  Dynamical decoupling sequences with multiple pulses can be considered to
exhibit filter functions for the time evolution of a qubit superposition state.
They contribute to the improvement of coherence time and qubit-phase
accumulation due to a time-varying field and can thus achieve
high-frequency-resolution spectroscopy. Such behaviors find useful application
in highly sensitive detection based on qubits for various external fields such
as a magnetic field. Hence, decoupling sequences are indispensable tools for
quantum sensing. In this study, we experimentally and theoretically
investigated the effects of finite-width pulses in the sequences on AC
magnetometry utilizing nitrogen-vacancy centers in an isotopically-controlled
diamond. We revealed that the finite pulse widths cause a deviation of the
optimum time to acquire the largest phase accumulation due to the sensing field
from that expected by filter functions neglecting the pulse widths, even if the
widths are considerably shorter than the time period of the sensing field.
Moreover, we experimentally demonstrated that the deviation can be corrected by
an appropriate time-frequency conversion. Our results provide a guideline for
the detection of an AC field with an accurate frequency and linewidth in
quantum sensing with multiple-pulse sequences.
