SPECTRUM OF THERMAL FLUCTUATION NOISE IN-DIFFUSION AND PHONON COOLED HOT-ELECTRON MIXERS

A systematic study of the intermediate frequency noise bandwidth of Nb thin-film superconducting hot-electron bolometers is presented. We ha ve measured the spectrum of the output noise as well as the conversion efficiency over a very broad intermediate frequency range (from 0.1 t o 7.5 GHz) for devices varying in length from 0.08 mu m to 3 mu m. Loc al oscillator and rf signals from 8 to 40 GHz were used. For a device of a given length, the spectrum of the output noise and the conversion efficiency behave similarly for intermediate frequencies less than th e gain bandwidth, in accordance with a simple thermal model for both t he mixing and thermal fluctuation noise. For higher intermediate frequ encies the conversion efficiency decreases; in contrast, the noise dec reases but has a second contribution which dominates at higher frequen cy. The noise bandwidth is larger than the gain bandwidth, and the mix er noise is low, between 120 and 530 K (double side band). (C) 1998 Am erican Institute of Physics.