1. Field of the Invention
The present invention relates to frequency modulators, more particularly, a frequency modulator and a frequency modulating method for performing frequency modulation using a PLL (Phase Locked Loop), and a wireless circuit using the frequency modulator and the frequency modulating method.
2. Description of the Background Art
As exemplary methods for performing frequency modulation using a PLL, the following two methods are known. In a first method, a reference signal source is subjected to frequency modulation. In a second method, a modulation signal is supplied to a frequency control terminal of a voltage controlled oscillator. In general, the modulation signal used in digital communications, etc., has flat frequency characteristics from low to high frequencies within a predetermined band. Thus, in the case where frequency modulation is performed based on such a modulation signal, flat characteristics have to be obtained when the modulation signal spans over a range of low to high frequencies.
The above-described first method, however, requires a reference signal source having a frequency modulation function, and makes it difficult to modulate over a frequency band wider than a PLL loop bandwidth. In this case, a widened PLL loop bandwidth enables modulation over a wide range of frequencies. However, the widened loop bandwidth generally deteriorates a C/N (Carrier to Noise Ratio) obtained at a point away from zero by a width corresponding to the loop bandwidth. Thus, a PLL does not allow its loop bandwidth to be sufficiently widened.
On the other hand, the above-described second method makes it difficult to modulate over a frequency lower than the PLL loop bandwidth. In this case, a narrowed PLL loop bandwidth enables modulation over a lower frequency band. However, the narrowed loop bandwidth generally slows the loop response speed of the PLL. Thus, a PLL does not also allow its loop bandwidth to be sufficiently narrowed.
In order to solve the above-described problems, a third method that is a combined adoption of the above-described first and second methods is known. FIG. 13 is a block diagram illustrating the structure of a conventional frequency modulator using the third method. The frequency modulator shown in FIG. 13 includes a voltage controlled oscillator 1, a variable frequency divider 2, a phase comparator 3, a loop filter 4, and a reference signal source 10. The reference signal source 10 generates a reference signal with a predetermined frequency. The voltage controlled oscillator 1, the variable frequency divider 2, the phase comparator 3, and the loop filter 4 form the PLL, which will be described below.
The variable frequency divider 2 divides the frequency of an output signal of the voltage controlled oscillator 1 based on provided frequency division factor data M. The phase comparator 3 compares the phases of an output signal of the reference signal source 10 and the output signal of the variable frequency divider 2. The output signal of the phase comparator 3 is inputted into the voltage controlled oscillator 1 after passing through the loop filter 4. The voltage controlled oscillator 1 oscillates at a frequency appropriate to the output signal of the loop filter 4. This PLL performs feedback control so as to keep a center frequency of the output signal of the voltage controlled oscillator 1 at a predetermined value. The reference signal source 10 and the voltage controlled oscillator 1 are each provided with a frequency modulation terminal, to which an analog modulation signal Xa is supplied.
FIG. 14 is an illustration showing frequency modulation characteristics of the frequency modulator shown in FIG. 13. In FIG. 14, low-pass characteristics shown in solid line represent the degree of modulation of the output signal against a modulation signal supplied to the reference signal source 10, and high-pass characteristics shown in dashed line represent the degree of modulation of the output signal against a modulation signal supplied to the voltage controlled oscillator 1. In this case, it is possible to obtain frequency modulation characteristics that are flat over a wide range of frequencies by adding the low-pass characteristics (solid line) and the high-pass characteristics (dashed line). Thus, even if a bandwidth (shaded portion) of a modulation signal is wider than a PLL loop bandwidth, it is possible to obtain good frequency modulation characteristics.
However, the frequency modulator using the above-described third method has to be supplied with an analog modulation signal, which results in the need of a high-precision D/A converter that converts digital modulation data into the analog modulation signal. Furthermore, the modulation signal is supplied to both the reference signal source and the voltage controlled oscillator, which results in the need of individual adjustment of the level of the modulation signal in both the reference signal source and the voltage controlled oscillator in order to obtain good frequency modulation characteristics.