  We describe a real-time data processing and frequency control method to track
peaks in optically detected magnetic resonance of nitrogen-vacancy centers in
diamond. This procedure allows us to measure magnetic field continuously with
sensitivity ~6 uT/Hz^{1/2} and to track resonances at sweep rates exceeding 110
uT/s. We use a custom-built differential photon rate meter and active feedback
control to "lock" the microwave excitation frequency of the magnetic resonance.
Our scheme covers a broad magnetic field range, limited by the frequency range
of the microwave generator. This measurement procedure automates the processing
of voltage pulse outputs from the photon counters, and it provides sensitivity
comparable to traditional photon counting methods
