Patent ID: 7199698

Claim:
A digitally-controlled reference oscillator that generates a reference signal with a reference frequency that corresponds to coarse and fine digital command signals, the oscillator comprising: an amplifier having an input port and configured to provide said reference signal with a frequency that corresponds to varactor capacitance across said input port; a crystal coupled across said input port; M varactor systems that each includes N sets of varactor transistors wherein each set has a tuning terminal and capacitor terminals and wherein: in each of M-1 of said varactor systems, the tuning terminals of all sets are coupled to receive a respective one of coarse tuning potentials and the capacitance terminals of all sets are coupled across said input port to provide a coarse varactor capacitance that is a function of said respective coarse tuning potential; and in a remaining one of said varactor systems, the tuning terminal of each of said sets is coupled to receive a respective one of fine tuning potentials and the capacitance terminals of all sets are coupled across said input port to provide a fine varactor capacitance that is a function of said respective fine tuning potential; a coarse decoder that, in response to successive states of said coarse digital command signal, steps the coarse tuning potential of successive ones of said M-1 varactor systems from a maximum analog value to a minimum analog value to thereby successively add coarse varactor capacitances across said input port; N digital-to-analog converters that each provide a respective one of said fine tuning potentials; and a fine decoder that, in response to successive first states of said fine digital command signal, enables successive ones of said converters so that each of said converters, in response to successive second states of said fine digital command signal, successively increases the fine tuning potential of a respective one of said sets along a transfer-function shape from said maximum analog value to said minimum analog value to thereby successively increase a fine varactor capacitance across said input port; the capacitance across said input port thereby monotonically increasing in a substantially-linear progression.