     a demodulation circuit that performs demodulation into a signal from the frequency conversion circuit. 
     (16) 
     The semiconductor device according to (15), further including: 
     a frequency divider that divides a frequency of the oscillation signal from the oscillation circuit; and 
     a control circuit section that switches a number of frequencies divided by the frequency divider. 
     (17) 
     The semiconductor device according to (16), in which the control circuit section switches the number of divided frequencies in accordance with a frequency of an inputted signal. 
     (18) 
     A wireless communication apparatus including 
     a semiconductor device including
         an oscillation circuit including a plurality of capacitors provided on a semiconductor substrate,   a conversion circuit that converts an analog signal into a digital signal, and   a switch circuit that switches the capacitors on the basis of the digital signal,   an oscillation frequency linearly varying with respect to a variation in the analog signal.       

     REFERENCE SIGNS LIST 
     
         
           10 ,  10   a ,  90   a ,  90   b  oscillator 
           20  LC oscillator 
           30  AD converter 
           40 ,  40   a  negative resistance circuit 
           50  constant current source 
           60 ,  60   a ,  60   b  radio receiver 
           70  local oscillation signal generator 
           80  variable resistor 
           100 ,  100   a ,  900  capacity bank 
           102  capacitor 
           104  fixed capacitor 
           106  capacitor pair 
           202 ,  208  switch 
           204  coupling line 
           206  node 
           210  controller 
           300  inductor 
           400  n-type MOS transistor 
           402  p-type MOS transistor 
           502  variable capacitor 
           602  variable condenser 
           702  RF amplifier 
           704  mixer circuit 
           706  crystal oscillator 
           708  phase comparator 
           710 ,  710   a ,  710   b ,  710   c  frequency divider 
           712  charge pump 
           714  loop filter 
           716  PLL synthesizer 
           718  filter 
           720  IF amplifier 
           722  demodulator 
           800  semiconductor substrate 
           802  inductor formation region 
           804  capacity bank formation region 
           806  negative resistance circuit formation region 
           808  frequency divider formation region