  We use an electric-dipole laser-driven transition to precisely measure the
cyclotron-frequency ratios of the pairs $^{42}$Ca$^+$-$^{40}$Ca$^+$,
$^{44}$Ca$^+$-$^{40}$Ca$^+$ and $^{48}$Ca$^+$-$^{40}$Ca$^+$ in a 7-tesla
Penning trap. A single laser-cooled ($T\approx 1$~mK) ion serves, together with
photon-counting and/or photon-imaging units, as a radiofrequency detector
covering a broad-band frequency spectrum, in the present case from kHz to a few
MHz. Such detectors ($^{40,42,44,48}$Ca$^{\scriptsize{+}}$) allow measuring
extremely small forces, with measured normalized sensitivities down to
$7.4(3.5)$ yN$/\sqrt{\text{Hz}}$ and $24.9(9.9)$ yN$/\sqrt{\text{Hz}}$ in the
MHz and kHz regime, respectively. The direct determination of the ions'
amplitudes makes a cyclotron-frequency measurement process more robust against
inhomogeneities of the magnetic field and/or deviations of the electric
quadrupole field due to mechanical imperfections of the trap.
