  Exquisite sensitivities are a prominent advantage of quantum sensors. Ramsey
sequences allow precise measurement of direct current fields, while
Hahn-echo-like sequences measure alternating current fields. However, the
latter are restrained for use with high-frequency fields (above approximately
$1$ kHz) due to finite coherence times, leaving less-sensitive noncoherent
methods for the low-frequency range. In this paper, we propose to bridge the
gap with a fitting-based algorithm with a frequency-independent sensitivity to
coherently measure low-frequency fields. As the algorithm benefits from
coherence-based measurements, its demonstration with a single nitrogen-vacancy
center gives a sensitivity of $9.4$ nT Hz$^{-0.5}$ for frequencies below about
$0.6$ kHz down to near-constant fields. To inspect the potential in various
scenarios, we apply the algorithm at a background field of tens of nTs, and we
measure low-frequency signals via synchronization.
