Arcetri Technical Report 4/2018
SKA Project Series
Quantization effects and nonlinearities in multi-bit correlation
S. Chiarucci, G. Comoretto
INAF Osservatorio Astrofisico di Arcetri
Abstract
An astronomic correlator analyses a signal that has a Gaussian bivariate statistics, quantized with a limited number of bits. The correlator response to the quantized signal is linear only in a limited range of the signal amplitude, but the relation between the quantized and unquantized correlation is well known, and most present day correlators correct for this effects. In large interferometers, with hundreds of antennas and thousands of spectral channels, it can be advantageous to limit the signal amplitude to the linear region, and completely avoid the quantization correction. Nonlinearities have been computed for quantization schemes from 4 to 9 bits. When 4 or 5 bits are used, keeping the signal in the linear region causes a degradation in the quantization efficiency. Autocorrelation is always affected when using quantization with less than 8 bits, and must be corrected. The effect of rounding in requantization is also considered.
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