  We describe a new time-of-flight Fourier spectrometer for investigation of
UCN diffraction by a moving grating. The device operates in the regime of a
discrete set of modulation frequencies. The results of the first experiments
show that the spectrometer may be used for obtaining UCN energy spectra in the
energy range of 60$\div$200 neV with a resolution of about 5 neV. The accuracy
of determination of the line position was estimated to be several units of
$10^{-10}$ eV
