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Integrated varactors are becoming a common feature for many RF designs and in particular RF voltage controlled oscillators (VCOs). Optimization of the quality of both the inductor and the varactor from the VCO core is essential. This work details the characterization and optimization of a number of varactor types available on a typical submicron BiCMOS process. Engineering of the bottom plate of the varactor was used to optimize the quality factor of the varactor. No additional mask layers or processing steps were required to achieve this. Integrated isolated diode varactors with quality factors of 30 at 2 GHz have been demonstrated with tuning capacitance ranges of 2.5. Integrated MOS capacitor varactors with quality factors of 50 at 2 GHz have been demonstrated with tuning capacitance range of 5. A spice model for one of the varactor types is further developed in this paper. Accurate prediction of varactor performance over voltage bias and frequency was achieved.