A Varactor Model Including Avalanche Noise Source for VCOs Phase Noise Simulation

This work studies an important area of oscillator operation - VCO biased at varactor breakdown. In this operation, the oscillator's phase noise is primarily determined by avalanche noise generated in the varactor, i.e., the phase noise can be modulated by the incremental shot noise generated at varactor breakdown. A simple experimental diode model was optimized for modeling varactor-breakdown current and an empirical shot noise model based on Hine's theory presents good agreement with noise power measurements of varactors. The developed varactor noise-model has successfully been used to simulate the phase noise of a single-ended Colpitts InGaP HBT VCO over and beyond the varactor's breakdown voltage. The VCO presents a phase noise below - 100dBc/Hz for varactor voltages lower than breakdown voltage. Beyond the breakdown the phase noise gradually increases to - 60 dBc/Hz@100kHz with a centre frequency deviating less than 0.6% from a 7.38GHz carrier.