A voltage controlled oscillator is provided. The voltage controlled oscillator includes a current controlled oscillator, a voltage to current conversion circuit and a noise cancellation circuit. The current controlled oscillator is configured to receive a bias current and generate an oscillating signal with an oscillating frequency according to the bias current. The voltage to current conversion circuit is coupled to a power supply voltage and configured to generate a supply current according to an input voltage. The noise cancellation circuit is configured to receive a bias voltage and the supply current from the voltage to current conversion circuit, and configured to generate a noise cancellation current in response to power supply voltage variation and cancel the noise cancellation current from the supply current to generate the bias current. The bias voltage of the noise cancellation circuit is coupled to an internal voltage of the voltage to current conversion circuit.

In an example, a voltage-controlled oscillator (VCO) includes: an oscillator having a supply input; and a voltage regulator, coupled to the supply input. The voltage regulator includes: a first transistor and a second transistor providing a first source-coupled transistor pair, and a third transistor and a fourth transistor providing a second source-coupled transistor pair; an active load coupled to drains of the first, second, third, and fourth transistors; a first current source coupled to sources of the first and second transistors, and a second current source coupled to sources of the third and fourth transistors; a fifth transistor having a source and a drain coupled to the source and the drain, respectively, of the first transistor; and a sixth transistor having a source and a drain coupled to the source and the drain, respectively, of the third transistor.

Methods, systems, and circuits for providing compensation for voltage variation are disclosed. A system includes: a voltage comparator configured to assert a control signal in response to detecting that one or more of power supply voltages droops below a threshold amount; a phase locked loop (PLL) configured to divide an output frequency for the PLL in response to the assertion of the control signal; a plurality of voltage sensors corresponding to the plurality of power supply voltages, the voltage sensors configured to output respective digital signals indicative of a voltage level of its corresponding power supply voltage; and a control circuit configured to control an oscillator frequency in the PLL during the open-loop mode responsive to the respective digital signals.

Methods, systems, and circuits for providing compensation for voltage variation are disclosed. A system includes: a voltage comparator configured to assert a control signal in response to detecting that one or more of power supply voltages droops below a threshold amount; a phase locked loop (PLL) configured to divide an output frequency for the PLL in response to the assertion of the control signal; a plurality of voltage sensors corresponding to the plurality of power supply voltages, the voltage sensors configured to output respective digital signals indicative of a voltage level of its corresponding power supply voltage; and a control circuit configured to control an oscillator frequency in the PLL during the open-loop mode responsive to the respective digital signals.