Direct Measurement of the Input RF Noise of Superconducting Hot Electron Bolometer Receivers

A good understanding of input radio-frequency (RF) noise contribution is of particular importance to the development of high-sensitivity superconducting hot electron bolometer (HEB) receivers. In terms of simulation results based on the hot spot model, we found that the output noise of HEB mixers is nearly linearly decreased with the mixer's bath temperature when the HEB mixer is operated at its optimum DC-bias regime under appropriate local-oscillator signal pumping, and the effect of this decrease on the receiver output power can be well corrected. Thus, we used an intersecting lines technique to measure the input RF noise temperature of a quasi-optical superconducting NbN HEB receiver, and the measured input RF noise temperature of the HEB receiver is around 300 K at 850 GHz. For comparison, we also evaluated the losses and equivalent noise temperatures of the quasi-optical components in the RF signal path. The evaluated input RF noise temperature is found to be smaller than the measured one.