Abstract: One aspect of the invention is directed to on-chip high-frequency, low-jitter clock circuits including long Josephson junction (LJJ) oscillators usable as clock sources. LJJ oscillators embodying the invention may be formed using either “linear” or “annular” long Josephson junctions. This invention enables the generation and distribution of a stable high-frequency on-chip single flux quantum (SFQ) clock. The on-chip clock circuit may include a clock selector circuit and a clock distribution scheme and may be integrated with RSFQ circuits and/or with a wideband analog-to-digital converter (ADC) comparator. The invention also includes a new fluxon “sender” circuit suitable for synchronizing the LJJ oscillator with another oscillator, either on-chip or external to the chip. The new sender circuit may also enable the realization of a novel phase-locked loop (PLL) circuit.