PIXIE-4 User’s Manual
V2.69
©
XIA
2015. All rights reserved.
xxxiii
circular, i.e. old data is overwritten once it has been processed. If the circular buffer fills up
before the data can be processed, no further raw data is read from the RTPUs.
In this scheme, the greatest processing power is located in the RTPUs. Implemented in FPGAs
each of them processes the incoming waveforms from its associated ADC in real time and
produces, for each valid a event, a small set of distilled data from which pulse heights and
arrival times can be reconstructed. The computational load for the DSP is much reduced, as it
has to react only on an event-by-event basis and has to work with only a small set of numbers
for each event.
5.4 PCI Interface
The PCI interface through which the host communicates with the Pixie-4 is implemented in a
PCI slave IC together with an FPGA. The configuration of this PCI IC is stored in a PROM,
which is placed in the only DIP-8 IC-socket on the Pixie-4 board. The interface conforms to
the commercial PCI standard. It moves 32-bit data words at a time.
The interface does not issue interrupt requests to the host computer. Instead, for example to
determine when data is ready for readout, the host has to poll a Control and Status Register
(CSR) in the interface logic, also called communication FPGA.
The communication FPGA links the PCI slave with the DSP and the on-board memory. The
host can read out the memory without interrupting the operation of the DSP. This allows
updates of the MCA spectrum while a run is in progress. The communication FPGA also
distributes triggers and coincidence signals to other modules using the PXI backplane
connections.
6 Theory of Operation
6.1 Digital Filters for
-ray Detectors
Energy dispersive detectors, which include such solid state detectors as Si(Li), HPGe, HgI
2
,
CdTe and CZT detectors, are generally operated with charge sensitive preamplifiers as shown
in Figure 6.1 (a). Here the detector D is biased by voltage source V and connected to the input
of preamplifier A which has feedback capacitor C
f
and feedback resistor R
f
.
The output of the preamplifier following the absorption of an
-ray of energy E
x
in detector D
is shown in Figure 6.1 (b) as a step of amplitude V
x
(on a longer time scale, the step will decay
exponentially back to the baseline, see section 6.3). When the
-ray is absorbed in the detector
material it releases an electric charge Q
x
= E
x
/
, where
is a material constant. Q
x
is integrated
onto C
f
, to produce the voltage V
x
= Q
x
/C
f
= E
x
/(
C
f
). Measuring the energy E
x
of the
-ray
therefore requires a measurement of the voltage step V
x
in the presence of the amplifier noise
, as indicated in Figure 6.1 (b).