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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 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. Index Terms-Broad-band communications, phase noise, radiofrequency integrated circuits, SiGe, voltage-controlled oscillators (VCO). I. INTRODUCTIONR ECENTLY, there has been much interest in the prospect of combining voice-over-IP (VOIP) services with cable television and internet access. This means that consistent operation meeting Data-Over-Cable Service Interface Specification (DOCSIS) over a wide range of operating temperatures ( 40 C to 85 C) will be the driving specification for any telecommunications chips that will be used in these systems.This has led to much interest in doing highly integrated cable tuner receivers, capable of delivering high-quality 256 QAM demodulated signals to the baseband processor. Target specifications for phase noise in these systems is usually better than 100 dBc/Hz at 100-kHz offset. Thus, the voltage-controlled oscillator (VCO) in the system must be of good quality. Also, since cable is broadband (47-870 MHz), these VCOs must be tunable over a broad range (1.897-2.72 GHz for an intermediate frequency of 1.85 GHz) and, even so, several VCOs may be necessary to cover the whole range. Automatic-amplitude control is desired because without an adaptive loop it would be impossible for the designer to set the VCO current at an optimal level over all conditions [1]. In this brief, the design of two VCOs with two different approaches to this will be presented. II. GENERAL VCO DESIGN CONSIDERATIONSThe purpose of this design was to create a VCO with a wide tuning range, good phase noise, and very robust performance over process and temperature variations. The requirement for as Manuscript

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