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Context: Fourier transform (or lag) correlators in radio interferometers canserve as an efficient means of synthesising spectral channels. However aliasingcorrupts the edge channels so they usually have to be excluded from the dataset. In systems with around 10 channels, the loss in sensitivity can besignificant. In addition, the low level of residual aliasing in the remainingchannels may cause systematic errors. Moreover, delay errors have been widelyreported in implementations of broadband analogue correlators and simulationshave shown that delay errors exasperate the effects of aliasing. Aims: Wedescribe a software-based approach that suppresses aliasing by oversampling thecross-correlation function. This method can be applied to interferometers withindividually-tracking antennas equipped with a discrete path compensatorsystem. It is based on the well-known property of interferometers where thedrift scan response is the Fourier transform of the source's band-limitedspectrum. Methods: In this paper, we simulate a single baseline interferometer,both for a real and a complex correlator. Fringe-rotation usually compensatesfor the phase of the fringes to bring the phase centre in line with thetracking centre. Instead, a modified fringe-rotation is applied. This enablesan oversampled cross-correlation function to be reconstructed by gatheringsuccessive time samples. Results: Simulations show that the oversampling methodcan synthesise the cross-power spectrum while avoiding aliasing and worksrobustly in the presence of noise. An important side benefit is that itnaturally accounts for delay errors in the correlator and the resultingspectral channels are regularly gridded.

A spectral synthesis method to suppress aliasing and calibrate for delay errors in Fourier transform correlators