  Spectral analysis of electron spin resonance (ESR) is a powerful technique
for various investigations including characterization of spin systems,
measurements of spin concentration, and probing spin dynamics. The
nitrogen-vacancy (NV) center in diamond is a promising magnetic sensor enabling
improvement of ESR sensitivity to the level of a single spin. Therefore,
understanding the nature of NV-detected ESR (NV-ESR) spectrum is critical for
applications to nanoscale ESR. Within this work we investigate the linewidth of
NV-ESR from single substitutional nitrogen centers (called P1 centers). NV-ESR
is detected by a double electron-electron resonance (DEER) technique. By
studying the dependence of the DEER excitation bandwidth on NV-ESR linewidth,
we find that the spectral resolution is improved significantly and eventually
limited by inhomogeneous broadening of the detected P1 ESR. Moreover, we show
that the NV-ESR linewidth can be as narrow as 0.3 MHz.
