Abstract:
The functionality and performance of a voltage-controlled oscillator are improved by enabling continuously adjusting an inductor by a control signal. The oscillator has a variable inductor unit with variable inductance; a variable capacitance device connected to the variable inductor unit; an output unit that oscillates at an oscillation frequency determined by the inductance of the variable inductor unit and the capacitance of the variable capacitance device; and a control signal generator for modulating the oscillation frequency. The variable inductor unit has a first inductor; a current signal generator for detecting an electric signal denoting current flowing to the first inductor, and generating a current signal based on the electric signal; and a second inductor that receives the current signal. The first inductor and second inductor are disposed to a predetermined magnetically coupled position, and the variable inductor unit sets the inductance of the first inductor desirably.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of Technology  
         [0002]     The present invention relates to technology for a voltage-controlled oscillator that is used in wireless communication devices having a portable terminal, and to a PLL circuit that uses the voltage-controlled oscillator.  
         [0003]     2. Description of Related Art  
         [0004]     A voltage-controlled oscillator (VCO) is used in portable wireless devices such as cell phones for frequency conversion operations converting transmission signals to high frequency signals for transmission and converting reception signals to low frequency signals for demodulation. These applications require a wide oscillation frequency range, the ability to freely adjust the oscillation frequency, and a high carrier-to-noise (C/N) ratio at the oscillation frequency.  
         [0005]     Semiconductor devices used in the communications industry today often have an internal voltage-controlled oscillator. Spiral inductors are generally used when the inductor is also built in to the IC device. A wide oscillation frequency band is achieved in the built-in voltage-controlled oscillator by switching between spiral inductors.  
         [0006]     An example of this type of conventional voltage-controlled oscillator is the oscillation circuit and inductance load difference circuit shown in  FIG. 20  and taught in Japanese Unexamined Patent Appl. Pub. 2004-266718 corresponding to United States Patent Appl. Pub. US 2004/0183606 A1.  
         [0007]     The oscillation circuit shown in  FIG. 20  is composed of a differential inductance-capacitance resonance circuit and positive feedback circuit where the resonance circuit comprises capacitor C 1  and an inductance load difference circuit comprising variable inductance units Lvar 1  and Lvar 2 , and the positive feedback circuit comprises n-channel MOS transistors M 1  and M 2 . The variable inductance units Lvar 1  and Lvar 2  each have first and second input/output (I/O) terminals with the second I/O terminals connected to a common external power supply node Vdd. The first I/O terminals are connected to output nodes OUT and OUTB, respectively. The capacitor C 1  is also connected to the first I/O terminals of the variable inductance units Lvar 1  and Lvar 2 . The oscillation frequency of the voltage-controlled oscillator can be determined from the inductance of the variable inductance unit and the capacitance.  
         [0008]     The variable inductance units Lvar 1  and Lvar 2  vary the inductance and control the oscillation frequency by switching a plurality of switch circuits SW 1 , SW 2 , SW 3 , SW 1   d,  SW 2   d,  and SW 3   d  disposed between a plurality of selected positions on the spiral wiring layer and the I/O terminals. The variable inductance units Lvar 1  and Lvar 2  form an inductor pair when switch SWndd of switch circuits SW 1 , SW 2 , SW 3  connected between the first I/O terminals is ON at the same time as switch circuits SWn and SWnd.  
         [0009]     See Japanese Unexamined Patent Appl. Pub. 2004-266718 corresponding to United States Patent Appl. Pub. US 2004/0183606 A1.  
         [0010]     The variable inductance units taught in the patents cited above are composed of serial-parallel circuits comprising a plurality of inductors and a plurality of switch circuits, and changes the overall inductance in steps by turning the switch circuits on or off. As a result, the oscillation frequency of the voltage-controlled oscillator also changes in steps.  
         [0011]     This arrangement enables increasing the bandwidth of the voltage-controlled oscillator to some degree but does not afford sufficiently fine-tuning the oscillation frequency because correcting for variation in the inductors built in to the IC device is deficient. The oscillation frequency band can also not be freely set, and correcting for the capacitance-voltage nonlinearity and temperature characteristic of a varactor diode is not possible.  
         [0012]     The nonlinearity of the varactor diode also makes directly modulating the voltage-controlled oscillator difficult.  
       SUMMARY OF THE INVENTION  
       [0013]     The present invention is directed to solving these problems, and an object of the invention is to improve the functionality and performance of a voltage-controlled oscillator and a PLL circuit that uses the voltage-controlled oscillator by enabling continuously controlling the inductor by a control signal.  
         [0014]     To achieve this object, an oscillator according to a preferred aspect of the invention comprises: a variable inductor unit with variable inductance; a variable capacitance device connected to the variable inductor unit; an output unit that comprises one or more active elements, oscillates at an oscillation frequency determined by the inductance of the variable inductor unit and the capacitance of the variable capacitance device, and generates a VCO signal; and a control signal generator operable to generate a frequency control signal to modulate the oscillation frequency.  
         [0015]     The variable inductor unit comprises a first inductor; a current signal generator operable to detect an electric signal denoting current flowing to the first inductor or the voltage at both ends of the first inductor, and to generate a current signal based on the electric signal; and a second inductor that receives the current signal. The first inductor and second inductor are disposed to a predetermined magnetically coupled position, and the variable inductor unit sets the inductance of the first inductor desirably. The current signal generator controls the amplitude of the current signal based on the frequency control signal.  
         [0016]     A PLL circuit according to another aspect of the invention comprises an oscillator according to this invention, a phase generator operable to adjust the phase of the VCO signal, and a loop filter operable to filter output from the phase generator and to output a capacitance control signal. The oscillator controls the voltage at both ends of the variable capacitance device based on the capacitance control signal, and sets the capacitance of the variable capacitance device desirably.  
         [0017]     An oscillator and PLL circuit according to the present invention can correct the capacitance characteristic, frequency band characteristic, and temperature characteristic of the oscillator, and thereby achieve a constant conversion gain Kv across a wide frequency band and temperature range, by inputting different control signals to the variable inductor unit having a continuously variable output function. As a result, the invention enables directly modulating the oscillation frequency of the oscillator based on the frequency control signal, and thus affords a high precision, direct modulation oscillator. A mixer circuit is thus unnecessary, and power consumption can be reduced during transmission.  
         [0018]     The lockup time and C/N characteristic of the PLL incorporating this voltage-controlled oscillator are also constant to the oscillation frequency, and a stable oscillation characteristic can be achieved.  
         [0019]     Other objects and attainments together with a fuller understanding of the invention will become apparent and appreciated by referring to the following description and claims taken in conjunction with the accompanying drawings. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0020]      FIG. 1  is a block diagram of a voltage-controlled oscillator according to a first embodiment of the invention.  
         [0021]      FIG. 2  is a partial circuit diagram of the voltage-controlled oscillator according to the first embodiment of the invention.  
         [0022]      FIG. 3  is a block diagram of a voltage-controlled oscillator according to a first variation of the first embodiment of the invention.  
         [0023]      FIG. 4  is a circuit diagram of a voltage-controlled oscillator according to a third variation of the second embodiment of the invention.  
         [0024]      FIG. 5  shows the relationship between the capacitance of the varactor diode and the capacitance control signal.  
         [0025]      FIG. 6  shows the relationship between the oscillation frequency of the voltage-controlled oscillator and the capacitance control signal.  
         [0026]      FIG. 7  shows the relationship between the capacitance control signal and the oscillation frequency using the frequency band signal is a parameter.  
         [0027]      FIG. 8  shows the relationship between the oscillation frequency and capacitance control signal using temperature as a parameter.  
         [0028]      FIG. 9  is a circuit diagram of the voltage-current conversion circuit in a first variation of the first embodiment of the invention.  
         [0029]      FIG. 10  is a block diagram of a voltage-controlled oscillator according to a second variation of the first embodiment.  
         [0030]      FIG. 11  is a block diagram of a voltage-controlled oscillator according to a third variation of the first embodiment.  
         [0031]      FIG. 12  is a block diagram of a PLL circuit according to a second embodiment of the invention.  
         [0032]      FIG. 13  is a block diagram of a PLL circuit according to a first variation of the second embodiment of the invention.  
         [0033]      FIG. 14  is a block diagram of a PLL circuit according to a second variation of the second embodiment of the invention.  
         [0034]      FIG. 15  is a block diagram of a PLL circuit according to a third variation of the second embodiment of the invention.  
         [0035]      FIG. 16  is a block diagram of a PLL circuit according to a fourth variation of the second embodiment of the invention.  
         [0036]      FIG. 17  is a block diagram of a PLL circuit according to a fifth variation of the second embodiment of the invention.  
         [0037]      FIG. 18  is a circuit diagram of the charge pump in the second embodiment of the invention.  
         [0038]      FIG. 19  is a circuit diagram of the loop filter in the second embodiment of the invention.  
         [0039]      FIG. 20  is a block diagram of a voltage-controlled oscillator according to the prior art. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0040]     Preferred embodiments of the present invention are described below with reference to the accompanying figures. The first embodiment below describes a voltage-controlled oscillator according to the present invention, and the second embodiment describes a PLL circuit according to the present invention.  
         [0041]     Numeric values shown in the following embodiments are used by way of example only to describe the invention, and the invention is not limited to using these values.  
       First Embodiment  
       [0042]      FIG. 1  is a block diagram of a voltage-controlled oscillator  110  according to a first embodiment of the invention.  
         [0043]     As shown in  FIG. 1 , the voltage-controlled oscillator  110  comprises an output unit  80  having transistors  7 A and  7 B as active elements, a variable capacitor unit  81  comprising varactor diodes  6 A and  6 B as variable capacitance elements and fixed capacitors  10 A,  11 A,  10 B,  11 B, and a variable inductor unit  82  having spiral inductors  9 A and  9 B. The output unit  80  oscillates using the inductance-capacitance parallel resonance circuit comprising variable inductor unit  82  and variable capacitor unit  81  as the load.  
         [0044]     In the output unit  80  the transistors  7 A and  7 B are connected with the base of one connected to the collector of the other, and the output signals Pout 1  and Pout 2  of the voltage-controlled oscillator  110  output from these two nodes. The emitters of transistors  7 A and  7 B go to ground through current source  8 . This cross connection of the collectors and bases of the two transistors renders a positive feedback operation that oscillates at the resonance frequency of the inductance-capacitance parallel resonance circuit including the variable inductor unit  82  and variable capacitor unit  81 .  
         [0045]     Two transistors are used as the output unit  80  in this first embodiment of the invention, but the same effect can be achieved using two MOS transistors.  
         [0046]     The anodes of the varactor diodes  6 A and  6 B in the variable capacitor unit  81  are connected to each other, and capacitance control signal  302  is input to this node. The cathodes of the varactor diodes  6 A and  6 B are connected to one end of first inductors  1 A and  1 B and to current detection circuits  3 A and  3 B, respectively. The voltage applied to both ends of the varactor diodes  6 A and  6 B varies according to the capacitance control signal  302 , and the capacitance of the variable capacitor unit  81  is thus continuously variable.  
         [0047]     Fixed capacitors  10 A,  11 A, and  10 B,  11 B are connected in parallel to varactor diodes  6 A and  6 B, and switches  12 A,  13 A and  12 B,  13 B are disposed in series to these fixed capacitors. Because the voltage-controlled oscillator  110  has a differential arrangement, the capacitance of fixed capacitors  10 A,  11 A and fixed capacitors  10 B,  11 B is the same, and switches  12 A,  13 A and switches  12 B,  13 B operate in conjunction with each other.  
         [0048]     By appropriately switching fixed capacitors  10 A,  11 A, the capacitance can be varied in four steps.  
         [0049]     Variable capacitance elements of which the capacitance can be controlled by the voltage of a varactor diode, for example, can be used instead of fixed capacitors  10 A,  11 A,  10 B,  11 B and switches  12 A,  13 A,  12 B,  13 B to switch the fixed capacitors.  
         [0050]     The arrangement and operation of the variable inductor unit  82  are described next.  
         [0051]     The current flowing through first inductors  1 A and  1 B also flows through current detection circuits  3 A and  3 B, and the frequency, phase, and current amplitude of these currents are detected by the current detection circuits  3 A and  3 B. The current sources  4 A and  4 B generate current signals of the same frequency, same phase, and current amplitude of a predetermined current-amplitude ratio K 1  to the current detected by the current detection circuits  3 A and  3 B. The resulting current signals flow to second inductors  2 A and  2 B.  
         [0052]     The value of the current-amplitude ratio K 1  is positive, negative, or zero, and is constant relative to the current amplitude of the input current, but also varies according to the current amplitude control signal  300  input to both current sources  4 A and  4 B.  
         [0053]     The first inductors  1 A and  1 B and second inductors  2 A and  2 B constituting spiral inductors  9 A and  9 B, respectively, are disposed to positions that are magnetically coupled by mutual induction. Depending on the sign of the current-amplitude ratio K 1 , the magnetic flux produced by the second inductors  2 A and  2 B works to increase or decrease the magnetic flux produced by the first inductors  1 A and  1 B. In this embodiment of the invention the second inductors  2 A and  2 B work to increase the magnetic flux from the first inductors  1 A and  1 B when current-amplitude ratio K 1  is positive, and to decrease the magnetic flux of first inductors  1 A and  1 B when the current-amplitude ratio K 1  is negative.  
         [0054]     The amplitude of the current signals flowing through the second inductors  2 A and  2 B varies continuously according to the current amplitude control signal  300  input to the current sources  4 A and  4 B in the variable inductor unit  82  thus arranged. As a result, the current-amplitude ratio K 1  of the current signal or the amplitude of the current signal flowing through the second inductors  2 A and  2 B can be continuously controlled by the current amplitude control signal  300 . The apparent inductance of the first inductors  1 A and  1 B reflecting the mutual induction of the magnetic flux from the second inductors  2 A and  2 B on the magnetic flux produced by the first inductors  1 A and  1 B is the inductance of the variable inductor unit  82 , and the inductance of the variable inductor unit  82  changes continuously and can be set as desired by the current amplitude control signal  300 .  
         [0055]     Furthermore, when current-amplitude ratio K 1  is positive, the inductance of the variable inductor unit  82  increases and the resistance of the first inductors  1 A and  1 B does not change. As a result, the Q of the inductance of the variable inductor unit  82  increases compared with the first inductors  1 A and  1 B alone.  
         [0056]     A frequency band signal  303  is also input to the current sources  4 A and  4 B. The current sources  4 A and  4 B can control the amplitude of the current signals or the current-amplitude ratio K 1  of the current signals flowing to second inductors  2 A and  2 B continuously by the current amplitude control signal  300  and in steps by the frequency band signal  303 . As a result, the inductance of the variable inductor unit  82  varies continuously according to the current amplitude control signal  300  and varies in steps according to the frequency band signal  303 . The inductance of the variable inductor unit  82  can thus be set as desired.  
         [0057]     The current amplitude control signal  300  and the frequency band signal  303  are generated by the control signal generator  310 . The current amplitude control signal  300  and frequency band signal  303  are set automatically in the control signal generator  310  by an arrangement including the correction information generator  44  shown in  FIG. 16 . These signals can alternatively be manually set as desired by the operator, or preset to a constant factory setting prior to shipping. Note, further, that current detection circuit  3 A and current source  4 A together render one current signal generator, and current detection circuit  3 B and current source  4 B together render another current signal generator.  
         [0058]      FIG. 2  is a partial circuit diagram of the voltage-controlled oscillator  110  according to the first embodiment of the invention.  FIG. 2  shows specific examples of the current detection circuits  3 A and  3 B and current sources  4 A and  4 B, which are described in further detail below with reference to current detection circuit  3 A and current source  4 A.  
         [0059]     As shown in  FIG. 2 , the current flowing through the first inductor  1 A flows between the collector and emitter of transistor T 10 A. A current signal substantially proportional to the size ratio of transistor T 11 A to transistor T 10 A flows between the collector and emitter of the transistor T 11 A whereby a current mirror circuit is rendered with transistor T 10 A. The collector of transistor T 11 A is connected to second inductor  2 A and to DC source  59 A through current source T 12 A, and the current signal flowing to the transistor T 11 A also flows to second inductor  2 A.  
         [0060]     The gate voltage of MOS transistor T 13 A changes and the ON resistance between the drain and source of MOS transistor T 13 A changes continuously according to the current amplitude control signal  300 . As a result, the current signal flowing to the second inductor  2  also changes continuously. The current-amplitude ratio K 1  of the current signal or the amplitude of the current signal flowing through second inductor  2 A can therefore be controlled continuously based on the current amplitude control signal  300 .  
         [0061]     The parts relating to the current detection circuit  3 B and current source  4 B can be described in the same way.  
       Major Parts of a First Variation of the First Embodiment  
       [0062]      FIG. 3  is a block diagram of the voltage-controlled oscillator  210  in a first variation of the first embodiment of the invention.  
         [0063]     This voltage-controlled oscillator  210  comprises an output unit  80  composed of transistors  7 A and  7 B as active elements, a variable capacitor unit  81  comprising varactor diodes  6 A and  6 B as variable capacitance elements and fixed capacitors  10 A,  11 A,  10 B,  11 B, and a variable inductor unit  83  having spiral inductors  9 A and  9 B. The output unit  80  oscillates using the inductance-capacitance parallel resonance circuit comprising variable inductor unit  83  and variable capacitor unit  81  as the load.  
         [0064]     The arrangement and operation of the output unit  80  and variable capacitor unit  81  are the same as described in the first embodiment.  
         [0065]     The arrangement and operation of the variable inductor unit  83  are described next. The voltage applied to the ends of the first inductors  1 A and  1 B that are not connected to each other is also input to the voltage-current conversion circuit  5 . The voltage-current conversion circuit  5  generates a current signal having a current amplitude of a predetermined voltage-current conversion ratio K 2  and the same frequency as the input voltage, and this current signal flows to second inductors  2 A and  2 B. The value of the voltage-current conversion ratio K 2  is positive, negative, or zero, and is constant relative to the voltage amplitude of the input voltage, but also depends on the voltage-current conversion control signal  301  input to the voltage-current conversion circuit  5 .  
         [0066]     The first inductor  1 A and second inductor  2 A constituting spiral inductor  9 A, and the first inductor  1 B and second inductor  2 B constituting spiral inductor  9 B, are disposed to positions that are magnetically coupled by mutual induction. Depending on the sign of the voltage-current conversion ratio K 2 , the magnetic flux produced by the second inductors  2 A and  2 B works to increase or decrease the magnetic flux produced by the first inductors  1 A and  1 B. In this embodiment of the invention the magnetic flux increases when voltage-current conversion ratio K 2  is positive, and decreases when the voltage-current conversion ratio K 2  is negative.  
         [0067]     In the variable inductor unit  83  thus comprised, the amplitude of the current signals flowing to the second inductors  2 A and  2 B varies continuously according to the voltage-current conversion control signal  301  input to the voltage-current conversion circuit  5 . As a result, the voltage-current conversion ratio K 2  of the current signals or the amplitude of the current signals flowing to the second inductors  2 A and  2 B can be continuously controlled based on the voltage-current conversion control signal  301 . The apparent inductance of the first inductors  1 A and  1 B reflecting the mutual induction of the magnetic flux from the second inductors  2 A and  2 B on the magnetic flux from the first inductors  1 A and  1 B is the inductance of the variable inductor unit  83 , and varies continuously and can be set as desired by the voltage-current conversion control signal  301 .  
         [0068]     Furthermore, when voltage-current conversion ratio K 2  is positive, the inductance of the variable inductor unit  83  increases and the resistance of the first inductors  1 A and  1 B does not change. As a result, the Q of the inductance of the variable inductor unit  83  increases compared with first inductors  1 A and  1 B alone.  
         [0069]     Frequency band signal  303  is also input to the voltage-current conversion circuit  5 . The voltage-current conversion circuit  5  varies the voltage-current conversion ratio K 2  of the current signals or the amplitude of the current signals flowing to the second inductors  2 A and  2 B continuously according to the voltage-current conversion control signal  301  and in steps according to the frequency band signal  303 . As a result, the inductance of the variable inductor unit  83  varies continuously according to the voltage-current conversion control signal  301  and in steps according to the frequency band signal  303 . The inductance of the variable inductor unit  83  can thus be set as desired.  
         [0070]     The voltage-current conversion control signal  301  and frequency band signal  303  are generated by the control signal generator  310 . The voltage-current conversion control signal  301  and frequency band signal  303  are set automatically in the control signal generator  310  by an arrangement including the correction information generator  44  shown in  FIG. 17  and further described below. These signals can alternatively be manually set as desired by the operator, or preset to a constant factory setting prior to shipping. Note, further, that the voltage-current conversion circuit  5  constitutes a current signal generator in the accompanying claims.  
         [0071]      FIG. 4  is a partial circuit diagram of the voltage-controlled oscillator in the first variation of the first embodiment of the invention.  FIG. 4  shows a specific example of the voltage-current conversion circuit  5 , which is described in further detail below.  
         [0072]     The voltage applied to the ends of the first inductors  1 A and  1 B that are not connected to each other is also input to the base of transistor T 20  and transistor T 21 , and a current signal proportional to the voltage is produced by the differential amplifier composed of transistor T 20  and transistor T 21  and is supplied to second inductors  2 A and  2 B.  
         [0073]     The emitter of transistor T 23  goes to ground through resistance R 20 , voltage-current conversion control signal  301  is applied to the base, and the collector current of the transistor T 23  varies continuously according to the voltage-current conversion control signal  301 . As a result, the current signal flowing to second inductors  2 A and  2 B is also continuously variable. The arrangement shown in  FIG. 4  can therefore continuously control the voltage-current conversion ratio K 2  of the current signal or the amplitude of the current signal flowing to the second inductors  2 A and  2 B based on the voltage-current conversion control signal  301 .  
       Oscillation Frequency Characteristic in the First Embodiment  
       [0074]     Factors affecting the oscillation frequency of the voltage-controlled oscillator are described below.  
         [0075]      FIG. 5  shows the relationship between the capacitance of the varactor diode and capacitance control signal  302 . Because both cathodes of the varactor diodes  6 A and  6 B are connected to DC power source  70 , the voltage applied to both ends of the varactor diodes  6 A and  6 B decreases as capacitance control signal  302  increases from V 4  to V 3 , V 2 , V 1 . The capacitance characteristic representing the relationship between the capacitance of varactor diodes  6 A and  6 B and capacitance control signal  302  is ideally linear and rises to the right as denoted by dot-dash line BD 0 .  
         [0076]      FIG. 6  schematically shows the relationship between the oscillation frequency of the voltage-controlled oscillator and capacitance control signal  302 .  
         [0077]     If half the inductance of the variable inductor unit is L and the capacitance of one of varactor diodes  6 A and  6 B is C, the ideal oscillation frequency fc of the differential operating voltage-controlled oscillator  110 ,  210  can be derived from equation (1). 
 
 fc= 1/(2π*sqrt( L*C ))   (1) 
 
         [0078]     If the capacitance of varactor diodes  6 A and  6 B varies linearly on a right-rising curve as denoted by BD 0  in  FIG. 5 , the oscillation frequency of the voltage-controlled oscillator ideally decreases linearly to the right as denoted by FC 0  in  FIG. 6 .  
         [0079]      FIG. 7  schematically shows the relationship between the capacitance control signal  302  and the oscillation frequency of the voltage-controlled oscillator where the frequency band signal is a parameter. The ideal response corresponding to the ideal characteristic FC 0  shown in  FIG. 6  is line FB 0 , which represents the frequency band characteristic denoting the relationship when the frequency band signal is varied based on line FB 0 . If the inductance of the variable inductor unit is assumed to increase monotonically to frequency band signals FB 1 , FB 2 , FB 3 , FB 4 , four frequency bands can be rendered as shown in  FIG. 7 . As a result, the oscillation frequency band increases and operation can switch between a plurality of frequency bands, and the present invention can be applied in, for example, cell phones that use a plurality of frequency bands.  
         [0080]     In practice, however, the capacitance of the varactor diode is nonlinear with respect to the capacitance control signal  302  as indicated by curve BDR in  FIG. 5 . As a result, the oscillation frequency of the voltage-controlled oscillator also varies nonlinearly to the capacitance control signal  302  as indicated by curve FCR in  FIG. 6 . The conversion gain Kv of the voltage-controlled oscillator is expressed as the degree of change in the oscillation frequency to the change in the capacitance control signal  302 , but in this case varies dependently upon the value of the capacitance control signal  302 . A PLL incorporating this voltage-controlled oscillator will exhibit an unstable lockup time and C/N characteristic depending on the oscillation frequency.  
         [0000]     Solving the Nonlinearity of the Oscillation Frequency Characteristic  
         [0081]     To solve this problem, the nonlinearity induced by the varactor diode as denoted by curve FCR in  FIG. 6  is corrected by the variable inductance function of the variable inductor unit. The temperature characteristic of the varactor diode and fixed capacitor is also corrected in the same way.  
         [0082]     If VT (unit=volts) denotes the level of the capacitance control signal, FB is the number of the frequency band signal, and TM (unit=degrees) is temperature, the actual oscillation frequency fc 1  can be derived from equation (2) as compares with the ideal oscillation frequency fc shown in equation (1). 
 
 fc 1=1/(2π*sqrt( L*A 1( VT )* A 2( FB )* A 3( TM )* C ))   (2) 
 
         [0083]     A 1 (VT), A 2 (FB), and A 3 (TM) are nonlinear functions that are uniquely determined by VT, FB, and TM, and represent the offset from the ideal capacitance, frequency band, and temperature characteristics. Capacitance C denotes the capacitance of the varactor diode or fixed capacitor, is offset from the ideal characteristic by nonlinearity and the temperature characteristic, and is (A 1 (VT)*A 2 (FB)*A 3 (TM))*C. In this case, the ideal oscillation frequency fc can be achieved as shown in equation (1) by changing the half inductance L of the variable inductor unit to L/(A 1 (VT)*A 2 (FB)*A 3 (TM)) as shown in equation (3). 
 
 fc= 1/(2π*sqrt( L /( A 1( VT )* A 2( FB )* A 3( TM ))*( A 1( VT )* A 2( FB )* A 3( TM ))* C ))   (3) 
 
         [0084]     If the actual capacitance characteristic BDR shown in  FIG. 5  is divided into three parts, the capacitance characteristic will be approximated by line BD 1  of slope B 1  when the capacitance control signal VT is in the range from V 1  to V 2 , by line BD 2  of slope B 2  when VT is in the range from V 2  to V 3 , and line BD 3  of slope B 3  when VT is in the range from V 3  to V 4 . If B 0  is the slope of the ideal characteristic BD 0 , the correction coefficient for the capacitance characteristic is defined by equations (4), (5), and (6). 
 
 A 1( VT )= B 0 /B 1( V 2 ≦VT≦V 1)   (4) 
 
 A 1( VT )= B 0 /B 2( V 3 ≦VT≦V 2)   (5) 
 
 A 1( VT )= B 0 /B 3( V 4 ≦VT≦V 3)   (6) 
 
 Linear approximation is used for correction coefficient A 1 (VT) here, but a quadratic approximation or table based on the actual curve could be used. 
 
         [0085]     The correction coefficient for the frequency band characteristic is as shown in equations (7), (8), and (9) where B 1  is the slope of the line when the frequency band signal FB is FB 1  in  FIG. 7 , B 2  is the slope when frequency band signal FB is FB 2 , B 3  is the slope when frequency band signal FB is FB 3 , and B 0  is the slope for the ideal characteristic FB 0 . 
 
 A 2( FB 1)= B 0 /B 1   (7) 
 
 A 2( FB 2)= B 0 /B 2   (8) 
 
 A 2( FB 3)= B 0 /B 3   (9) 
 
         [0086]     The temperature characteristic is described next.  
         [0087]      FIG. 8  schematically shows the relationship between the oscillation frequency of the voltage-controlled oscillator and capacitance control signal  302  using temperature as a parameter. The ideal temperature characteristic corresponding to ideal characteristic FC 0  in  FIG. 6  is line TM 0  representing the values at a normal temperature of 25 degC. TM 1  corresponds to a high temperature of 100 degC., and TM 2  to a low temperature of −40 degC.  
         [0088]     If B 1  is the slope when temperature TM is TM 1 , B 2  is slope when TM is TM 2 , and B 0  is the slope of the ideal characteristic TM 0 , the temperature characteristic correction coefficient is as shown in equations (10) and (11). 
 
 A 3( TM 1)= B 0 /B 1   (10) 
 
 A 3( TM 2)= B 0 /B 2   (11) 
 
         [0089]     This temperature characteristic correction is applied not only to the varactor diode, but also the fixed capacitors  10 A,  11 A,  10 B,  11 B shown in  FIG. 1  and  FIG. 3 .  
         [0090]     By correcting the half inductance L of the variable inductor unit to L/(A 1 (VT)*A 2 (FB)*A 3 (TM)) by the control signals, the nonlinearity of the varactor diode and the temperature characteristic of the varactor diode and fixed capacitors can be corrected to the ideal characteristic. Because the conversion gain Kv of the voltage-controlled oscillator is constant regardless of capacitance control signal  302 , the lockup time and C/N characteristic of a PLL incorporating the voltage-controlled oscillator are constant to the oscillation frequency, and a stable oscillation characteristic can be achieved.  
       Control Signal Generator  310  in the First Embodiment and First Variation of the First Embodiment  
       [0091]     The current amplitude control signal  300 , voltage-current conversion control signal  301 , and frequency band signal  303  input as control signals from the control signal generator  310  to the current sources  4 A and  4 B in  FIG. 1  and the voltage-current conversion circuit  5  in  FIG. 3  are described above. Another control signal and the control signal generator  310  therefore are described below.  
         [0092]     The actual capacitance characteristic BDR can be reflected in the inductance correction by storing the capacitance characteristics of the varactor diodes  6 A and  6 B to the capacitance control signal  302  as shown in  FIG. 5  in a storage circuit and reading data from the storage circuit according to the capacitance control signal  302 .  
         [0093]     The ideal oscillation frequency characteristic can be achieved by multiplying the inductance of the variable inductor unit  82 ,  83  by 1/(A 1 (VT)*A 2 (FB)) by capacitance control signal  302  and frequency band signal  303 .  
         [0094]     The capacitance control signal  302  is generated by the control signal generator  310 . The control signal generator  310  sets the capacitance control signal  302  automatically by an arrangement comprising a loop filter  37  such as shown in  FIG. 12  and  FIG. 13  and described below. These signals can alternatively be manually set as desired by the operator, or preset to a constant factory setting prior to shipping.  
         [0095]     The temperature characteristic signal  304  is generated by an arrangement including a temperature sensor  23  and storage circuit  22 . The temperature sensor  23  detects the temperature of at least one of the varactor diodes  6 A and  6 B and fixed capacitors  10 A,  11 A,  10 B,  11 B, and inputs the detected temperature to the storage circuit  22 , which stores the previously measured temperature characteristic.  
         [0096]     The capacitance control signal  302  is also input to the storage circuit  22 , and the change in capacitance at the detected temperature and capacitance control signal  302  level is generated as temperature characteristic signal  304  based on the input from the temperature sensor  23  and the capacitance control signal  302 .  
         [0097]     The ideal oscillation frequency characteristic can thus be acquired using capacitance control signal  302 , frequency band signal  303 , and temperature characteristic signal  304  by multiplying the inductance of the variable inductor unit  82  by 1/(A 1 (VT)*A 2 (FB)*A 3 (TM)).  
         [0098]     The temperature sensor  23  and storage circuit  22  are part of the control signal generator  310 , and the temperature characteristic signal  304  is generated by the control signal generator  310 . The temperature characteristic signal  304  is set in this aspect of the invention by the control signal generator  310 , but can alternatively be manually set as desired by the operator, or preset to a constant factory setting prior to shipping.  
         [0099]     The voltage-controlled oscillator  110  or  210  can be directly modulated by inputting the frequency control signal  305  directly to current sources  4 A and  4 B or voltage-current conversion circuit  5 , respectively. More specifically, like the current amplitude control signal  300  or voltage-current conversion control signal  301 , the frequency control signal  305  is input to the gate of transistor T 13 A and transistor T 13 B in  FIG. 2  or the base of transistor T 23  in  FIG. 4 , causing the inductance of variable inductor unit  82  or  83  to vary according to the frequency control signal  305 . The oscillation frequency can thus be directly modulated according to the frequency control signal  305 , and a direct-modulation oscillator can be provided.  
         [0100]     A mixer circuit is thus unnecessary, and power consumption can be reduced during transmission.  
         [0101]     The frequency control signal  305  is generated by the control signal generator  310 . In the control signal generator  310  the frequency control signal  305  is, for example, a cell phone baseband signal, and is a packetized signal containing headers and error correction parity added to compressed video or audio information. Specific examples of a control signal generator  310  therefore include error correction coding devices and packetizing devices.  
       Circuit Diagram for the Voltage-Current Conversion Circuit  5  in a First Variation of the First Embodiment  
       [0102]      FIG. 9  is a circuit diagram of the voltage-current conversion circuit  5  in a first variation of the first embodiment.  
         [0103]     As with the arrangement shown in  FIG. 4 , the bases of transistor T 20  and transistor T 21  in a differential arrangement are connected to the bases of transistors  7 A and  7 B, respectively, and the collectors are respectively connected to one side of second inductors  2 A and  2 B. A current supply source is connected to the emitters of transistors T 20  and T 21 . These four current supply sources are switched by the control signal.  
         [0104]     The capacitance control signal  302  is divided by range dividing circuit  75  into three bands from V 1  to V 2 , V 2  to V 3 , and V 3  to V 4  as shown in  FIG. 5 , and is input as three signals, one of which is HIGH, to the characteristic correction circuit  77 . Based on the signals from the range dividing circuit  75 , the frequency band signal  303 , and the temperature characteristic signal  304 , the characteristic correction circuit  77  calculates correction coefficient 1/(A 1 (VT)*A 2 (FB)*A 3 (TM)) and outputs four control signals controlling switches S 33 P, S 33 Q, S 33 R, S 33 S.  
         [0105]     The four current supply sources T 33 P, T 33 Q, T 33 R, T 33 S have a weighted current setting, and set the voltage-current conversion ratio K 2  according to the current level. By appropriately switching these four current supply sources, the inductance of the variable inductor unit  83  is corrected to the inductance times 1/(A 1 (VT)*A 2 (FB)*A 3 (TM)). At least one of switches S 33 P, S 33 Q, S 33 R, S 33 S is ON, and a plurality of these switches could be ON. By thus switching four current supply sources, the inductance can be precisely corrected in fifteen steps.  
         [0106]     The voltage-current conversion control signal  301  is used for precisely adjusting the four current supply sources or for correcting another parameter.  
       Second and Third Variations of the First Embodiment  
       [0107]      FIG. 10  is a block diagram of a voltage-controlled oscillator  110  according to a second variation of the first embodiment.  
         [0108]     This voltage-controlled oscillator  110  differs from the voltage-controlled oscillator  110  shown in  FIG. 1  in that the output unit  80  comprises an amplitude controller  24  for controlling signal amplitude. Signals Pout 1  and Pout 2  are input to the amplitude controller  24 , which modulates the amplitude based on the amplitude control signal  306  and outputs the result as signal Pout. Because signals Pout 1  and Pout 2  are frequency modulated based on frequency control signal  305 , and the signal amplitude is then modulated based on the amplitude control signal  306 , this aspect of the invention can generate a modulated signal such as QAM carrying both phase and amplitude information.  
         [0109]      FIG. 11  is a block diagram of a voltage-controlled oscillator  210  according to a third variation of the first embodiment.  
         [0110]     This voltage-controlled oscillator  110  differs from the voltage-controlled oscillator  110  shown in  FIG. 3  in that the output unit  80  comprises an amplitude controller  24  for controlling signal amplitude. Signals Pout 1  and Pout 2  are input to the amplitude controller  24 , which modulates the amplitude based on the amplitude control signal  306  and outputs the result as signal Pout. Because signals Pout 1  and Pout 2  are frequency modulated based on frequency control signal  305 , and the signal amplitude is then modulated based on the amplitude control signal  306 , this aspect of the invention can generate a modulated signal such as QAM carrying both phase and amplitude information.  
       Fourth Variation of the First Embodiment  
       [0111]     In the voltage-controlled oscillators  110  and  210  according to the first embodiment and the first to third variations of the first embodiment the inductance of the variable inductor unit  82 ,  83  varies according to the current signal flowing from the second inductors  2 A and  2 B, and the Q of the variable inductor unit  82 ,  83  thus also varies. As a result, the optimum resonance frequency flowing to the first inductors  1 A and  1 B varies with the change in Q.  
         [0112]     In order to optimize the resonance frequency flowing to the first inductors  1 A and  1 B, the amplitude of the current signal flowing to the second inductors  2 A and  2 B is detected by a current signal detector, and the current flow to the current source  8  is varied according to the amplitude of the current signal. The relationship between the amplitude of the current signal flowing to the second inductors  2 A and  2 B and the optimum current flow to the current source  8  is predetermined and stored in the storage circuit, and is referenced when adjusting the current flow to the current source  8 .  
       Effect of the First Embodiment  
       [0113]     Various problems with the voltage-controlled oscillator  110 ,  210  can be solved by inputting various control signals to a variable inductor unit  82 ,  83  with a continuously variable output function.  
         [0114]     First, the inductance of the variable inductor unit  82 ,  83  can be varied continuously by applying current amplitude control signal  300  and voltage-current conversion control signal  301 , respectively, and can be varied in steps by applying frequency band signal  303 . As a result, the voltage-controlled oscillator  110 ,  210  has a frequency band selection function and can select any desired frequency within the selected frequency band.  
         [0115]     In addition, the current amplitude control signal  300  and voltage-current conversion control signal  301  enable accurately and precisely adjusting the oscillation frequency, and can thus reduce manufacturing variation in the voltage-controlled oscillator  110 ,  210  to a negligible level that poses no problem in practical operation.  
         [0116]     The nonlinearity of the varactor diodes  6 A and  6 B can also be corrected by inputting the capacitance control signal  302  to the variable inductor unit  82 ,  83 . By also considering the frequency band signal  303 , nonlinearity can be corrected across a wide frequency band. As a result, the conversion gain Kv of the voltage-controlled oscillator  110 ,  210  is constant irrespective of the capacitance control signal  302  and frequency band signal  303 . As a result, the lockup time and C/N characteristic of the PLL incorporating this voltage-controlled oscillator  110 ,  210  are constant relative to the oscillation frequency, thus affording a stable oscillation characteristic.  
         [0117]     Yet further, the capacitance control signal  302  and temperature characteristic signal  304  enable correcting change in the temperature characteristic according to the voltage at both ends of the varactor diode, and correcting the temperature characteristic of the fixed capacitors. A stable oscillation characteristic can thus be achieved over a wide temperature range.  
         [0118]     Because the oscillation frequency characteristic of the voltage-controlled oscillator  110 ,  210  is thus linear over a wide frequency range, the oscillation frequency of the voltage-controlled oscillator  110 ,  210  can be directly modulated using the frequency control signal  305 . As a result, the invention enables directly modulating the oscillation frequency of the oscillator based on the frequency control signal, and thus affords a high precision, direct modulation oscillator. A mixer circuit is thus unnecessary, and power consumption can be reduced during transmission.  
         [0119]     Another effect of this aspect of the invention is that because the Q of the variable inductor unit  82 ,  83  can be increased, the C/N ratio of the oscillation frequency of the voltage-controlled oscillator  110 ,  210  can also be improved.  
         [0120]     In addition, by also using a function for varying the capacitance using a varactor diode and variable inductor unit  82 ,  83 , the oscillation frequency can be changed without greatly varying the inductance to capacitance ratio. As a result, the oscillation frequency band is increased and a stable oscillation characteristic is achieved over a broad frequency range.  
       Second Embodiment  
       [0121]     A PLL circuit according to a second embodiment of the invention is described next.  
         [0122]      FIG. 12  is a block diagram of a PLL circuit according to this second embodiment of the invention.  
         [0123]     The voltage-controlled oscillator  110  generates and outputs VCO signal  307 , which is also 1/N frequency divided by 1/N frequency divider  32 . The output of the reference signal oscillator  33  is 1/R frequency divided by 1/R frequency divider  34 . The phase comparator  35  then compares the outputs from 1/N frequency divider  32  and 1/R frequency divider  34 , and outputs the phase difference of the advance or delay in the input signals as the phase signal. Based on the sign of this phase difference, the charge pump  36  converts the phase signal to a positive or negative current. The loop filter  37  then converts the integral of this current to voltage, and removes high frequency distortion and noise. The output of loop filter  37  is input as capacitance control signal  302  to voltage-controlled oscillator  110 , which generates the VCO signal  307  at an oscillation frequency determined by the capacitance control signal  302 .  
         [0124]     The phase comparator  35  is also referred to a phase generator herein.  
         [0125]      FIG. 18  is a circuit diagram of the charge pump in the second embodiment of the invention.  
         [0126]     The current mirror unit  52  connected to Vcc supply voltage  410  and ground voltage  411 , together with current mirror unit  50  and current mirror unit  51 , constitute a current mirror circuit. Current of the same level flows to current mirror unit  50  and current mirror unit  51 . This current level can be varied by adjusting the current of current mirror unit  52 . The phase signal generated by phase comparator  35  is input to switch  53  and through inversion circuit  55  to switch  54 . The phase signal from the phase comparator  35  causes switch  53  and switch  54  to turn on and off in a seesaw pattern, and the positive current from current mirror unit  50  and the negative current from current mirror unit  51  are alternately supplied to the loop filter  37 .  
         [0127]      FIG. 19  is a circuit diagram of the loop filter in the second embodiment of the invention. High frequency distortion and noise are removed from the output of the charge pump  36  in  FIG. 18  by the low-pass filter of the loop filter  37  comprising resistance R 1 , resistance R 2 , and capacitor C as shown in  FIG. 19 . The difference between the positive current and the negative current alternately supplied from the charge pump  36  is accumulated in capacitor C, converted to voltage, and supplied to the voltage-controlled oscillator  110 .  
         [0128]     The arrangement of the voltage-controlled oscillator  110  in  FIG. 12  is as shown in  FIG. 10 . The nonlinearity of the varactor diode can be corrected, the temperature characteristic can be corrected, a direction modulation function is afforded, and a frequency band switching function is afforded by applying control signals such as the current amplitude control signal  300 , capacitance control signal  302 , frequency band signal  303 , temperature characteristic signal  304 , and frequency control signal  305 .  
         [0129]     Because the linearity of the voltage-controlled oscillator  110  in this PLL circuit is maintained over a wide frequency band, a direct modulation function can be achieved by applying the frequency control signal  305 , and a high precision modulation signal is output as VCO signal  307 . A mixer circuit is therefore unnecessary, and power consumption during transmission can be reduced.  
       First Variation of the Second Embodiment  
       [0130]      FIG. 13  is a block diagram of a PLL circuit according to a first variation of the second embodiment.  
         [0131]     The voltage-controlled oscillator  210  generates and outputs VCO signal  307 , which is also 1/N frequency divided by 1/N frequency divider  32 . The output of the reference signal oscillator  33  is 1/R frequency divided by 1/R frequency divider  34 . The phase comparator  35  then compares the outputs from 1/N frequency divider  32  and 1/R frequency divider  34 , and outputs the phase difference of the advance or delay in the input signals as the phase signal. Based on the sign of this phase difference, the charge pump  36  converts the phase signal to a positive or negative current. The loop filter  37  then converts the integral of this current to voltage, and removes high frequency distortion and noise. The output of loop filter  37  is input as capacitance control signal  302  to voltage-controlled oscillator  210 , which generates the VCO signal  307  at an oscillation frequency determined by the capacitance control signal  302 .  
         [0132]     The phase comparator  35  is also referred to a phase generator herein.  
         [0133]     The charge pump and loop filter are as shown by the circuit diagrams in  FIG. 18  and  FIG. 19 , respectively, and operate to the same effect as described above.  
         [0134]     The arrangement of the voltage-controlled oscillator  210  in  FIG. 13  is as shown in  FIG. 11 . The nonlinearity of the varactor diode can be corrected, the temperature characteristic can be corrected, a direction modulation function is afforded, and a frequency band switching function is afforded by applying control signals such as the voltage-current conversion control signal  301 , capacitance control signal  302 , frequency band signal  303 , temperature characteristic signal  304 , and frequency control signal  305 .  
         [0135]     Because the linearity of the voltage-controlled oscillator  210  in this PLL circuit is maintained over a wide frequency band, a direct modulation function can be achieved by applying the frequency control signal  305 , and a high precision modulation signal is output as VCO signal  307 . A mixer circuit is therefore unnecessary, and power consumption during transmission can be reduced.  
       Second and Third Variations of the Second Embodiment  
       [0136]      FIG. 14  is a block diagram of a PLL circuit according to a second variation of the second embodiment of the invention.  
         [0137]     This PLL circuit differs from the PLL circuit shown in  FIG. 12  in that the capacitance control signal  302  is not limited to the output signal from the loop filter  37 . More particularly, a predetermined voltage source  38  can be selected by switch  39  as the capacitance control signal  302 .  
         [0138]     When the switch  39  switches to the voltage source  38 , the PLL circuit is an open loop, and the capacitance control signal  302  is held to the specific voltage from the voltage-source  38 . When the capacitance control signal  302  is fixed, the voltage-controlled oscillator  110  can be directly modulated according to the frequency control signal  305 , and direct modulation oscillator that is unaffected by the frequency lock characteristic of the PLL circuit can be provided.  
         [0139]      FIG. 15  is a block diagram of a PLL circuit according to a third variation of the second embodiment of the invention.  
         [0140]     This PLL circuit differs from the PLL circuit shown in  FIG. 13  in that the capacitance control signal  302  is not limited to the output signal from the loop filter  37 . More particularly, a predetermined voltage source  38  can be selected by switch  39  as the capacitance control signal  302 .  
         [0141]     When the switch  39  switches to the voltage source  38 , the PLL circuit is an open loop, and the capacitance control signal  302  is held to the specific voltage from the voltage source  38 . When the capacitance control signal  302  is fixed, the voltage-controlled oscillator  210  can be directly modulated according to the frequency control signal  305 , and direct modulation oscillator that is unaffected by the frequency lock characteristic of the PLL circuit can be provided.  
       Fourth and Fifth Variations of the Second Embodiment  
       [0142]      FIG. 16  is a block diagram of a PLL circuit according to a fourth variation of the second embodiment of the invention.  
         [0143]     This PLL circuit differs from the PLL circuit shown in  FIG. 14  in that various characteristics of the voltage-controlled oscillator  110  are measured in advance and based on the results of these measurements the characteristics are corrected by the control signals. These differences are further described below.  
         [0144]     Switch  39  is first switched to the variable voltage source  38 P side so that the PLL circuit is an open loop. The current amplitude control signal  300  and frequency band signal  303  are fixed to predetermined values, the current-amplitude ratio K 1  is therefore also constant, and the inductance of the variable inductor unit  82  of the voltage-controlled oscillator  110  is thus fixed. If the voltage from the variable voltage source  38 P changes in this state, the capacitance control signal  302  varies accordingly and the oscillation frequency of the voltage-controlled oscillator  110  thus varies.  
         [0145]     The voltage-controlled oscillator  110  generates and outputs VCO signal  307 , which is 1/N frequency divided by 1/N frequency divider  32  and input to frequency detector  41 . The reference signal from the reference signal oscillator  33  is 1/R frequency divided by 1/R frequency divider  34  and input to frequency detector  41 . The frequency detector  41  determines the frequency of the VCO signal  307  from the frequency difference of the VCO signal  307  to the reference signal. The operating circuit  42  measures the capacitance characteristic, frequency band characteristic, and temperature characteristic at the frequency of the VCO signal  307 , and the resulting measurement data is stored in storage circuit  43 .  
         [0146]     The switch  39  then switches to the loop filter  37  side so that the PLL circuit is a closed loop. Based on the measurement data read from storage circuit  43 , the correction information generator  44  generates the current amplitude control signal  300  and frequency band signal  303  causing the inductance L of the variable inductor unit  82  to go to L/(A 1 (VT)*A 2 (FB)*A 3 (TM)), and inputs the resulting current amplitude control signal  300  and frequency band signal  303  to the voltage-controlled oscillator  110 .  
         [0147]     The capacitance characteristic, frequency band characteristic, and temperature characteristic of the VCO signal  307  generated by the voltage-controlled oscillator  110  are thus corrected, and the conversion gain Kv is constant with respect to changes in the capacitance control signal  302 . The lockup time and C/N characteristic of the PLL incorporating this voltage-controlled oscillator are also constant to the oscillation frequency, and a stable oscillation characteristic can be achieved. A high precision directly modulated oscillator can thus be achieved by applying the frequency control signal  305  to the voltage-controlled oscillator  110 . A mixer circuit is therefore unnecessary, and power consumption during transmission can be reduced.  
         [0148]      FIG. 17  is a block diagram of a PLL circuit according to a fifth variation of the second embodiment of the invention.  
         [0149]     This PLL circuit differs from the PLL circuit shown in  FIG. 15  in that various characteristics of the voltage-controlled oscillator  210  are measured in advance and based on the results of these measurements the characteristics are corrected by the control signals. These differences are further described below.  
         [0150]     Switch  39  is first switched to the variable voltage source  38 P side so that the PLL circuit is an open loop. The voltage-current conversion control signal  301  and frequency band signal  303  are fixed to predetermined values, the voltage-current conversion ratio K 2  is therefore also constant, and the inductance of the variable inductor unit  83  of the voltage-controlled oscillator  110  is thus fixed. If the voltage from the variable voltage source  38 P changes in this state, the capacitance control signal  302  varies accordingly and the oscillation frequency of the voltage-controlled oscillator  210  thus varies.  
         [0151]     The voltage-controlled oscillator  210  generates and outputs VCO signal  307 , which is 1/N frequency divided by 1/N frequency divider  32  and input to frequency detector  41 . The reference signal from the reference signal oscillator  33  is 1/R frequency divided by 1/R frequency divider  34  and input to frequency detector  41 . The frequency detector  41  determines the frequency of the VCO signal  307  from the frequency difference of the VCO signal  307  to the reference signal. The operating circuit  42  measures the capacitance characteristic, frequency band characteristic, and temperature characteristic at the frequency of the VCO signal  307 , and the resulting measurement data is stored in storage circuit  43 .  
         [0152]     The switch  39  then switches to the loop filter  37  side so that the PLL circuit is a closed loop. Based on the measurement data read from storage circuit  43 , the correction information generator  44  generates the voltage-current conversion control signal  301  and frequency band signal  303  causing the inductance L of the variable inductor unit  83  to go to L/(A 1  (VT)*A 2 (FB)*A 3 (TM)), and inputs the resulting voltage-current conversion control signal  301  and frequency band signal  303  to the voltage-controlled oscillator  210 .  
         [0153]     The capacitance characteristic, frequency band characteristic, and temperature characteristic of the VCO signal  307  generated by the voltage-controlled oscillator  110  are thus corrected, and the conversion gain Kv is constant with respect to changes in the capacitance control signal  302 . The lockup time and C/N characteristic of the PLL incorporating this voltage-controlled oscillator are also constant to the oscillation frequency, and a stable oscillation characteristic can be achieved. A high precision directly modulated oscillator can thus be achieved by applying the frequency control signal  305  to the voltage-controlled oscillator  210 . A mixer circuit is therefore unnecessary, and power consumption during transmission can be reduced.  
       Effect of the Second Embodiment  
       [0154]     The capacitance characteristic, frequency band characteristic, and temperature characteristic of the voltage-controlled oscillator  110 ,  210  are corrected, and a constant conversion gain Kv is achieved over a wide frequency band and temperature range. The lockup time and C/N characteristic of the PLL incorporating this voltage-controlled oscillator  110 ,  210  are also constant to the oscillation frequency, and a stable oscillation characteristic can be achieved.  
         [0155]     A high precision directly modulated oscillator can thus be achieved by applying the frequency control signal  305  to the voltage-controlled oscillator  110 ,  210 . A mixer circuit is therefore unnecessary, and power consumption during transmission can be reduced.  
         [0156]     An oscillator according to the present invention and a PLL circuit comprising this oscillator can be used in communication modules using a wireless or wired communication path, to communication equipment incorporating such communication modules, to cell phones and other portable terminals incorporating this communication module and to portable communication systems comprising such portable terminals.  
         [0157]     It will also be noted that the embodiments described above are used for illustration only, and the invention is not limited to these embodiments.  
         [0158]     The present invention can be used in oscillators and in PLL circuits that use such an oscillator.  
         [0159]     Although the present invention has been described in connection with the preferred embodiments thereof with reference to the accompanying drawings, it is to be noted that various changes and modifications will be apparent to those skilled in the art. Such changes and modifications are to be understood as included within the scope of the present invention as defined by the appended claims, unless they depart therefrom.