Linewidth measurements and phase locking of Josephson oscillatorsusing RSFQ circuits

We present a technique for linewidth measurement and phase-locking of Josephson oscillators using digital rapid single-flux-quantum (RSFQ) circuits. The oscillator consists of a resistively shunted 6 μm×6 μm Nb/AlO_x/Nb Josephson tunnel junction that is integrated with RSFQ input and output circuits. A cascade of RSFQ T flip-flops is used to directly monitor the output of the Josephson oscillator. Spectral characteristics have been measured directly for oscillator frequencies ranging from 10-50 GHz. The linewidth can be reduced by over 100 times by phase-locking the oscillator to an RSFQ pulse train generated by an external sinusoidal signal. These Josephson oscillators can be used as on-chip stable high frequency clocks for RSFQ circuits