Linewidth of a resistively shunted high-temperature-superconductor Josephson heterodyne oscillator

High-temperature-superconducting (HTS) resistively shunted superconducting quantum interference devices, consisting of two yttrium-barium-copper-oxide (YBCO) grain boundary junctions shunted by gold-film resistors of similar to 20 mu Omega, have been prepared and characterized at temperatures from 1 7 to 74 K. The small value and stability of the resistor allows narrow-band heterodyne oscillations to be generated between the Josephson oscillations in the individual junctions. The frequency can be precisely controlled in accordance with the ac Josephson effect by adjustment of an external curren t applied to the YBCO-Au-YBCO shunt resistor, and the linewidth is determin ed in principle only by Johnson noise in the resistor. Off-chip detection a nd spectral analysis of the heterodyne signal generated in this way has not been previously reported. Measurements of the signal power (similar to nW) , the tunability of the center frequency from 5 to 50 MHz, the frequency st ability and the linewidth of the heterodyne Josephson oscillation are descr ibed. Although linewidth broadening due to nonthermal fluctuations is obser ved, the linewidth of similar to 30 kHz at T = 17 K, at a center frequency of 24 MHz, is believed to be the narrowest reported for Josephson oscillati ons in an HTS device of this type. (C) 2000 American Institute of Physics. [S0003-6951(00)04113-9].