Substrate noise coupling effect characterization for RF CMOS LC VCOs

A generic, mixed-mode 0.18-/spl mu/m substrate is studied using a finite-element device simulator. The isolation properties of the device are extracted for guard ring structures of varying width. The effect of the substrate filtering on randomly switching noise is studied in the context of a 5-GHz CMOS LC-VCO's phase noise performance. Poor high frequency noise damping (/spl sim/ -20 dB) in the substrate due to inductive blocking is responsible for significant degradation of oscillator phase noise in simulation. Measurement results of wideband substrate noise effects on oscillator phase noise are included.