1.  Field of the Invention
The present invention relates to an FM modulator, and more particularly to an FM modulator using a voltage controlled oscillator. 
2.  Description of the Background Art
FIG.  17 is a diagram illustrating a structure of a conventional analog FM modulator.  As shown in FIG.  17, the FM modulator 90 comprises a reference signal generator 91, a control circuit 92, and a voltage controlled oscillator 93 (hereinafter, referred to as a VCO 93).  The control circuit 92 includes a phase comparator 911, a low pass filter (hereinafter, referred to as an LPF) 912, an adder 913, a frequency divider 914, a VCO gain correction section 915, and multipliers 916 and 917. 
With reference to FIG.  17, the control circuit 92 and the VCO 93 constitutes a phase locked loop circuit (hereinafter, referred to a PLL circuit).  The PLL circuit detects for a phase difference between a reference signal and a signal obtained by frequency-dividing an output signal outputted by the VCO so as to synchronize a phase of the reference signal and a phase of the signal obtained by frequency-dividing the output signal outputted by the VCO, thereby locking a phase of the output signal outputted by the VCO. 
The VCO 93 outputs the output signal in accordance with a control voltage applied to a VCO control terminal.  A relationship between the control voltage and the output signal outputted by the VCO in accordance with the control voltage is referred to as an f-V characteristic.  The VCO 93 shown in FIG.  17 receives, from the VCO gain correction section 915, a correction signal used for correcting a nonlinear f-V characteristic of the VCO 93, so as to perform an output equivalent to that from a VCO having a linear characteristic. 
The phase comparator 911 compares a phase of an output signal from the multiplier 917 with a phase of an output signal from the frequency divider 914, thereby outputting a pulse signal based on a comparison result.  Specifically, the phase comparator 911 detects for a phase difference between a signal outputted by the multiplier 917 and a signal outputted by the frequency divider 914, thereby outputting a pulse signal having a pulse width corresponding to the time difference. 
The LPF 912 eliminates a high frequency component from the pulse signal outputted by the phase comparator 911 by using, for example, an integration and averaging, so as to output only a DC component.  Through this operation, the LPF 912 is capable of converting the pulse signal into a DC signal based on a magnitude of a phase difference.  The DC signal obtained through the conversion is outputted to the VCO gain correction section 915 through the adder 913. 
When the f-V characteristic of the VCO 93 is nonlinear, the VCO gain correction section 915 outputs a corrected signal so as to obtain an output equivalent to that from a VCO having a linear characteristic.  In order to output the corrected signal, the VCO gain correction section 915 stores, for example, a correction table in which the control voltages applied to the VCO control terminal are associated with the output signals from the VCO 93, respectively. 
The output signal from the VCO 93 is inputted to the multiplier 916 and multiplied by a modulation signal which is inputted to the FM modulator 90 from outside thereof, and a signal obtained through the multiplication is inputted to the frequency-divider 914.  The frequency-divider 914 subjects a frequency of the signal having been received to 1÷N frequency division, and outputs a frequency-divided signal.  Hereinafter, N is referred to as a frequency dividing ratio.  Further, a signal obtained by adding channel data to the modulation signal is inputted to the frequency divider 914.  The frequency divider 914 determines the frequency dividing ratio based on the channel data contained in the added signal having been received.  A frequency of a carrier wave outputted by the FM modulator 90 shown in FIG.  17 is determined based on a product of the frequency dividing ratio used by the frequency divider 914 and a frequency of the reference signal generated by the reference signal generator 91.  For example, when the frequency of the reference signal generated by the reference signal generator 91 is 100 kHz and the frequency dividing ratio N is 50, the frequency of the carrier wave is determined as 100 kHz×50=5.0 MHz. 
The PLL circuit shown in FIG.  17 feeds back the output signal from the VCO 93 to the control circuit 92 via the frequency divider 914 so as to reduce, to almost zero, the phase difference between the output signal from the VCO 93 and the output signal from the multiplier 917, thereby enabling synchronization between the modulation signal inputted to the FM modulator 90 from outside thereof and the output signal from the VCO 93. 
The FM modulation described with reference to FIG.  17 refers to a change of a frequency of the reference signal in accordance with a frequency signal inputted to the FM modulator from outside thereof.  Specifically, the FM modulation is realized by applying, to the VCO control terminal, the control voltage which varies based on a certain voltage as time passes.  In FIG.  17, a plurality of modulation signals are simultaneously inputted to the FM modulator 90 from outside thereof so as to obtain low-frequency to high-frequency modulation characteristics of the PLL circuit including the control circuit 92 and the VCO 93.  The modulation signals are inputted to the multiplier 917, the frequency divider 914, and the multiplier 916 so as to obtain the low frequency modulation characteristic.  On the other hand, the modulation signal is inputted to the adder 913 so as to obtain the high frequency modulation characteristic. 
FIG.  18 is a diagram illustrating an FM modulation performed by the VCO 93.  In FIG.  18, a straight line C1 represents a linear characteristic of a VCO.  That is, a value obtained by dividing the VCO output frequency by the VCO control terminal voltage is constant.  A curved line C2 represents a nonlinear characteristic of the VCO 93.  A curved line S1 represents the control voltage applied to the VCO control terminal of the VCO 93.  The curved line S2 represents an output, corresponding to the curved line S1, obtained by the VCO having the characteristic represented by C1.  The curved line S3 represents an output, corresponding to the curved line S1, obtained by the VCO 93 having the characteristic represented by C2.  In FIG.  18, V[V] represents the control voltage applied to the VCO control terminal, and f[Hz] represents an oscillation frequency of the VCO.  Vcc [V] represents an upper limit value of the control voltage applied to the VCO. 
When the f-V characteristic of the VCO 93 is a nonlinear one as represented by the curved line C2, the output corresponding to the control voltage S1 applied to the VCO 93 has an unfavorable wave form as represented by the curved line S3.  The unfavorable output degrades the characteristic of the FM modulator 90. 
FIG.  19 is a diagram illustrating a correction method performed by the VCO gain correction section 915.  In a conventional art shown in FIG.  17, when the VCO 93 is powered on, the VCO gain correction section 915 applies, to the VCO 93, the control voltages of 0[V] to Vcc [V], that is, all voltages which can be applied to the VCO 93.  The control voltages are sequentially applied in accordance with a value obtained by dividing, into a plurality of portions, the range of the voltages which can be applied to the VCO 93.  Next, the VCO gain correction section 915 stores, in the correction table thereof, the output signals from the VCO 93 corresponding to the respective applied voltages.  The curved line C4 shown in FIG.  19 represents the measured f-V characteristic of the VCO 93. 
When the storage of the control voltages and the output signals from the VCO 93 has been completed, the VCO gain correction section 915 derives an equation representing a linear characteristic so as to output the corrected signal.  The equation is derived based on equation (1).f=A×V+B  (1)where f represents a VCO output frequency [Hz], V represents a VCO control terminal voltage [V], and each of A and B is a constant. 
The VCO gain correction section 915 solves simultaneous equations so as to derive the equation representing the linear characteristic.  The simultaneous equations consist of an equation obtained by substituting, into equation 1, a VCO oscillation frequency obtained when the VCO control terminal voltage is 0 [V], and an equation obtained by substituting, into equation 1, the VCO oscillation frequency obtained when the VCO control terminal voltage is Vcc[V]. 
The VCO gain correction section 915 solves the obtained two equations as simultaneous equations so as to calculate the constants represented as A and B in equation 1, thereby deriving the equation representing a linear characteristic.  In FIG.  19, a curved line S4 represents a control voltage, a curved line S5 represents a desired frequency signal that corresponds to curved line S4, and a straight line C3 represents the linear characteristic obtained from the derived equation.  When the FM modulation is started, the VCO gain correction section 915 starts the correction operation.  At this time, a voltage represented as V2 is applied to the VCO, and the VCO gain correction section 915 substitutes the voltage V2 into equation 1 having constant A and B having been calculated, thereby obtaining a frequency f2 to be outputted by the VCO 93.  Next, the VCO gain correction section 915 finds the control voltage V1 corresponding to the frequency f2 using the curved line C4 shown in FIG.  19 representing the measured f-V characteristic of the VCO 93, thereby applying the control voltage V1 to the VCO 93.  Through the aforementioned operation, the VCO 93 is capable of performing an output equivalent to that performed by a VCO having a linear f-V characteristic. 
The aforementioned method in which a characteristic of a voltage controlled oscillator is measured when the voltage controlled oscillator is powered on, and the correction table is created based on the characteristic having been measured, and an output from the voltage controlled oscillator is corrected using the created correction table, is disclosed in Japanese Laid-Open Patent Publication No.  10-115677 and Japanese Laid-Open Utility Model Publication No.  5-25810. 
However, in a conventional art, when the VCO 93 is powered on, the control voltages corresponding to all the voltages which can be applied to the VCO 93 are applied to the VCO 93 so as to measure the f-V characteristic of the VCO 93.  Therefore, the f-V characteristic is measured over a wide voltage range, so that a time period required for measuring the characteristic is extended.