1. Field of the Invention
The present invention relates generally to a frequency modulator, and in particular, to a digital frequency modulator for accumulating input digital signals and changing the phase of the accumulated signal.
2. Description of the Related Art
In general, a wireless communication system transmits on a radio channel a relatively high-frequency carrier with a relatively low-frequency input signal. This is called modulation. In particular, when the information of the input signal is contained in the frequency and phase of the carrier signal, the modulation is called frequency modulation (FM).
An analog frequency modulator illustrated in FIG. 1 is usually used as a frequency modulator. The analog frequency modulator is comprised of a gain controller 110, a digital-to-analog converter (DAC) 120, an FM filter 130, a voltage controlled oscillator (VCO) 150, and a frequency synthesizer 140.
Referring to FIG. 1, the gain controller 110 multiplies an input digital signal Vin by a predetermined gain G. The DAC 120 converts the output of the gain controller 110 to an analog signal. The FM filter 130 filters the analog signal and feeds the filtered signal as a voltage control signal to the VCO 150. Meanwhile, the frequency synthesizer 140 generates a carrier signal with a predetermined carrier frequency. The VCO 150 controls the frequency of the carrier signal received from the frequency synthesizer 140 with the voltage control signal, thereby outputting an FM signal.
As described above, the analog frequency modulator effects FM by converting an input digital signal to an analog signal and then controlling the VCO 150 with the analog signal.
A distinctive shortcoming of the typical analog frequency modulator is that the VCO being an analog device makes performance optimization difficult due to performance variation and use of additional analog devices for stabilizing the characteristics of the VCO increases cost. The VCO has a non-linear frequency output characteristic with respect to an input signal and exhibits very different characteristics depending on devices. Thus to achieve desirable FM characteristics, compensation or adjustment is required for the VCO. As a solution to this problem, implementation of a digital frequency modulator has been considered. However, the digital frequency modulator also has the limitations of difficulty in achieving desirable resolution with respect to various frequency inputs and implementation complexity.