Patent ID: 7639092

Claim:
A method for tuning a frequency of a crystal oscillator, the method comprising: adjusting the tuning frequency of the crystal oscillator from a nominal frequency via a first switched-capacitor frequency tuning circuit, the crystal oscillator comprising an amplifier and a crystal and the first switched-capacitor frequency tuning circuit being coupled to a first node of the crystal, wherein the first switched-capacitor frequency tuning circuit comprises a plurality of switchable sections to adjust the tuning of the crystal oscillator, and wherein the plurality of switchable sections in the first switched-capacitor frequency tuning circuit comprises an analog control input and a digital control input; controlling the analog control input coupled to a node within each of the plurality of switchable sections, wherein each of the plurality of switchable sections in the first switched-capacitor frequency tuning circuit comprises a fixed capacitor in series with a varactor and a switch, and wherein the node within each of the plurality of switchable sections comprises a node to control the varactor within each of the plurality of switchable sections; and controlling the digital control input, wherein the digital control input is configured to electrically connect or disconnect one or more of the switchable sections from the first node of the crystal, wherein a capacitance for a switchable section i of the plurality of switchable sections comprises C section — i , wherein C section — i is equivalent to C fix — i *C var — i /(C fix — i +C var — i ), wherein i represents a positive number from 1 to n, wherein n represents a number of bits for the digital control input, wherein C fix — i represents a capacitance of the fixed capacitor in the switchable section i, and C var — i represents a variable capacitance range value of C var — i for the varactor in the switchable section i, wherein a total load capacitance for the switchable section i comprises C L — i , wherein C L — i is equivalent to k i *C section — i , wherein k i represents a positive numerical value for the switchable section i that is a function of the digitally-controlled input, wherein a total load capacitance of the first switched-capacitor frequency tuning circuit comprises C total , wherein C total is equivalent to Σ i=1 . . . n C L — i , and wherein C total is equivalent to Σ i=1 . . . n k i *C fix — i *C var — i /(C fix — i +C var — i ), wherein k i further represents an average on and off switching ratio for the switchable section i, and wherein the total load capacitance is scaled by a factor of (r n −1)/(r−1) for which the average on and off switching ratio is scaled higher by a factor of r for consecutive switchable sections in which r>1, or the total load capacitance is scaled by a factor of (1−r n )/(1−r) for which the average on and off switching ratio is scaled by a factor of r for consecutive switchable sections in which r<1.