A frequency synthesizer is described that includes: a voltage controlled oscillator, VCO; a VCO bias circuit, operably coupled to the VCO and configured to provide a controllable bias current of the VCO; a temperature sensor, located in the frequency synthesizer, configured to determine an operating temperature of the frequency synthesizer; an analog-to-digital converter, ADC, operably coupled to the temperature sensor and configured to provide a digital representation of the determined operating temperature; and a bias control circuit operably coupled and configured to provide a bias control signal to the VCO bias circuit based on the determined operating temperature of the frequency synthesizer. The VCO bias circuit is configured to adjust the controllable bias current applied to the VCO based on the bias control signal.