1. A frequency modulator comprising: a digitally-controlled oscillator (DCO), arranged for producing a frequency deviation in response to a modulation tuning word and a phase-locked loop (PLL) tuning word, wherein said PLL tuning word comprises an integer part and a fractional part, and at least one of said integer part and said fractional part is obtained through asynchronous samplings; wherein said modulation tuning word comprises an integer part and a fractional part, and said fractional part of said modulation tuning word is obtained through asynchronous sampling.
5. The frequency modulator of claim 1 , wherein said modulation tuning word is transmitted to said DCO via a first path, said PLL tuning word is transmitted to said DCO via a second path, and said first path and said second path are merged in a capacitive domain of said DCO.