Below is the lag spectrum (correlation function) produced when observing a strong calibrator in continuum (bandwidth 128MHz) mode with 32 channels. This function is the Fourier transform of the spectrum, so for a pure square wave type spectrum you expect a sinc function.
Below is the lag spectrum produced when observing a weak calibrator in spectral line (bandwidth 8MHz) mode using 1024 channels. This function is the Fourier transform of the spectrum, so for a pure square wave type spectrum you expect a very narrow sinc function.