A study of digital and analog automatic-amplitude control circuitry for voltage-controlled oscillators

This paper presents both analog and digital automatic-amplitude control techniques for voltage-controlled oscillators (VCOs). These feedback mechanisms help to keep the VCOs at an optimum amplitude over temperature, process, and voltage variations. The VCOs were fabricated in a 50-GHz SiGe BiCMOS process. They use MOS varactors and achieve a 600-MHz tuning range in the 2-GHz band. The phase noise of the VCO with analog control was measured to be -99 dBc/Hz at 100-kHz offset from the carrier. The digital loop allows for a more optimized VCO core that achieves a phase noise of -108.5 dBc/Hz at 100-kHz offset in a low-gain mode. Techniques for suppressing the phase noise in regions of high gain are also presented. The VCOs draw between 4 and 8 mA from a 3.3-V supply.