The function that relates the true correlation of analog signals to the measured correlation of digitized signals has been analyzed for two and four quantization levels. The analysis has been carried out for both XF and FX correlators, including a mixed two-four case when a two-level digitizer is used for one input signal and a four-level digitizer is used for the other input signal. It is shown that the same parameters of a four-level digitizer optimize both the two-four case and the four-four case. The signal-to-noise ratio of the two-four case is ∼17% worse than that of the four-four. The fringe stopping procedure, which is typical for the interferometry application of a correlator, linearizes the correction function. So different correction functions should be used for autocorrelation and cross correlation.