PIXIE-4 User’s Manual
V2.69
©
XIA
2015. All rights reserved.
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one clock cycle. Special firmware is required to combine the two input streams into one;
contact XIA for details.
The RTPUs apply digital filtering to perform essentially the same action as a shaping amplifier.
The important difference is in the type of filter used. In a digital application it is easy to
implement finite impulse response filters, and we use a trapezoidal filter. The flat top will
typically cover the rise time of the incoming signal and makes the pulse height measurement
less sensitive to variations of the signal shape.
Secondly, the RTPUs contain a pileup inspector. This logic ensures that if a second pulse is
detected too soon after the first, so that it would corrupt the first pulse height measurement,
both pulses are rejected as piled up. The pileup inspector is, however, not very effective in
detecting pulse pileup on the rising edge of the first pulse, i.e. in general pulses must be
separated by their rise time to be effectively recognized as different pulses. Therefore, for high
count rate applications, the pulse rise times should be as short as possible, to minimize the
occurrence of pileup peaks in the resulting spectra.
If a pulse was detected and passed the pileup inspector, a trigger may be issued. That trigger
would notify the DSP that there are raw data available now. If a trigger was issued the data
remain latched until the RTPU has been serviced by the DSP.
The third component of the RTPU is a FIFO memory, which is controlled by the pile up
inspector logic. The FIFO memory is continuously being filled with waveform data from the
ADC, only stopped to avoid overwriting of data for valid events. On a trigger the read pointer
is positioned such that it points to the beginning of the pulse that caused the trigger. When the
DSP collects event data, it can read any fraction of the stored waveform, up to the full length
of the FIFO.
5.3 Digital Signal Processor (DSP)
The DSP controls the operation of the Pixie-4, reads raw data from the RTPUs, reconstructs
true pulse heights, applies time stamps, prepares data for output to the host computer, and
increments spectra in the on-board memory.
The host computer communicates with the DSP, via the PCI interface, using a direct memory
access (DMA) channel. Reading and writing data to DSP memory does not interrupt its
operation, and can occur even while a measurement is underway.
The host sets variables in the DSP memory and if necessary calls DSP functions to apply them
to the hardware. Through this mechanism all gain and offset DACs are set and the filter settings
are applied to the RTPUs.
The RTPUs process their data without support from the DSP, once they have been set up. When
any one or more of them generate a trigger, an interrupt request is sent to the DSP. It responds
with reading the required raw data from the RTPUs and storing those in an intermediate buffer.
It then returns from the interrupt routine without processing the data to minimize the DSP
induced dead time. The event processing routine works from the data in the buffer to generate
the requested output data. There are different implementations of the intermediate buffer for
the different run types. In standard list mode runs, intermediate and I/O buffer are the same, to
avoid moving long waveforms inside the DSP. In other run types, the intermediate buffer is