Patent ID: 7047146

Claim:
A method for automatically calibrating the frequency range of a phase lock loop (PLL), the method comprising: providing a loop filter for accumulating charge to generate a loop-filter voltage; providing a voltage controlled oscillator (VCO) having a plurality of frequency ranges, the VCO receiving the loop-filter voltage and generating an output signal having a frequency according to the loop-filter voltage and a currently selected VCO frequency range; connecting an input of the loop filter to a constant voltage; and selecting an optimal VCO frequency range by comparing the frequency of a PLL feedback signal for a plurality of the VCO frequency ranges with the frequency of a reference signal, the PLL feedback signal being generated according to the VCO output signal; wherein the optimal VCO frequency range comprises either a first VCO frequency range or an adjacent second VCO frequency range such that the frequency of the PLL feedback signal is faster than the frequency of the reference signal for the first VCO frequency range and the frequency of the PLL feedback signal is slower than the frequency of the reference signal for the second VCO frequency range; selecting the optimal VCO frequency range further comprises synchronizing the PLL feedback signal with the reference signal; the input of the loop filter is connected to the medium voltage and selecting the optimal VCO frequency range further comprises conducting a binary search starting from a middle VCO frequency range and proceeding until the optimal VCO frequency range is found; and the optimal VCO frequency range comprises the first VCO frequency range when the time duration between the second rising edges of the reference signal and the PLL feedback signal for the first VCO frequency range is shorter than that of the second VCO frequency range, otherwise comprises the second VCO frequency range when the time duration between the second rising edges of the reference signal and the PLL feedback signal for the second VCO frequency range is shorter than that of the first VCO frequency range.