For various reasons, in some applications, the analog-processed signal does not yet possess the characteristics suitable for measurement. In such cases, digital processing is employed, transforming the digitized signal using appropriate filters to achieve the desired performance.
In the case of an X-ray detector, for example, one might wish to alter the pulse shaping based on the photon arrival rate to strike the right balance between energy resolution and dead time. The signal-to-noise ratio depends on noise characteristics, which can vary according to external conditions (such as the presence of interference). Under these conditions, the digital filter can be modified to optimize energy resolution.
The figure shows the analog signal from one channel of the AXPiDe detector – developed for the XAFS beamline at Elettra Sincrotrone Trieste – following digitization and parametrization via a fit of the analog circuit’s transfer function. Also displayed are three signals derived using DSP: the differentiated signal, useful for triggering acquisition upon the arrival of an event and performing the measurement at the appropriate moment; and two examples of pseudo-trapezoidal shaping for photon energy measurement using different shaping times (as the peaking time increases, noise rises while dead time decreases).
