A fully-differential CMOS Clapp VCO for IEEE 802.11a applications

A fully integrated 5-6GHz differential VCO derived from the classical Clapp topology is presented in 180nm CMOS. The Clapp architecture and its characteristics are reviewed, and the location of critical portions of the circuit such as the LC-tank and varactors is discussed. The use of a Clapp VCO topology allows a larger voltage swing, which improves spectral purity and phase noise. The use of a symmetrical inductor for differential operation results in a higher Q with lower area, while providing common-mode noise rejection. The VCO achieves a simulated phase noise of -98dBc/Hz @ 100kHz & -123.1dBc/Hz @ 1MHz offsets, and draws 5mA from a 1.8V power supply. A high tuning range of 18% and FOM of 189dBc/Hz are reported. Finally, system-level simulations of a frequency synthesizer for the IEEE 802.11a bands using the Clapp VCO are presented