Signal processing for low-light-level, high-precision CCD imaging

CCD signal-processing schemes attempt to reduce the effect of KTC, 1/f, and broadband noise on the output signal. A number of schemes have been reported over the years. These schemes employ time delay and subtraction to eliminate KTC noise and attenuate 1/f noise. They also include a low-pass function to reduce the effect of broadband noise. Signal processing schemes include dual-slope integration, double-correlated sampling, a variation of double-correlated sampling referred to in this paper as "switched exponential filtering," and transverse filters. Transfer functions for signal and noise are presented for each of these schemes. Performance comparisons are given with emphasis on their applicability to relatively high-speed CCD readout applications (readout rates of 1 megapixel per second and faster). Recent technology has produced CCDs with high-output gains in terms of volts per electron. Such devices feature very small output gate capacitance and hence, reduced KTC noise. Signal processing that does not use delay and subtraction to eliminate KTC noise is discussed. Again, the emphasis is on applicability to relatively high-speed readout.