Pulse Height Spectroscopical Measurements
3 Pulse Height Spectroscopical Measurements
3.1 Introduction to Digital Signal Processing
The main task of a multi-channel analyzer is to measure the height of voltage steps. In a
conventional analog MCA the voltage step is converted by a Gaussian bandpass filter into
a pulse with several microseconds pulse width, a peak detector converts this to a DC
voltage, which is then measured with a single conversion of an analog to digital converter
(ADC). Typical there is also a fast channel, which converts the signal to very narrow
pulses for counting, triggering, and rejecting events which are in too short time distance to
be measured correctly (Pile up rejection, PUR).
In a digital MCA the input signal is digitized with a high rate, and the amplitude is
calculated from a multitude of measurements. The MCA527 samples the input signal with
a 14 bit ADC and a rate of 10MS/s, this is one voltage measurement every 100ns.
Digital filtering with a finite impulse response (FIR) filter is now just multiplying the
incoming train of measurement values with a fixed row of numbers and adding up all the
results. This row of numbers is the digital filter. A signal processor is especially designed
to perform such a task.
Also with a digital MCA the signal processing is split up into a fast and a spectroscopical
(slow) channel. A short trigger filter is applied to the incoming signal every 100ns, whereas
the long spectroscopical filter is only applied if a valid event was found.
The simplest method to measure the step voltage is to take one value before the step and
one after the step and subtract them from each other. This would correspond to a digital
filter looking like -1, 0, 0, 0, 0, 0, 0, 0, 0, 1. But such a filter would be quite unusual as the
accuracy of this would be rather bad compared to what is obtainable.
The main errors of an ADC are amplitude noise and timing error. So for minimizing the
errors, it is advisable to use more than one voltage value before and after the step for
averaging, and to avoid using voltage values where the voltage is changing quickly. So
typically between 10 and 100 values before and after the rise are averaged. This time
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Figure 9: Example of a digital filter applied to a voltage step. Here the flattop time is 1µs
(10*100ns) and the shaping time 0.5µs (
≙
1µs rise time)
