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fractional sampling rate conversion system implementation in WSRT

Submitter: Ronald de Wild
Description: In the Westerbork Synthesis Radio Telescope (WSRT), one of the key functions in the Tied Array Distribution Unit (TADU) is the conversion of the sampling frequency of the signal that originates from the radio telescope system into sampling frequencies that meet the requirements of various astronomical user groups.

For the sampling-rate conversion (SRC) function, two candidate concepts have been analyzed and compared with respect to circuit-size, processing clock-rate, scalability and numerical accuracy: the Time-Variant Filter (TVF) and the Poly-Phase Filter (PPF). The TVF applies a direct re-sampling process, for which no reliable literature exists. Especially the impact of non-linear distortion on the quality of the celestial signal was underestimated.

A structured top-down design methodology that makes use of the hardware description language VHDL has led to fully synthesizable, technology-independent models for both candidates. Scaling parameters enable re-configurability for a wide range of sampling-rate conversion ratios.

With respect to computational efficiency (circuit-size, processing clock-rate and scalability) the TVF is the better candidate. With respect to numerical accuracy (distortionless dynamic range), the more conventional PPF is to be preferred. The latter candidate is successfully operating in WSRT since spring this year.
Copyright: ASTRON
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