We review the studies of phase stabilization of a pulsed relativistic backward-wave oscillator (BWO) excited by the feed voltage with a steep front. Results of radiation phase stabilization are compared with the results of in-phase excitation of two independent nanosecond relativistic microwave backward-wave oscillators of the 8-mm wavelength range. Stable and controlled (by correcting the voltage front) synchronization of two channels with identical high-current electron beams is demonstrated for the case of generation power of up to 230 MW and a pulse duration of up to 100 oscillation periods in each beam.