  The quantum diamond microscope (QDM) is a recently developed technology for
near-field imaging of magnetic fields with micron-scale spatial resolution. In
the present work, we integrate a QDM with a narrowband measurement protocol and
a lock-in camera; and demonstrate imaging of radiofrequency (RF) magnetic field
patterns produced by microcoils, with spectral resolution $\approx1$\,Hz. This
RF-QDM provides multi-frequency imaging with a central detection frequency that
is easily tunable over the MHz-scale, allowing spatial discrimination of both
crowded spectral peaks and spectrally well-separated signals. The present
instrument has spatial resolution $\approx2\,\mathrm{\mu m}$, field-of-view
$\approx300\times300\,\mathrm{\mu m^2}$, and per-pixel sensitivity to
narrowband fields $\sim{1}\,$nT$\cdot$Hz$^{-1/2}$. Spatial noise can be reduced
to the picotesla scale by signal averaging and/or spatial binning. The RF-QDM
enables simultaneous imaging of the amplitude, frequency, and phase of
narrowband magnetic field patterns at the micron-scale, with potential
applications in real-space NMR imaging, AC susceptibility mapping, impedance
tomography, analysis of electronic circuits, and spatial eddy-current-based
inspection.
