Design of Low-Noise $K$ -Band SiGe Bipolar VCOs: Theory and Implementation

A study of K-band SiGe bipolar VCOs is reported in this paper. The design challenges related to the operation in the K-band and the use of a pure bipolar technology are discussed with particular emphasis to achieving low phase noise while using varactor diodes. Two different VCOs have been designed and fabricated. In the designs, the varactor is coupled to the active element by means of a magnetic transformer to avoid the use of tuning voltages exceeding the supply voltage. All the VCOs are operated in class-C. One of the designs features dynamic biasing to ensure robust start-up conditions. The VCOs feature a phase noise as low as -137 dBc/Hz at 10 MHz offset from the carrier. The VCOs show a state-of-the-art FOM of -189 dBc/Hz, and an excellent FOMT of -193 dB/Hz.