Embodiments herein relate to an apparatus and method for calibrating a notch filter which filters a power supply signal for a voltage-controlled oscillator (VCO). In one aspect, a control circuit performs a number of calibration cycles for the filter to determine a value of a calibration code for the filter which minimizes a change in a frequency of the output signal of the VCO due to a change in the voltage of the power supply signal. After each calibration cycle, the calibration code is adjusted based on whether the frequency of the output signal increase or decreases. The calibration cycles can therefore converge on an optimal calibration code which minimizes the change in frequency due to the change in voltage. This minimizes a sensitivity of the VCO to noise in the power supply signal.

Embodiments herein relate to an apparatus and method for calibrating a notch filter which filters a power supply signal for a voltage-controlled oscillator (VCO). In one aspect, a control circuit performs a number of calibration cycles for the filter to determine a value of a calibration code for the filter which minimizes a change in a frequency of the output signal of the VCO due to a change in the voltage of the power supply signal. After each calibration cycle, the calibration code is adjusted based on whether the frequency of the output signal increase or decreases. The calibration cycles can therefore converge on an optimal calibration code which minimizes the change in frequency due to the change in voltage. This minimizes a sensitivity of the VCO to noise in the power supply signal.