Patent ID: 6873214
Filing Date: 2005-03-29
Classification: H03L

Abstract:
1. A phase locked loop comprising an input, an output, a charge generator, a loop filter, an oscillator and a frequency divider, the charge generator having a first input coupled to the input of the phase locked loop and an output coupled to an input of the loop filter, the loop filter having an output coupled to the input of the oscillator, the oscillator having an output coupled to both an input of the frequency divider and the output of the phase locked loop, and the frequency divider having an output coupled to a second input of the charge generator, wherein: the frequency divider is arranged to output a frequency equal to the frequency output by the oscillator divided by a selectable divider ratio; the charge generator is arranged to output a charge dependent upon the difference in phase between the signals at the first and second inputs to the charge generator, the output of the charge generator being such that, when the phase locked loop is in lock, the first and second inputs to the charge generator have a substantially constant phase relationship; the loop filter has a first capacitor for integrating the charge produced by the charge generator, the capacitance of the first capacitor being selectable; the circuit comprises selection means for selecting the divider ratio and the capacitance of the first capacitor so that they are proportional to each other; and the frequency at the output of the oscillator is dependent on the voltage at the output of the loop filter.