  A normal metal - superconductor hybrid single-electron trap with tunable
barrier is utilized as a tool for spectrum analysis at the extremely low signal
levels, using only well filtered cryogenic microwave background as a photon
source in the frequency range from about 50 to 210 GHz. We probe millimeter
wave propagation in two superconducting systems: a Josephson junction array
around its plasma frequency, and a superconducting titanium film in the limit
when the photon energies are larger than the superconducting energy gap. This
regime is relevant for improving the performance of cryogenic quantum devices
but is hard to access with conventional techniques. We show that relatively
simple models can be used to describe the essential properties of the studied
components.
