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The paper presents design optimization strategies and a comparison of the performance of SiGe HBT fundamental and push-push Colpitts and Colpitts-Clapp voltage-controlled oscillators (VCOs), with and without doublers and buffers, as possible solutions for efficient milliwatt-level, low-noise signal sources at submillimeter-wave frequencies. The fundamental frequency Colpitts VCO covers a 12% tuning range between 218 and 246 GHz (the highest for SiGe HBTs) with up to -3.6-dBm output power and 0.8% efficiency. The 300-GHz signal source, consisting of a Colpitts-Clapp VCO followed by a buffer amplifier and a doubler, shows -1.7-dBm output power around 290 GHz, -101-dBc/Hz phase noise at 10-MHz offset, 7.5% tuning range, and 0.4% efficiency. Finally, the push-push Colpitts-Clapp VCO exhibits the highest operation frequency, from 309 to 325 GHz, but with reduced efficiency of only 0.07% and 5% tuning range. It was concluded that the differential cascode buffer placed between the VCO and doubler was instrumental in achieving the best phase noise and output power with good efficiency and without compromising tuning range.