  We present the development and characterization of a generic, reconfigurable,
low-cost ($<$ 350 USD) software-defined digital receiver system (DRS) for
temporal correlation measurements in atomic spin ensembles. We demonstrate the
use of the DRS as a component of a high resolution magnetometer. Digital
receiver based fast Fourier transform spectrometers (FFTS) are generally
superior in performance in terms of signal-to-noise ratio (SNR) compared to
traditional swept-frequency spectrum analyzers (SFSA). In applications where
the signals being analyzed are very narrow band in frequency domain, recording
them at high speeds over a reduced bandwidth provides flexibility to study them
for longer periods. We have built the DRS on the STEMLab 125-14 FPGA platform
and it has two different modes of operation: FFT Spectrometer and real time raw
voltage recording mode. We evaluate its performance by using it in atomic spin
noise spectroscopy (SNS). We demonstrate that the SNR is improved by more than
one order of magnitude with the FFTS as compared to that of the commercial
SFSA. We also highlight that with this DRS operating in the triggered data
acquisition mode one can achieve spin noise (SN) signal with high SNR in a
recording time window as low as 100 msec. We make use of this feature to
perform time resolved high-resolution magnetometry. While the receiver was
initially developed for SNS experiments, it can be easily used for other
atomic, molecular and optical (AMO) physics experiments as well.
