  We have built a vacuum double crystal spectrometer, which coupled to an
electron-cyclotron resonance ion source, allows to measure low-energy x-ray
transitions in highly-charged ions with accuracies of the order of a few parts
per million. We describe in detail the instrument and its performances.
Furthermore, we present a few spectra of transitions in Ar$^{14+}$, Ar$^{15+}$
and Ar$^{16+}$. We have developed an ab initio simulation code that allows us
to obtain accurate line profiles. It can reproduce experimental spectra with
unprecedented accuracy. The quality of the profiles allows the direct
determination of line width.
