A 2-V 2.5-GHz-104-dBc/Hz at 100 kHz fully integrated VCO with wide-band low-noise automatic amplitude control loop

The automatic amplitude control (AAC) loop is an indispensable element for the practical realization of VCOs embedded in a complete transceiver. Its n oise however can unacceptably degrade the single-sideband-to-carrier ratio (SSCR) performance of the oscillator, this problem being even exacerbated i n low-voltage circuits. This paper addresses the design issues of a low-vol tage low-noise differential LC-VCO with AAC, tunable within a 2.3-2.8-GHz f requency range, fully integrated in bipolar technology with 2-V power suppl y. First, the mechanisms through which the AAC noise affects the output phase are identified as the poor indirect stability and the AM-to-PM conversion d ue to the varactors. The effect of the AAC noise is discussed and substanti ally reduced with suitable design choices. We show that the achievable nois e-to-signal ratio is bounded by the shot noise coming from the bias source of the differential oscillator, an intrinsic limit set by the low supply vo ltage which does not allow for degeneration of the tail transistor. Second, the design of the AAC is discussed. A large gain-bandwidth product (GBWP), about 100 MHz, has been implemented in order to correct for the fast oscil lation amplitude variations and reduce the effect of the ground line distur bances. The expected value of the phase noise level, SSCR at 100 kHz = -104 dBc/Hz, is tightly matched by the experimental results. The core oscillator dissip ates 7 mA, while less than 600 muA are drawn by the AAC circuit.