We propose an original analog method to perform instantaneous and quantitative spectral analysis
of microwave signals. An ensemble of nitrogen-vacancy (NV) centers held in a diamond plate is
pumped by a 532 nm laser. Its photoluminescence is imaged through an optical microscope and
monitored by a digital camera. An incoming microwave signal is converted into a microwave field
in the area of the NV centers by a loop shaped antenna. The resonances induced by the magnetic
component of that field are detected through a decrease of the NV centers photoluminescence. A
magnetic field gradient induces a Zeeman shift of the resonances and transforms the frequency information
into spatial information, which allows for the simultaneous analysis of the microwave
signal in the entire frequency bandwidth of the device. The time dependent spectral analysis of an
amplitude modulated microwave signal is demonstrated over a bandwidth of 600 MHz, associated
to a frequency resolution of 7MHz , and a refresh rate of 4 ms. With such integration time, a field
of a few hundreds of lW can be detected. Since the optical properties of NV centers can be
maintained even in high magnetic field, we estimate that an optimized device could allow
frequency analysis in a range of 30 GHz, only limited by the amplitude of the magnetic field
gradient. In addition, an increase of the NV centers quantity could lead both to an increase of the
microwave sensitivity and to a decrease of the minimum refresh rate down to a few ls
