Soliton oscillators enable the generation of short-time pulses, applicable in sampling of fast signals or as probe signals in radar systems. The soliton oscillator is made up of a feedback loop containing an amplifier stage and a nonlinear transmission line. In this paper, an efficient technique for the harmonic balance design of these oscillators is presented. The bifurcation behavior of this kind of oscillators is analyzed in depth, with a detailed study of their oscillation modes. The sources of non ideality of the soliton-train waveform have also been investigated. The techniques have been applied to a first prototype at 1 GHz with very good agreement with the measured results