Low frequency noise in resonant Josephson soliton oscillators

The noise in the resonant soliton mode of long and narrow Josephson tunnel junctions (Josephson transmission lines or JTLs) have been measured in the frequency range from 0.1 Hz to 25 kHz by means of a DC SQUID. The measured white noise was found, to within a factor of two, to be equal to the Nyquist voltage noise in a resistance equal to the dynamic resistance R/sub D/ of the current-voltage characteristic of the bias point. In contrast, measurements of the linewidth of the microwave radiation from the same JTL showed that the spectral density of the underlying noise voltage scaled as R/sub D//sup 2//R/sub S/ where R/sub S/ is the static resistance. The origin of the different behavior is not known.