  The Arcminute Microkelvin Imager (AMI) telescopes located at the Mullard
Radio Astronomy Observatory near Cambridge have been significantly enhanced by
the implementation of a new digital correlator with 1.2 MHz spectral
resolution. This system has replaced a 750-MHz resolution analogue lag-based
correlator, and was designed to mitigate the effects of radio frequency
interference, particularly from geostationary satellites that contaminate
observations at low declinations. The upgraded instrument consists of 18 ROACH2
Field Programmable Gate Array platforms used to implement a pair of real-time
FX correlators -- one for each of AMI's two arrays. The new system separates
the down-converted RF baseband signal from each AMI receiver into two 2.3
GHz-wide sub-bands which are each digitized at 5-Gsps with 8 bits of precision.
These digital data streams are filtered into 2048 frequency channels and
cross-correlated using FPGA hardware, with a commercial 10 Gb Ethernet switch
providing high-speed data interconnect. Images formed using data from the new
digital correlator show over an order of magnitude improvement in dynamic range
over the previous system. The ability to observe at low declinations has also
been significantly improved.
