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Semi-Gaussian Pulse Shaping By replacing the simple RC integrator with a more complicated active integrator network (Fig. 13), the signal-to-noise ratio of the pulse-shaping amplifier can be improved by 17% to 19% at the noise corner time constant. This is important for semiconductor detectors, whose energy resolution at low energies and short shaping time constants is limited by the signal-to-noise ratio. Amplifiers incorporating the more complicated filters are typically called "semi-Gaussian shaping amplifiers" because their output pulse shapes crudely approximate the shape of a Gaussian curve [Fig. 14(a)]. A further advantage of the semi-Gaussian pulse shaping is a reduction of the output pulse width at 0.1% of the pulse amplitude. At the noise corner time constant, semi-Gaussian shaping can yield a 22% to 52% reduction in output pulse width compared with the CR-RC filter. This leads to better baseline restorer performance at high counting rates. The reduction in pulse width corresponds to a 9% to 13% reduction in the amplifier dead time per pulse. Although the unipolar output pulse from a semi-Gaussian shaping amplifier is normally the better choice for energy spectroscopy [Fig. 14(a)], a bipolar output is typically also available [Fig. 14(b)]. The bipolar output is useful in minimizing baseline shift with varying counting rates when the electronic circuits following the amplifier are ac-coupled. It is also convenient for zero-crossover timing applications. The drawbacks inherent in the bipolar output relative to the unipolar output are a longer pulse duration and a worse signal-to-noise ratio.
Figure 13. Pulse Shaping in the Semi-Gaussian Shaping Amplifier.
Figure 14. Typical (a)
Unipolar, and (b) Bipolar Output Pulse Shapes |
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