Compensating for oversampling effects in polyphase channelizers: A radio astronomy application

In order to maximize science returns in radio astronomy there is a constant drive to process ever wider instantaneous bandwidths. A key function of a radio telescope signal processing system is to divide a wide input bandwidth into a number of narrow sub-bands for further processing and analysis. The polyphase filter-bank channelizer has become the primary technique for performing this function due to its flexibility and suitability for very efficient implementation in FPGA hardware. Furthermore, oversampling polyphase filter-banks are gaining popularity in this role due to their ability to reduce spectral image components in each sub-band to very low levels for a given prototype filter response. A characteristic of the oversampling operation in a polyphase filterbank, however, is that the resulting sub-band outputs are in general no longer band centered on DC (as is the case for a maximally decimated filterbank) but are shifted by an amount that depends on the index of the sub-band. In this paper we present the structure of the oversampled polyphase filterbank used for the new Australian Square Kilometer Array Pathfinder (ASKAP) radio telescope and describe a technique used to correct for the sub-band frequency shift brought about by oversampling.