Third generation of correlators for six antennas

The technical evolution of the correlators of the Plateau de Bure interferometer since the first fringes, 14 years ago, is shortly presented. The progressive addition of antennas over this period has allowed the Grenoble correlator group to undertake several 'start-from-scratch' designs, which have replaced on-site equipment as it came obsolete. The tradeoff between design cycle time and lifetime of such equipment is discussed. The latest design is described in detail. The new correlator can be set to analyze up to eight simultaneous windows, adjustable in size and center frequency, thanks to a 2 X 220 MHz image rejection mixer. Advantages of analog IF processing are presented. The frequency plan of the IF processor has been designed to be fully compatible with MarkIV VLBI recording. The correlator is then used to sum up the signals of the 6 antennas over 256 MHz. The digital section mainly uses an IRAM-designed low-power, low-cost ASIC. Delay lines use FPGA's and phase rotators use DDS's. Surface-mount technology is used everywhere. A commercial CPU module runs the real-time software under Linux. A 21-slot VME chassis hosts the hardware. Test results and measurements of performance on the full-size machine are presented. The difficulties encountered in achieving this kind of machine within schedule in today's industrial environment are retrospectively analyzed.