  Ensembles of nitrogen-vacancy (NV) centers are used as sensors to detect NMR
signals from micron-sized samples at room temperature. In this scenario, the
regime of large magnetic fields is especially interesting as it leads to a
large nuclear thermal polarisation -- thus, to a strong sensor response even in
low concentration samples -- while chemical shifts and J-couplings become more
accessible. Nevertheless, this regime remains largely unexplored owing to the
difficulties to couple NV-based sensors with high-frequency nuclear signals. In
this work, we circumvent this problem with a method that maps the relevant
energy shifts in the amplitude of an induced nuclear spin signal that is
subsequently transferred to the sensor. This stage is interspersed with
free-precession periods of the sample nuclear spins where the sensor does not
participate. Thus, our method leads to high spectral resolutions ultimately
limited by the coherence of the nuclear spin signal.
