Thermal noise of microwave superconducting filter-amplifier combinations

Superconducting filters in radioastronomical receiver frontends at cm-wavelengths may eliminate nearby terrestrial interference signals since they offer very high frequency selectivity and extremely small insertion loss. Moreover, power in unwanted intermodulation signals is sufficiently small at the expected input levels. On the other hand thermal noise outside the passband of a filter-amplifier combination may affect the receiver performance. The overall thermal noise of superconducting filters with Butterworth or quasi-elliptic characteristic connected via a 50 Ω transmission line of finite length with a low noise amplifier is investigated. Arrangements with very low noise outside the passband are described. Simulated and measured thermal noise spectra are in reasonable agreement.