A hybrid-level digital correlation-spectrometer for radio-astronomy
Creators: name: Hovey, Gary J.
Publisher: name: University of British Columbia
Year: 2009

description: This thesis describes the design, development, testing, and construction of a hybrid-level digital correlation-spectrometer built for the seven-element synthesis and 26m paraboloid radio-telescopes at the Dominion Radio-Astrophysical Observatory near Penticton, British Columbia. Included is an analysis of the theoretical performance of the correlator and formulae for linearizing its output response. This new instrument replaces an existing, circa 1975, spectrometer system at DRAO and while it is functionally similar to its predecessor, it is more capable and flexible, and costs less to construct. These advantages were achieved largely by improvements in correlator architecture and are not solely due to, say, scaling-up an existing design using advanced technology. Following a review of correlator architectures, it is shown that the architecture developed has a number of advantages over many correlation-spectrometers. Unlike many correlation-spectrometers, this one can auto-correlate or cross-correlate both Nyquist and double-Nyquist sampled data using all lags so that spectral resolution is always maximized. As well, it is a hybrid-level correlator—as it uses 14-level correlators, for measuring total power, and three-level (multi-lag) correlators for measuring spectral power. A review of the theoretical performance of a three-level correlator is given. These results are extended for a 14-level correlator. It is shown that the sensitivity of a 14-level correlator is close to a perfect analog correlator; however, its input-output response is non-linear (over a 3dB range). These non-linear effects are analyzed and inversion formulae for correcting them are developed. The resulting corrected, or linearized, hybrid-level correlator is nearly ideal for measuring total power, while its system cost is low as three-level correlators are used for making spectral measurements. The main component in the system is a four-lag correlator gate-array IC, that was designed by the author. Its architectural advantages are: a) the multiplier load present at a correlator delay tap is minimal and b) it has a readout scheme that allows sampling the most significant accumulator bits in parallel. These features yield a high-performance flexible architecture that uses IC gates efficiently and maximizes sample rate and data-transfer performance. The basic system fits in a 19-inch rack and consists of a microprocessor based controller, eight analog to digital converters (ADCs), several support-logic cards, eight (single-lag) 14-level autocorrelators, and twenty (512-lag) three-level correlators. The system is normally controlled by a host computer that sends ASCII commands to the system and receives and stores integrated correlation data. For testing purposes, the system can also be operated directly via a VT-100 type terminal. A two-rack cross-correlator system was constructed for the synthesis telescope, while a smaller halfrack auto-correlation system was built for the 26m telescope. These systems are now in routine service and make astronomical measurements almost 24 hours per day.