PuMa, the first fully digital pulsar machine

Pulsars are neutron stars, rapidly rotating remains of supernova explosions, emitting bundles of broadband electromagnetic radiation. Researching these signals yields tests for fundamental physics theories and insight in the evolution of stars. To carry out pulsar observations, two hurdles have to be overcome. Typically, the signal-to-noise ratio is poor, requiring long observations and large bandwidths. Next there is dispersion, causing the pulsating signals to smear out and calls for narrow signal bands. Using many parallel narrow signal bands resolves this dilemma. Traditionally, pulsar machines use tens of parallel (analog) heterodyne receivers. Though impractical, it is desirable to have many more receivers. PuMa, the first Dutch pulsar machine, uses digital signal processing to split the incoming signal in up to thousands of narrow bands. The processor based design also increases flexibility as it allows different observational modes by loading the appropriate software into the signal processors. In total 192 SHARC processors (ADSP 21062) deliver the processing capacity. For PuMa a general purpose 6-processor SHARC board was developed, optimized for concurrent use of data busses. Other parts are commercially available components and all is joined in a VME environment. Mid 1998 PuMa was installed at the Westerbork Synthesis Radio Telescope in the Netherlands and its commissioning is completed.