The problem of computing pairwise correlation as a function of frequency can be solved by two mathematically equivalent but distinct ways. By using Discrete Fourier Transform (DFT) it is possible to analyze signals in the time domain as well as in the spectral domain. The first approach is "XF" correlation because it first cross-correlates antennas (the "X" operation) using a time-domain "lag" convolution, and then computes the spectrum (the "F" operation) for each resulting baseline. The second approach "FX" takes advantage of the fact that convolution is equivalent to multiplication in Fourier domain. It first computes the spectrum for each individual antenna (the F operation), and then multiplies pairwise all antennas for each spectral channel (the X operation). A FX correlator has an advantage over a XF correlators in that the computational complexity is O(N2). Therefore, FX correlators are more efficient for larger arrays.