This paper presents a generalized field analysis method for calculating the external circuit impedances looking outside from the active device of a common millimeter wave broadband harmonic oscillator. A Set of numerical results of the external circuit impedances varying with the structure parameters are given, and then some useful conclusions for millimeter-wave harmonic oscillator design are derived. Based on the theoretical study, Wband broadband harmonic oscillators are developed. In the frequency range 75~100 GHz, power output of 2~22.5 mW is achieved.