A 2GHz Digital Filterbank Correlator

The combination of traditional polyphase filter architecture with a new multiplier-rich FPGA family provides the means to build wideband (GHz), flexible and high dynamic range filterbanks and FX correlators using a small number of commodity ICs. A 2GHz 4k-channel unit, serving as a prototype for next generation ATNF correlators and as an SKA demonstrator, is described. Polyphase Digital Filterbanks Whereas the ‘F’ in ‘XF correlator’ necessarily represents a Fourier transform, the ‘F’ in ‘FX correlator’ indicates a filterbank of which the ubiquitous FFT is but one, rather crude, example. Conventional uniform filterbanks comprise an array of quadrature downconverters. For each one the LO frequency defines the centre of the passband, and identical lowpass filters in the I and Q outputs define its shape. Such structures are at the heart of familiar VLBI systems such as the analog VCs and BBCs in MkIV and VLBA DARs respectively, and the digital IFPs in the LBA DAS. Direct implementation of large arrays of order 10 channels is expensive in any technology. Fortunately digital converters using FIR output filters can be described with simple algebra, thus providing an alternative form of ‘circuit design’. Comparison of the set of expressions for a filterbank reveals many common, ie redundant, operations. With a judicious choice of parameters the whole can be factorised and rearranged as a single instantiation of the FIR filter in polyphase form, and a matrix of complex modulators ek in the form of a DFT. The DFT is naturally evaluated as an FFT.