This paper presents a promising design of an Xray tube with field emission cathode-modulator unit based on oxide emitter for soft X-ray radiation. Carbon in the form of graphite is used in 'through-target' anode coating. Since oxide cathodes have been characterized by multi-peak energy spectra of emitted electrons, a digital model is proposed to simulate corresponding field emission electron-optical system taking into account appropriate initial-boundary conditions in the emission domain. Modification of cathodoluminescent UV radiation source is discussed by replacing the electron beam target, which inner surface coated by a luminophore.