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This brief characterizes one- and two-port passive networks from the viewpoint of oscillator design. A linear circuit theory derives the spectral spread due to RF noise from active devices. The formulation finds a frequency derivative function of logarithmic immittance that plays the key role of dominating the spectrum behavior. This leads to an explicit definition of the Q factor by referring to Leeson's basic oscillator model. The derived definition can provide an effective criterion for circuit topology and parameter optimization in low-noise oscillator design.