Patent ID: 8120394
Filing Date: 2012-02-21
Classification: H03L

Abstract:
1. An automatic frequency calibration circuit for a frequency synthesizer, the frequency synthesizer having a voltage-controlled oscillator (VCO) and a frequency dividing unit, wherein the VCO has a plurality of capacitor configurations, the automatic frequency calibration circuit comprising: a first frequency detecting unit, detecting a reference frequency; a second frequency detecting unit, detecting an output frequency of the frequency dividing unit; a comparator, receiving and comparing outputs of the first frequency detecting unit and the second frequency detecting unit, and outputting a comparison result; a judging interval unit, defining at least one judging period in a total comparison time, and enabling the comparator to output the comparison result during the judging period; and a state machine, selecting one of the capacitor configurations to set the VCO in a calibration mode, modifying a fractional part from a target division ratio for obtaining a modified fraction according to a precision, and combining the modified fraction with an integer part of the target division ratio to set a division ratio of the frequency dividing unit, wherein the state machine selects another one of the capacitor configurations to set the VCO when the comparison result shows that the reference frequency does not match the output frequency of the frequency dividing unit.