Tunable resonant circuit and voltage controlled oscillator using same

#Not Applicable

#Not Applicable

#Oscillators are used in a variety of electronic circuits, such as communication devices, to provide a variety of functions. These functions include, for example, the generation of a local oscillator signal. Radio communication devices, such as portable telephones and the like, require a stable operating frequency provided by the local oscillator. Typically, the output signal of the local oscillator is generated by a voltage controlled oscillator (VCO). It is often difficult to mass-produce such VCO circuits so that they all have the same basic operating frequency (i.e., the same relaxation frequency). This is due to the fact that the operating frequency is strongly dependent on parts tolerances and manufacturing techniques. In general, variation in component tolerance and parasitics as well as variation over temperature requires adjustment of the relaxation frequency in a post assembly process. More particularly, the resonant frequency of a tuned circuit is typically adjusted until the desired relaxation frequency is reached.

#The present invention is illustrated by way of example in the following drawings which disclose various embodiments of the present invention for purposes of illustration only and are not intended to limit the scope of the invention.

#FIG. 1A is a schematic diagram of a circuit that includes a tunable resonant circuit constructed in accordance with one embodiment of the present invention.

#FIG. 1B is a schematic diagram of a circuit that includes a tunable resonant circuit after it has been programmed to operate at a given resonant frequency.

#Although the tunable resonant circuit described herein can be used in a variety of different applications, the following discussion will principally focus on its use in the context of a voltage controlled oscillator (VCO). Given the teachings herein, adaptation of the tunable resonant circuit for use in other applications is within the scope of those skilled in the art.

#FIG. 1A illustrates a VCO, shown generally at 100, that includes one embodiment of a tunable resonant circuit constructed in accordance with the teachings of the present invention. In the embodiment of FIG. 1A, the VCO 100 is comprised of a first resonant circuit having a fixed resonant frequency, a second resonant circuit having a resonant frequency that varies in response to a tuning voltage input signal, and a frequency generator 105 that is responsive to resonant frequencies of the first and second resonant circuits. The first resonant circuit includes a plurality of capacitive circuits 110, 115, 120 that cooperate with an inductive element 125 to provide a circuit that is resonant at the desired relaxation frequency. The second resonant circuit includes, for example, a varactor 130 or other similarly variable capacitive element that likewise cooperates with the inductive element 125 to provide a circuit that is resonant at the desired tuning frequency. More particularly, the desired tuning frequency may be reached by providing a tuning voltage at input line 135 that effectively varies the capacitance of the varactor 130. This variation of the capacitance of the varactor 130, in turn, changes the resonant frequency of the second resonant circuit formed by the varactor 130 and inductive element 125. In the illustrated embodiment, the first and second resonant circuits share a common inductive element 125. However, other applications may indicate the use of separate inductive elements for generating higher-order filter responses.

#Each of the capacitive circuits 110, 115 and 120, include a capacitive element 140, 145 and 150 that is selectively connected to inductive element 125 to form the first resonant circuit. The capacitive elements 140, 145 and 150 may have the same capacitance value or different capacitance values depending on the particular application. Selective connection of the capacitive element 140, 145 and 150 is facilitated by a network of electrical elements respectively associated with each capacitive circuit 110, 115 and 120. In the embodiment of FIG. 1A, the network of electrical elements for each of the capacitive circuits 110, 115 and 120, includes voltage gate elements 155 through 180 and fuse elements 185 through 189. In the illustrated embodiment, the fuse elements 185 through 189 connect the capacitive elements 140 through 150, respectively, to a common electrical node with inductive element 125. The voltage gate elements 155 through 180 are each connected to receive a programming signal that is provided at input line 191. The voltage gate elements 155 through 180 are interconnected with the fuse elements 185 through 189 so that the fuse elements effectively blow or remain intact depending on the voltage level of the programming signal.