In the last few years, the operating modes of Gunn oscillators for frequencies above 60 GHz have been discussed controversially. In this context, a general theoretical circuit model for oscillators operating in the fundamental and in the second-harmonic modes is studied. The model employs a simple cubic I - V characteristic of the active element and separate embedding circuits for the fundamental and second-harmonic frequencies. The current and voltage waveforms of both modes are contrasted. The oscillator source impedances and the external Q of the second-harmonic mode oscillator are calculated.