  We study the performance of an hot-electron bolometer (HEB) operating at THz
frequencies based on superconducting niobium nitride films. We report on the
voltage response of the detector over a large optical bandwidth carried out
with different THz sources. We show that the impulse response of the fully
packaged HEB at 7.5 K has a 3 dB cut-off around 2 GHz. Remarkably, detection
capability is still observed above 30 GHz in an heterodyne beating experiment
using a THz quantum cascade laser frequency comb. Additionally, the HEB
sensitivity has been evaluated and an optical noise equivalent power NEP of 0.8
pW/sqrt(Hz) has been measured at 1 MHz.
