Loop filter with noise cancellation

Abstract

A loop filter with noise cancellation includes first and second signal paths, an operational amplifier (op-amp), and a noise cancellation path. The first signal path provides a first transfer function (e.g., a lowpass response) for a first signal. The second signal path provides a second transfer function (e.g., an integration response) for a second signal. The second signal is a scaled version of, and smaller than, the first signal by a factor of alpha, where alpha is greater than one. A capacitor in the second signal path may be scaled smaller by a factor of alpha. The op-amp couples to the first and second signal paths and facilitates summing of signals from the first and second signal paths to generate a control signal having op-amp noise. The noise cancellation path couples to the op-amp and provides a noise cancellation signal used to cancel the op-amp noise in the control signal.

What is claimed is: 
     
         1 . An apparatus comprising:
 a loop filter comprising an operational amplifier (op-amp) and configured to generate a control signal having op-amp noise and to generate a noise cancellation signal used to cancel the op-amp noise in the control signal; and   a voltage-controlled oscillator (VCO) coupled to the loop filter and comprising at least one adjustable circuit element to vary an oscillating frequency of the VCO, the at least one adjustable circuit element being applied with the control signal and the noise cancellation signal from the loop filter.   
     
     
         2 . The apparatus of  claim 1 , wherein the loop filter is configured to receive first and second signals, to provide a first transfer function for the first signal, and to provide a second transfer function for the second signal. 
     
     
         3 . The apparatus of  claim 2 , wherein the first transfer function is a lowpass response, and wherein the second transfer function is an integration response. 
     
     
         4 . The apparatus of  claim 2 , wherein the second signal is a scaled version of the first signal and is smaller than the first signal by a factor of alpha, where alpha is greater than one, and wherein the loop filter comprises a capacitor used to obtain the second transfer function and scaled smaller by a factor of alpha. 
     
     
         5 . The apparatus of  claim 1 , further comprising:
 a charge pump comprising a first current source to provide the first signal and a second current source to provide the second signal, the second current source providing alpha times less current than the first current source, where alpha is greater than one.   
     
     
         6 . The apparatus of  claim 1 , wherein the at least one adjustable circuit element comprises at least one varactor. 
     
     
         7 . The apparatus of  claim 1 , further comprising:
 a divider configured to divide an oscillator signal from the VCO by an integer divider ratio and provide a feedback signal; and   a phase-frequency detector configured to determine phase error between the feedback signal and a reference signal.   
     
     
         8 . The apparatus of  claim 1 , further comprising:
 a divider configured to divide an oscillator signal from the VCO by a non-integer divider ratio and provide a feedback signal; and   a phase-frequency detector configured to determine phase error between the feedback signal and a reference signal.   
     
     
         9 . An integrated circuit comprising:
 a loop filter comprising an operational amplifier (op-amp) and configured to generate a control signal having op-amp noise and to generate a noise cancellation signal used to cancel the op-amp noise in the control signal; and   a voltage-controlled oscillator (VCO) coupled to the loop filter and comprising at least one adjustable circuit element to vary an oscillating frequency of the VCO, the at least one adjustable circuit element being applied with the control signal and the noise cancellation signal from the loop filter.   
     
     
         10 . The integrated circuit of  claim 9 , wherein the loop filter is configured to receive first and second signals, to provide a first transfer function for the first signal, and to provide a second transfer function for the second signal. 
     
     
         11 . The integrated circuit of  claim 10 , wherein the second signal is a scaled version of the first signal and is smaller than the first signal by a factor of alpha, where alpha is greater than one, and wherein the loop filter comprises a capacitor used to obtain the second transfer function and scaled smaller by a factor of alpha. 
     
     
         12 . A wireless device comprising:
 a loop filter comprising an operational amplifier (op-amp) and configured to generate a control signal having op-amp noise and to generate a noise cancellation signal used to cancel the op-amp noise in the control signal; and   a voltage-controlled oscillator (VCO) coupled to the loop filter and comprising at least one adjustable circuit element to vary an oscillating frequency of the VCO, the at least one adjustable circuit element being applied with the control signal and the noise cancellation signal from the loop filter.   
     
     
         13 . The wireless device of  claim 12 , wherein the VCO generates an oscillator signal used to derive clock signals for digital circuitry. 
     
     
         14 . The wireless device of  claim 12 , wherein the VCO generates an oscillator signal used for frequency upconversion or downconversion.