Voltage controlled oscillator (VCO) is an integral part of IoT wireless transceiver components. In this paper, VCOs operating around 2.4 GHz have been designed in CMOS technology. The relation between their components and specifications is studied for their performance optimization. Ultra-low power, less than 270 µW, has been obtained, while performing a frequency tuning range of about 10% between 2.1 and 2.4 GHz. Investigations on phase noise performance have been also achieved.