Patent ID: 7706495

Claim:
A frequency modulation apparatus comprising: a Phase Locked Loop (PLL) circuit; a frequency divider that is provided in the PLL circuit and determines a frequency division ratio in the PLL circuit based on a digital baseband modulation signal and a carrier frequency signal; an adder that is provided in the PLL circuit between a loop filter and a voltage controlled oscillator, inputs an output signal of the loop filter to a first adder input terminal, inputs a signal obtained by analog converting the digital baseband modulation signal, to a second adder input terminal, and outputs an added output to the voltage controlled oscillator; and a noise shaper that includes: a differentiator that differentiates the digital baseband modulation signal; a quantizer that is provided after the differentiator and outputs a signal obtained by quantizing an input signal; a feedback circuit that is provided parallel to the quantizer after the differentiator and inputs a signal obtained by adding the signal outputted from the quantizer and a signal outputted from the differentiator, to the quantizer; the differentiator, quantizer and feedback circuit cooperating to generate an analog signal, which is provided to the second adder input terminal, comprising the sum of: (1) the quantization noise, produced by the quantizer, multiplied by an integration transfer function whereby the quantization noise increases infinitely at or near a frequency zero and the quantization noise is attenuated in higher frequency domain, (2) the digital base band modulation signal, after conversion to analog form; and a post filter that is provided between the noise shaper and the adder and allows to pass only a low frequency component of the analog voltage signal outputted from the noise shaper.