Frequency down conversion and AC Josephson effect in HTS ramp-type junctions on MgO substrates

We report on the characterization at high frequencies of DyBa/sub 2/Cu/sub 3/O/sub 7-x/ thin film ramp-type junctions with a PrBa/sub 2/Cu/sub 3/O/sub 7-x/ barrier fabricated on MgO substrates using a thin SrTiO/sub 3/ buffer layer. Josephson emission has been detected over a wide temperature range from 4.2 K to 85 K at millimeter wavelengths using a sensitive receiver. Measured spectrum characteristics, emitted power and linewidth are well described by theoretical models. Signal frequency down conversion has been carried out in two different modes using external and internal local oscillators. Wide band electronically tunable Josephson emission allowed us to achieve the frequency down conversion in a very sensitive self-oscillator-mixing mode up to 2.5 THz. For the first time, for any type of Josephson junction, self-oscillator mixing has been achieved at ambient temperatures as high as 80 K.