FAST ComTec MCA4A, User Manual

Page: 36 / 107

Hardware Description
The available trigger modes are SINGLE, NORMAL and AUTO.
In SINGLE mode only the first trigger event is displayed. In NORMAL mode subsequent trigger events start a
new display. In AUTO mode the measurement is triggered by real trigger events or - in absence of those -
automatically.
For further evaluation FFT and software PHA are available (ref. chapter 5 MPANT Software).
Be aware that due to the large number of data being transferred the effective event rate is low compared to a
firmware based (onboard) pulse height analysis. Thus, the scope mode is mostly for setting up the
experiment.
3.3.2 Pulse height analysis
Onboard (firmware based) signal evaluation and pulse height analysis allows for high event rates and only
little conversion induced dead time.
The incoming ADC data stream is evaluated for a rising edge threshold crossing which is taken as the start
of a pulse. The input signal is sampled at 8ns intervals and the data is stored in the 32k input buffer FIFO.
This 32k buffer FIFO is segmentable in 1, 2, 4 or 8 equal sized segments. This provides for virtually no
analysis induced dead-time as a new pulse can be stored while a previous is still processed.
After the input signal fell below the threshold again the pulse sampling is finished and processing begins.
Optionally a hysteresis for the threshold crossings can be chosen. This might improve the spectrum for low
amplitudes, slowly falling input pulses and high noise levels.
Each PHA event is tagged with the RTC (1ms or optionally 8ns) and – if selected – the TAG inputs at the
pulse start. The tagging is executed at the leading edge threshold crossing.
After detecting a completed input pulse the stored data is checked for the minimum and maximum number of
samples. This enables to discard false (too short and too long) pulses (ref. Fig. 3.8). Then the data is
evaluated using the selected fitting algorithm.
Absolute maximum (none):
No calculation is necessary and the absolute maximum value is taken.
Fit for Gaussian shaped pulses:
A curve is fitted through the selected number of samples (2 x i1 + 1) around the absolute maximum. This
provides for a reduction of noise and thus, better peak widths in the acquired spectrum.
36
F
ComTec GmbH
Note:
In scope mode a small data FIFO size should be chosen (ref. Fig. 4.6 ) for better display response
Fig. 3.8: Pulsewidth check - discard "false" pulses