Digital pulse shaping

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Role of the pulse processor
Analog pulse shaping
Time variant shaping
Digital pulse shaping
Fixed process time
Adaptive pulse shaping
Resolution & count rate
Pulse pile-up protection
Comparing different pulse processors
Summary

The pulse processor

In a processor using digital shaping, the signal from the preamplifier is digitized at the input of the pulse processor, and shaping and noise reduction are achieved by digital computation. The preamplifier output is sampled continuously by an analog to digital converter (ADC) and X-ray pulse heights are measured by subtracting the average of one set of values, measured before an X-ray event, from that for another set, measured after the event. The resultant value of the step measurement is then sent directly to the computer multichannel analyzer. The noise on the voltage ramp from the detector is effectively filtered out by averaging the signal (Fig. 15). The time over which the waveform is averaged (often called the process time) is equivalent to the peaking time for an analog shaper (T_P).

By measuring the voltage ramp in this way the benefits of perfect baseline recovery are achieved without the complexities required for time variant shaping circuits. Therefore peaks should not shift with count rate. There is also the potential for measuring the zero baseline (‘strobing’) by averaging the digital output of the ramp when no events are present (Fig. 18a). This will provide not only automatic correction for changes in the slope of the voltage ramp caused by changes in detector leakage current, but will also measure the effective noise resolution. Calibration procedures for processors using digital pulse shaping should be straightforward and reliable. One energy calibration should be sufficient to guarantee accurate energy calibration at all count rates.

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