Differentially "bathtub"-tuned CMOS VCO using inductively coupled varactors

A new differential tuning concept for LC oscillator is introduced and demonstrated on a 4.5GHz CMOS VCO. It uses inductive coupling of varactors, giving highly linear differential tuning with good common-mode rejection. Only one type of varactor is used, allowing the designer to use the varactor-type with the highest C/sub max//C/sub min/ ratio and/or quality factor. It is shown that differential tuning offers a means to suppress the upconversion of common-mode bias noise, allowing for a significant power reduction in the VCO's bias circuitry. The realized VCO combines differential tuning constant K/sub VCO/ is 65MHz/V. The measured phase noise is lower than -124dBc/Hz at 1 MHz offset over the entire tuning range. The circuit, including bias, draws 5.5mA from a 2.5V supply.