  Microwave magnetometry is essential for the advancement of microwave
technologies. We demonstrate a broadband microwave sensing protocol using the
AC Zeeman effect with ensemble nitrogen-vacancy (NV) centers in diamond. A
widefield microscope can visualize the frequency characteristics of the
microwave resonator and the spatial distribution of off-resonant microwave
amplitude. Furthermore, by combining this method with dynamical decoupling, we
achieve the microwave amplitude sensitivity of $5.2 \, \mathrm{\mu T} /
\sqrt{\mathrm{Hz}}$, which is 7.7 times better than $40.2 \, \mathrm{\mu T} /
\sqrt{\mathrm{Hz}}$ obtained using the protocol in previous research over a
sensing volume of $2.77 \, \mathrm{\mu m} \times 2.77 \, \mathrm{\mu m} \times
30 \, \mathrm{nm}$. Our achievement is a concrete step in adapting ensemble NV
centers for wideband and widefield microwave imaging.

Quantum Physics;Mesoscale and Nanoscale Physics;Optics