hi-spec module

About TGIClick to enlarge
The HI-SPEC module is a spectroscopy module for highest resolution and throughput spectrum acquisition with HPGe detectors. Being DSP based, it digitizes the detector signal after a differentiation at sufficiently high precision and sampling rate and feeds it the Digital Signal Processor (DSP), which detects and filters events which go into a spectrum (typically also held in the DSP).
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Compared to old analog systems, the hi-spec module contains the main amplifier, the ADC and a part of the MCA. It plugs into one of the Nukebus slots of our Nukeman series computers, is a half-height factor module (measuring apr. 55 x 110 x 19 mm or 2.16 x 4.33 x 0.75 inches), consumes < 2W and gets the best performance out of a detector possibly achievable today.

setup display

-Excellent resolution & throughput
-Very low drift
-No offset 0
-Live scope display of signals
-Input count rate indicator
-100% electronically controlled

As seen from the block diagram, the input signal after differentiation is digitized at 9.2 megasamples per second (MSPS) with 14-bit precision. The resulting data stream is fed to core A (dual core DSP is actually two DSPs on a single chip with means for fast intercommunication) which processes it in real time. It finds the events employing a high-pass filter, corrects the zero-level of the sampled signal, and responds to commands from the host (the processor which accesses the Nukebus). When an event gets detected, if it comes at a moment non-overlapping the programmed pile-up intervals, it is sent to core B (the second DSP) for further processing. If the event is within the pile-up detect time frame, it is discarded. In either case the event is counted, so the host can at any moment see the number of incoming events, as well as the number of those who make it to core B; this ratio is used to determine the "busy" time. Strictly speaking, there is no busy time at all - there are only events rejected because of pile-up. Core B has always the resources to process the incoming stream from the detector up to hundreds of kiloherz.
As core B receives an event, it filters it to determine its height; the so calculated value is used to increment the respective spectrum channel by 1. Core B holds its own copy of the spectrum - it has enough memory for a 16K, 32 bits per channel spectrum. The host CPU holds another copy of the spectrum, which it updates every 100 milliseconds, thus ensuring a live spectrum display. Also, core B holds always available for the host a copy of the input data which have been passed from core A, as well as the resulting filtered data. When the live oscillosope display is in operation, the host copies these data into its memory for visualization in two traces (again up to 10 times/second). There is no interference between the scope and spectrum data, one can acquire spectrum and see it live in one or more windows while looking at the live input and filtered signals in the setup window.
The filtering can be changed from the setup menu selecting a name. Every filter selection results in uploading the program code to both DSP cores (possibly different codes for each filter name) and the filter coefficients. Typically, the program code is the same, and so are the core-A coefficients. The core-B coefficients come as different prepared filters, and can be viewed instead of the filtered signal trace in the scope window. Expert users can edit the coefficients as a curve using the filter_curve object; this allows access to all key parameters of both core A and B.

The filter editor in use

The filter curve can be uploaded into the respective module on a mouse click; this results in overwriting the files acore.coef and bcore.coef in the nuvi directory, and setting the filters of the respective module to acore/bcore. One can edit the filter curve in one window, watch the changes in the filtered signal in another (with a scope display), see the spectrum in a spectrum window and have the peak fit results. Well, one still has to wait for the statistics...
The filter can be programmed to resemble anything between 0.25 uS to 12 uS shaping time of analog systems. However, the hi-spec module is not a straight forward digital implementation of old analog systems so a direct translation to filtered signal forms is not viable.
The settings can be saved to a file together with the recorded scope traces, and subsequently retrieved. The nuvi software automatically saves the settings for all hi-spec modules upon exit and restores them same upon startup.

Technical Characteristics:
Spectrum size: Up to 16K
Gain: 5 to 3000 (32 steps coarse, fine 1 to 2.5 in 4096 steps)
INL (Integral Non-Linearity): < 0.025% over the top 99.8% of full scale
DNL (Differential Non-Linearity): < 1% of channel
Zero drift: None
Offset Zero: None
Gain Drift: < 15 ppm/C (typ. 10 ppm)
Pulse pair resolution: programmable down to 0.5 uS.
Pole-Zero correction: electronic. Can be adjusted automatically or manually (looking at the scope display).
Power consumption: < 2W

Signal in - signal from detector. Polarity selectable from the setup menu.
Gate I/O - TTL I/O, visible to the DSP (core A) as well as driven by it.
Preamp Power- detector preamp power, standard layout.As an option for
intelligent detectors serial interface can be had on pins 5 and 8.
Nukebus - connects to the Nukebus.
Supply voltages:
+5,+3.3,+/-15,+/-24 (+/- 24 only bypassed to the preamp power connector).

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