Correlators at Arecibo

The first correlator used for in radio astronomy at Arecibo was a hand-me-down machine from the Haystack Observatory near Boston. This instrument, which arrived around 1972, was essentially the same as the 1-bit grandfather of all correlators, built in the early sixties by Sandy Weinreb. Built with discrete transistors and housed in a 6ft. rack, it had 100 channels and a maximum clock rate of ___ MHz. This correlator could dump its data to a computer. (Weinreb's original correlator had electro-mechanical counters as final accumulators; the experimenter used a clipboard and pencil to record the totals after each integration cycle).

At the same time, Arecibo had a 50-channel 1-bit x 8-bit correlator for the measurement of the incoherent scatter spectrum. This unit used TTL chips, and around 1974 we built a TTL correlator with 1008 channels for radio astronomy. This correlator had a 20-MHz clock rate for 1-bit operation and a 10-MHz clock rate for 3-level ("1.6- bit") operation.

The 1008-channel machine was replaced around 1984 with a clone of a new Berkeley correlator: 2048 channels and a maximum clock rate of 40-MHz with either 1-bit or 1.6-bit operation. The Berkeley Correlator used the same TTL technology but achieved higher speed by virtue of better circuit design. The space factor was about the same: a 6-ft rack held 1024 channels and each channel required about a half dozen integrated circuits.

Arecibo continued to use this 2048-channel correlator into the 90's, while other observatories began building correlators based on a Dutch-designed semi-custom gate array chip that held 16 channels. Around 1994, NAIC entered into a collaboration with a NASA-sponsored VLSI design lab, then at the University of Idaho and later at the University of New Mexico, that resulted in the design of a chip holding 1024 1.6-bit correlator channels with a clock rate of 100 MHz. The chips were fabricated by Hewlett Packard and correlators with tens of thousands of channels have been built using these chips at Arecibo, NRAO, and elsewhere.