  We use Carr-Purcell-Meiboom-Gill (CPMG) dynamical decoupling to measure the
magnetic noise power spectra for ensembles of P1 and NV centers in diamond
using pulsed electron paramagnetic resonance (pEPR) at 2.5 GHz. The
stroboscopically detected pEPR experiments on NV centers were performed on an
HPHT (high pressure, high temperature) diamond sample at 13 mT and 190 mT,
while the experiments on P1 centers were performed on a CVD (chemical vapor
deposition) diamond sample at 89 mT. All power spectra show two distinct
features, a broad component that is observed to scale as approximately
$1/\omega$, and a prominent peak at the $^{13}$C Larmor precession frequency.
The broad $1/\omega$ behavior is consistent with an inhomogeneous distribution
of Lorentzian spectra due to clustering of P1 centers, which has recently been
shown to be prevalent in HPHT diamond. However, it is unknown if such
clustering occurs in CVD diamond. The maximum rate at which we can apply $\pi$
pulses is higher than the $^{13}$C frequency at 13 mT, but is lower than the
$^{13}$C frequency at 89 mT and 190 mT. We develop techniques that utilize the
higher harmonics of the CPMG filter function to improve our estimate of the
$^{13}$C contribution to the power spectrum at the higher fields. Surprisingly,
the $^{13}$C peak, when measured with higher harmonics of the CPMG filter,
appears larger than expected based on measurements with the lower harmonics. We
assess the robustness of our methods in the presence of finite pulse widths and
flip angle errors. These techniques could be used in a variety of ac
magnetometry and noise spectroscopy measurements such as chemical sensing and
nanoscale nuclear magnetic resonance.
