Electromagnetic susceptibility analysis of phase noise in VCOs

Voltage controlled oscillators (VCOs) are one of the most sensitive circuit block to electromagnetic noise. The phase noise of VCOs depends on the time when the noise is injected, so it has time-variant nature to electromagnetic interference (EMI). In this paper, we apply the phase noise theory in analyzing the electromagnetic susceptibility (EMS) of phase noise in VCOs in presence of noise in power supply lines. For two most common types of VCOs, ring and LC, impulse sensitivity function (ISF) when EMI is injected on the supply voltage node, is derived from netlist including the actual layout parasitics. The ISF comparisons show that LC VCO has better EM noise immunity than ring VCO. The EMS prediction from ISF is validated from eye diagram analysis using SPICE simulation.