Abstract: Disclosed herein is a fine capacitance tuning circuit for a digitally controlled oscillator. The tuning circuit has low and high frequency tuning banks formed by varactors that have their top plates connected to one another. A controller initially sets states of switches selectively connecting the bottom plates of the varactors of the low frequency bank to a low voltage, a high voltage, or to an RC filter, in response to an integer portion of a control word. A sigma-delta modulator initially sets the states of switches selectively connecting the bottom plates of the varactors of the high frequency bank to either the low voltage or the high voltage, in response to a fractional portion of the control word. The controller modifies the states of the switches of the tuning banks in a complementary fashion, based upon comparisons between the fractional portion of the control word and a series of thresholds.

Abstract: Disclosed herein is a fine capacitance tuning circuit for a digitally controlled oscillator. The tuning circuit has low and high frequency tuning banks formed by varactors that have their top plates connected to one another. A controller initially sets states of switches selectively connecting the bottom plates of the varactors of the low frequency bank to a low voltage, a high voltage, or to an RC filter, in response to an integer portion of a control word. A sigma-delta modulator initially sets the states of switches selectively connecting the bottom plates of the varactors of the high frequency bank to either the low voltage or the high voltage, in response to a fractional portion of the control word. The controller modifies the states of the switches of the tuning banks in a complementary fashion, based upon comparisons between the fractional portion of the control word and a series of thresholds.