A 2-V 2.5-GHz – 104-dBc/Hz at 100kHz 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 noise however can unacceptably degrade the single-sideband-to-carrier ratio (SSCR) performance of the oscillator, this problem being even exacerbated in low-voltage circuits. This paper addresses the design issues of a low-voltage low-noise differential LC-VCO with AAC, tunable within a 2.3-2.8-GHz frequency range, fully integrated in bipolar technology with 2-V power supply. 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 due to the varactors. The effect of the AAC noise is discussed and substantially reduced with suitable design choices. We show that the achievable noise-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 voltage 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 oscillation amplitude variations and reduce the effect of the ground line disturbances. 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 dissipates 7 mA, while less than 600 μA are drawn by the AAC circuit.