Reflection Coefficient Shaping of a 5-GHz Voltage-Tuned Oscillator for Improved Tuning

Negative resistance voltage-controlled oscillators (VCOs) are systematically designed to operate with loaded resonator networks that permit stable steady-state oscillation over a specified tuning bandwidth. Circuit parasitics, however, significantly affect tuning behavior and complicate straightforward design. This paper introduces a scheme that compensates for the effect of parasitics by introducing an embedding network that modifies the effective active device reflection coefficient and thus enables conventional one-port oscillator design techniques to be used. A common-base SiGe HBT VCO operating from 4.4 to 5.5 GHz demonstrates the technique. Phase noise is better than -85 dBc/Hz at 10-kHz offset from the carrier and the second harmonic is less than -20 dBc, while higher order harmonics are less than -40 dBc. The voltage-tuned oscillator demonstrates an oscillator figure-of-merit of at least -182 dBc/Hz over a 800-MHz tuning range. The phase-noise-bandwidth (in megahertz) product is -159 dBc/Hz.