A promising new source to generate a periodic series of coherent, ultra-short pulses in the millimeter and submillimeter frequency range is based on the method of passive mode locking . The basic principle is well known from laser physics . A realization for millimeter and sub-millimeter waves consists of an amplifier and a saturable absorber coupled in a feedback loop. In this paper, a coupling system for two single-window vacuum electron tubes in a mode-locked microwave oscillator is presented. Based on full-wave simulations, the key components of the proposed feedback system at 263 GHz (typical DNP-NMR frequency) are designed.