This instrument analyzes up to eight 800 MHz or up to 32 50 MHz baseband outputs of the GBT. A total of 262,144 correlator lag channels is provided. The instrument is at present in the GBT electronics room undergoing integration into the system in preparation for activation of the GBT. A similar instrument with half the lags was built for the Tucson 12-meter telescope and is in use as the primary spectrometer.
The spectrometer supports many modes of operation. An observer can select the number of active telescope baseband outputs in binary steps from 1 to 32 and allot the 262,144 available lags to analyze the outputs selected. Full Stokes parameters can be computed by cross correlation. Bandwidths in factors of 4 from 12.5 MHz to 800 MHz are available. Combinations of high and low speed baseband outputs are also possible.
The GBT spectrometer will also support pulsar observations. In experiments where a pulsar of known period is observed, the spectrometer will maintain separate integration results for 256 lags in up to 4096 time bins across the pulsar period. Time resolution as fine as 1.5 usec will be possible. In search observations where no pulsar period is known, unprocessed integration results will be available every 1.3 msec.
An Application Specific Integrated Circuit (ASIC) has been developed for use in the GBT spectrometer by the NASA Engineering Research Center for VLSI Systems Design at the University on New Mexico. This ASIC has 1024 lag channels with a 2-bit 3-level architecture, 1024 32-bit short terms integrators, and 1024 32-bit readout registers. The ASIC has a 100-MHz clock specification. The GBT spectrometer has 256 such ASIC's in its design.
RAM memory is included in the spectrometer design to deliver 1.3 msec bursts of contiguous 2-bit samples, originally taken at the 1.6 GHz sample rate, to an array of 16 correlator chips being clocked at 100 MHz. Each chip thus analyzes one sixteenth of a high speed sampler output.
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[an error occurred while processing this directive] Modified on [an error occurred while processing this directive] by Vince Summers. [an error occurred while processing this directive]