tnt digital pulse processor

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IEEE TRANSACTIONS ON NUCLEAR SCIENCE, VOL. 53, NO. 3, JUNE 2006 723 TNT Digital Pulse Processor L. Arnold, R. Baumann, E. Chambit, M. Filliger, C. Fuchs, C. Kieber, D. Klein, P. Medina, C. Parisel, M. Richer, C. Santos, and C. Weber Abstract—We report on the development of Tracking Numerical Treatment (TNT) boards, which are the basic bricks of a more am- bitious data acquisition system intended for online data acquisi- tion and treatment in future European experiments. The module makes extensive use of field programmable gate arrays (FPGA) for signal deconvolution and energy calculation with a minimum of data loss. Four channels can be processed simultaneously. Event collection is performed through a fast universal serial bus (USB2). The system can sustain an event rate of 100 kHz without dead time. Here we describe the practical implementation of pulse processing algorithms in a digital electronic module based on high density FPGAs for event processing. The overall system and its architec- ture are portrayed, along with some technical characteristics. Fi- nally, we present some results and future developments. Index Terms—Data acquisition, digital spectrometry, digitizer, FPGA, high counting rates, online deconvolution, online pulse pro- cessing. I. INTRODUCTION T NT is a digitizer board developed to meet the requirements of current data acquisition systems in nuclear physics. Low dead-time even at high counting rates and good energy and time resolution for a large range of energies are the main issues. It is primarily targeted at data processing from Germanium detec- tors. TNT relies on the use of a state of the art FPGA, powerful enough to implement a full digital spectrometer providing trigger and energy computation in real time. It can include fur- ther custom functionality as waveform capture for subsequent analysis. The work presented here focus on the choice and online implementation of existing algorithms to achieve the requisites of nowadays experiments. In this paper, we present an overview of the whole system. In Section III we describe the signal processing algorithms imple- mented in the FPGA and finally the two main operating modes. We also describe the performance of the card in two in going experiments. II. OVERVIEW TNT is a NIM-based card providing digital spectrometry and waveform acquisition of the input signals from detector’s preamplifier. It features four acquisition channels operating in parallel and offers good high-resolution spectroscopy. Each channel combines a fourth order anti-aliasing Nyquist filter (cut-off frequency of 40 MHz), high sampling speed (100 MHz) and 14-bits resolution data bus by using the AD6645 Flash Analog to Digital Converter (FADC) from Analog Devices [1]. Manuscript received June 3, 2005; revised March 2, 2006. The authors are with the IPHC, BP 28, F67037 Strasbourg Cedex 2, France. Digital Object Identifier 10.1109/TNS.2006.873712 On-board computation provides real time operation. A Xilinx Virtex II, three million gates FPGA [2] implements pulse pro- cessing and hardware control. The Virtex II architecture is char- acterized by dedicated resources, such as shift registers and em- bedded multipliers that are well suited for development of fast digital signal processing (DSP) algorithms. The parallel data treatment capabilities of FPGAs enables a higher throughput and reduces the event processing time, thanks to a large amount of on-chip memory. All those features are widely exploited in the implementation of algorithms in need of high logic inte- gration, extensive pipeline and First Input First Output (FIFO) memory. An additional Spartan II FPGA offers system reconfiguration, giving the possibility to update the main FPGA contents via the USB bus. An 8051 compatible micro controller (FX2, from Cypress [3]) is responsible for data readout and slow control. Transfer is performed through a 16 bits, 48 MHz universal serial bus (USB) interface with an observed maximum rate of 30–40 Mbytes/s. The number of TNT boards that can be used in parallel de- pends on the required performances, since the whole system must share the USB bandwidth. The acquisition software, TUC (TNT USB Control), is in charge of slow control and monitoring of the boards in addition to providing tools for data visualization and saving (Fig. 1). It has been conceived to manage any number of TNT modules, assisting the user in changing settings. Spying of the data and counting rate is possible thanks to its graphical user interface. In addition, TUC can handle the updating of the embedded func- tionalities running on the Virtex II FPGA. Written in java, it has been in use on Linux and Windows platforms. Finally, it should be noted that the communication protocol between TNT mod- ules and TUC is accessible and hence, TUC can be replaced by any homemade software. III. DSP ALGORITHMS Two well known DSP algorithms have been implemented in the form of recursive finite impulse response (FIR) filters. De- convolution of the preamplifier signal grants an accurate energy measure. A performing trigger is needed for precise event detec- tion in a wide range of amplitudes. Here we describe its practical implementation taking advantage of some advanced features in the FPGA. A. Deconvolution Digitization of the preamplifier output signals allows Trape- zoidal shaping [4], [5], replacing traditional analog Gaussian shaping. It provides a good compromise between counting rate and low dead time. In addition, it offers good energy resolution 0018-9499/$20.00 © 2006 IEEE

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IEEE TRANSACTIONS ON NUCLEAR SCIENCE, VOL. 53, NO. 3, JUNE 2006 723

TNT Digital Pulse ProcessorL. Arnold, R. Baumann, E. Chambit, M. Filliger, C. Fuchs, C. Kieber, D. Klein, P. Medina, C. Parisel,

M. Richer, C. Santos, and C. Weber

Abstract—We report on the development of Tracking NumericalTreatment (TNT) boards, which are the basic bricks of a more am-bitious data acquisition system intended for online data acquisi-tion and treatment in future European experiments. The modulemakes extensive use of field programmable gate arrays (FPGA)for signal deconvolution and energy calculation with a minimumof data loss. Four channels can be processed simultaneously. Eventcollection is performed through a fast universal serial bus (USB2).The system can sustain an event rate of 100 kHz without dead time.Here we describe the practical implementation of pulse processingalgorithms in a digital electronic module based on high densityFPGAs for event processing. The overall system and its architec-ture are portrayed, along with some technical characteristics. Fi-nally, we present some results and future developments.

Index Terms—Data acquisition, digital spectrometry, digitizer,FPGA, high counting rates, online deconvolution, online pulse pro-cessing.

I. INTRODUCTION

TNT is a digitizer board developed to meet the requirementsof current data acquisition systems in nuclear physics. Low

dead-time even at high counting rates and good energy and timeresolution for a large range of energies are the main issues. Itis primarily targeted at data processing from Germanium detec-tors.

TNT relies on the use of a state of the art FPGA, powerfulenough to implement a full digital spectrometer providingtrigger and energy computation in real time. It can include fur-ther custom functionality as waveform capture for subsequentanalysis. The work presented here focus on the choice andonline implementation of existing algorithms to achieve therequisites of nowadays experiments.

In this paper, we present an overview of the whole system. InSection III we describe the signal processing algorithms imple-mented in the FPGA and finally the two main operating modes.We also describe the performance of the card in two in goingexperiments.

II. OVERVIEW

TNT is a NIM-based card providing digital spectrometryand waveform acquisition of the input signals from detector’spreamplifier. It features four acquisition channels operating inparallel and offers good high-resolution spectroscopy. Eachchannel combines a fourth order anti-aliasing Nyquist filter(cut-off frequency of 40 MHz), high sampling speed (100 MHz)and 14-bits resolution data bus by using the AD6645 FlashAnalog to Digital Converter (FADC) from Analog Devices [1].

Manuscript received June 3, 2005; revised March 2, 2006.The authors are with the IPHC, BP 28, F67037 Strasbourg Cedex 2, France.Digital Object Identifier 10.1109/TNS.2006.873712

On-board computation provides real time operation. A XilinxVirtex II, three million gates FPGA [2] implements pulse pro-cessing and hardware control. The Virtex II architecture is char-acterized by dedicated resources, such as shift registers and em-bedded multipliers that are well suited for development of fastdigital signal processing (DSP) algorithms. The parallel datatreatment capabilities of FPGAs enables a higher throughputand reduces the event processing time, thanks to a large amountof on-chip memory. All those features are widely exploited inthe implementation of algorithms in need of high logic inte-gration, extensive pipeline and First Input First Output (FIFO)memory.

An additional Spartan II FPGA offers system reconfiguration,giving the possibility to update the main FPGA contents via theUSB bus.

An 8051 compatible micro controller (FX2, from Cypress[3]) is responsible for data readout and slow control. Transfer isperformed through a 16 bits, 48 MHz universal serial bus (USB)interface with an observed maximum rate of 30–40 Mbytes/s.The number of TNT boards that can be used in parallel de-pends on the required performances, since the whole systemmust share the USB bandwidth.

The acquisition software, TUC (TNT USB Control), is incharge of slow control and monitoring of the boards in additionto providing tools for data visualization and saving (Fig. 1). Ithas been conceived to manage any number of TNT modules,assisting the user in changing settings. Spying of the data andcounting rate is possible thanks to its graphical user interface.In addition, TUC can handle the updating of the embedded func-tionalities running on the Virtex II FPGA. Written in java, it hasbeen in use on Linux and Windows platforms. Finally, it shouldbe noted that the communication protocol between TNT mod-ules and TUC is accessible and hence, TUC can be replaced byany homemade software.

III. DSP ALGORITHMS

Two well known DSP algorithms have been implemented inthe form of recursive finite impulse response (FIR) filters. De-convolution of the preamplifier signal grants an accurate energymeasure. A performing trigger is needed for precise event detec-tion in a wide range of amplitudes. Here we describe its practicalimplementation taking advantage of some advanced features inthe FPGA.

A. Deconvolution

Digitization of the preamplifier output signals allows Trape-zoidal shaping [4], [5], replacing traditional analog Gaussianshaping. It provides a good compromise between counting rateand low dead time. In addition, it offers good energy resolution

0018-9499/$20.00 © 2006 IEEE

724 IEEE TRANSACTIONS ON NUCLEAR SCIENCE, VOL. 53, NO. 3, JUNE 2006

Fig. 1. TUC, graphical user interface to TNT.

and excellent baseline stability at high counting rates. The trape-zoidal shaping compensates the ballistic deficit effect during therise time. Finally, it can be conceived in a recursive way, whichmeans that any output sample can be obtained from the previousone plus a limited number of input samples. This form is wellsuited for an online implementation on dedicated hardware ([6],chapter 4), as shown in Fig. 2.

The design extensively uses the embedded high-speed multi-plier blocks and the fast carry chains for arithmetic operations.The abundance of SRL16 shift registers, an optimized Virtex IIresource, is a key feature too.

TNT benefits from FPGAs ability to perform several tasks inparallel. The FADC output is processed continuously using apipelined, fast architecture to generate a real time shaped pulse.A wide range of filter parameters is provided for accurate setupand greater flexibility. The rise time and flat top width adjust-ments can both be extended up to 10 s to fine-tune spectrom-eter performance. Peaking time goes from 100 ns to about 20

s, with adjustable flat top duration. Pole/zero cancellation per-form automatic fall time detection and correction.

Fig. 2. Oscilloscope snapshot showing the deconvolution and subsequenttrapezoidal shaping (signal 2) of the preamplifier output (signal 1).

ARNOLD et al.: TNT DIGITAL PULSE PROCESSOR 725

Fig. 3. Trigger scheme. The signal from the preamplifier is differentiated bythe delay-subtract unit (DS), and then fed onto a moving average (MA) FIRunit. This filter is composed of a series of a DS and an accumulator (ACC) unit.The output of this stage is compared to a threshold and the result is delayed andcombined using a logical and. This avoids false triggers from occurring.

Baseline restoration (BLR) is carried out through an expo-nential average of the baseline, authorizing its minute adjust-ment for applications requiring wide dynamic rate counting.BLR allows a proper suppression of the pedestal of the energyfilter. It also considerably improves the energy resolution bysuppressing the negative influence of detector leakage currentson base line stability.

The output of the pulse shaper and other test signals are routedto a 12 bits, 100 MHz. Digital to Analog Converter (DAC) fordiagnostic purposes.

B. Trigger

Low energy threshold is attainable thanks to the followingdedicated triggering scheme, specially designed for a digital,real time hardware implementation ([7] chapter 4, and [8]chapter 5). The first sub module, a delay-subtract unit, differen-tiates the digitized preamplifier signal to remove any offset. Theresulting signal is then fed onto a moving average unit, whichacts as a low pass filter, hence removing noise. Finally, thesignal is compared to a threshold value (leading edge trigger).

Delays from the delay-subtract unit can be adjusted up to 320ns. In order to prevent false trigger from signal glitches, thetrigger signal has to achieve a minimum width and therefore thetrigger filter has to exceed the threshold for a minimum time.The design is shown in Fig. 3.

C. Clocking

Each of the TNT modules includes an LVDS port, permit-ting the synchronization of several modules. All waveform dig-itizers, triggers and time stamps are driven by the same peri-odic signal through this port in a daisy chain configuration. TNTbenefits of the advanced clock management capabilities pro-vided by the AD9852 Direct Digital Synthesis (DDS) device[9]. The latter provides a stable, low jitter, high frequency reso-lution clock signal to feed the FADCs in order to obtain the bestpossible signal to noise ratio (SNR). Several general-purposeI/O NIM interfaces are available to define coincidence/veto ac-quisition schemes. Different delayed, variable length time-win-dows of up to 650 s can be synthesized. It is then possibleto acquire data in coincidence or opposition to some event. Fi-nally, an external event reference can be used to resynchronizeall time stamp counters. This is also useful for synchronizationwith other systems. A picture of the TNT module, along with anoutline of its main components is shown in Fig. 4.

IV. WORKING MODES

The working mode of TNT is the following: each channel ac-cepts signals directly from a detector preamplifier. The signalsare digitized in order to apply real time digital processing. Be-fore this, the offset can be altered to take advantage of the fulldynamic range and avoid saturation of the FADC. Triggering,pile-up inspection and filtering of the data stream are performedby the FPGA, as well as averaging and detection of peak ampli-tude (Fig. 5).

Every time a pulse is detected, parallel signal processing ex-ecutes and outputs event data. The data is then transmitted overthe USB interface to TUC. The latter acts as an event collectorand increments a 32k Multi Channel Analyzer (MCA) spec-trum, generating a histogram. This operating mode offers thebest bandwidth to data flow ratio.

A. Oscilloscope Mode

A TNT2 module can continuously acquire waveforms with14-bit precision, which are sent into the digital pulse shaper.This data flow is also stored into a circular buffer, implementedin an external memory to the FPGA. This memory has a depthof 1024K entries corresponding to 10.4 ms worth of event wave-form data, at sampling intervals of 10 ns. When an event triggeris received, the module can incorporate waveforms of arbitrarylength into its output data stream for offline analysis. As in adigital oscilloscope, it is possible to record pre-trigger wave-form data.

B. Processing Mode

With the aim of improving energy resolution, customizedhardware in the FPGA determines the average value at theflat top of the deconvoluted preamplifier signal. As shown inFig. 5, this average can be calculated using an arbitrary numberof samples. This general strategy is intended for better noisefiltering, and includes the particular case of a single sample.

The readout takes place over the USB using a low level blocktransfer protocol. This implies some buffering in order to avoiddata loss due to non continuous data readout. The flexibility ofTNT modules lies in the FPGA, which provides 96 blocks of18 Kbits, fully dual-port embedded memory for critical signalprocessing applications [2]. It is used as FIFO data buffers here,with independent management of the read and write pointersand fully synchronous and independent clock domains for theread and write ports. Count vectors provide visibility into thenumber of data currently in the FIFO. Fig. 6 shows how thisflag can be useful for inspection of possible data loss dependingon the counting rate.

Information from previously analyzed pulses is stored intothese FIFOs while the data flow is continuously processed. Nodead time occurs at this stage, since nor storage neither furthertreatment of the data flow are necessary for event generation.

The depth and length of the data buffers can be configuredaccording to the required information, up to the FPGA’s fullmemory capacity. Buffers are continuously filled with energyvalue, time of trigger occurrence (48 bits time stamp) and eventcounter on an event-by-event basis, which allows an offline in-spection of the number of events rejected. This configuration

726 IEEE TRANSACTIONS ON NUCLEAR SCIENCE, VOL. 53, NO. 3, JUNE 2006

Fig. 4. The TNT2 board.

Fig. 5. Pile-up rejection and energy averaging on the flat top. Trace 1 representsimpulsions from the preamplifier while trace 2 shows corresponding trapezoids.For each impulsion, there is a trigger (trace 3). The energy is computed as anaveraged value of flat top samples (trace 4). The two first measures are rejecteddue to trapezoid overlapping.

allows a maximum depth of 16K events. In parallel, this infor-mation is read out from the FIFO at up to 30 Mbytes/s.

Finally, waveform capture and energy processing modes mayrun separately or in parallel. It provides offline pulse shape anal-ysis of the full trace. For instance, position location and particlediscrimination can be studied.

Fig. 6. FIFO filling level with time. When the FIFO is full (high level), nofurther event storage is possible. The slope of the trace depends on the eventcounting rate. Events at different rates correspond to different slopes. Very highcounting rates saturate the event collector.

V. RESULTS

In order to study the capabilities of the digital system, theenergy resolution of a small planar Germanium detector wasinvestigated. We compare here TNT with the performances of aclassic spectroscopy array.

Conventional analog NIM electronics perform Gaussianshaping and sampling using a low frequency, 13 bits ADC.Time occupancy with this type of shaping is about six times thepeaking time, . The results of the tests with analog electronicsare 0.7 keV FWHM for the 60 keV 241Am peak and 2.23 keV

ARNOLD et al.: TNT DIGITAL PULSE PROCESSOR 727

TABLE IPERFORMANCE TEST WITH TNT

FWHM for the 1.33 MeV 60Co peak with a of 4.8 s. Whenusing a of 9.6 s, we measured 0.6 keV FWHM for the 60keV 241Am peak and 1.87 keV FWHM for the 1.33 MeV 60Copeak. In the last case, the pile-up rejection made dead timeincrease noticeably up to 20 %. The counting rate was in bothcases equal to 4 kHz.

Table I summarizes performances with digital electronics.The test shows excellent results both in term of energy reso-lution and dead time even at high counting rates. No data lossdue to the USB is observed at the highest counting rate, and allevent rejection is due to pile up. The energy resolution at highcounting rates is mainly limited by the detector. The dependenceof the energy resolution on the trapezoidal filter parameters wassystematically measured. The best energy resolution was ob-tained for a peaking time equal to 2.2 s, while still achievingan acceptable percentage of data rejection. The SNR decreaseswith low energies (low amplitudes), and in this case the contri-bution of noise from the electronics becomes more important.It makes the resolution very sensitive to sampling inaccuracies,such as differential non linearity (DNL). Promising results ([7],chapter 4) on DNL correction should be considered in furthertests.

TNT has been successfully used by the GRACE group forseveral months: the aim was to measure (n, xn) reaction cross-section induced by a neutron beam at GELINA (IRMM Geel)([6], chapter 3). The first concern was the separation time be-tween the gamma flash and the fastest neutrons which is notgreater than 2.5 s. The other one was the beam frequency(800Hz). So on one hand, one needs a fast way to compute theenergy within the 2.5 s time scale and on the other hand, oneneeds to overcome the acquisition dead time coming with thedata readout in case of offline analysis. TNT’s digital, shorterpulse shaping time offers higher counting rates, and online treat-ment of the energies lowers the bandwidth. Our system has beentested in the measurement of the 207Pb(n,2n) reaction. For a100% coaxial HPGe detector, the energy resolution on the 803keV transition in 206Pb is equal to 2.6 keV, with a 3 s deadtime. It allows the record of events corresponding to neutron en-ergies up to 14 MeV, or to 3.9 keV with a 2.5 s, thus allowingthe measurement of neutrons up to 20 MeV.

The digital electronics bring a unique opportunity to performspectroscopy of very heavy elements, where the productionscross section is less than afew 10 nb, with a high fission crosssection. Very high counting rates are expected from this kindof experiments. In this case, online spectroscopy with low datarejection becomes a main concern to reduce data storage while

Fig. 7. Colbalt energy spectra taken at JYFL Jyvaskyla, in keV. The figureshows a zoom of the full spectra. The Compton suppression can be observed insemilog y axis.

still retaining good energy resolution. TNT2 cards will be usedin 2 complementary projects: GABRIELLA at JINR DUBNAand JUROGAM II at JYFL Jyvaskyla [10]. For the latter project,the TNT cards will be linked to the existing TDR acquisitionsystem where each detector’s information is associated to atimestamp given by a 100MHz clock (from METRONOMEboards). Some tests regarding time alignment between the TNTcards and the TDR system have been successfully run recentlyat JYFL to assess its compatibility with current installation.The Compton suppression has also been tested with sources(Fig. 7) and with beam 36Ar on a107,109Ag target up to a 100kHz rate per TNT channel. Analysis of the correspondingdatais under progress.

Finally, the low noise of the analog stage, as well as the goodperformances in terms of SNR measures within the TNT makeit a reference design for the digitizer part of the AGATA [11]collaboration.

VI. FUTURE DEVELOPMENTS

In order to improve the TNT system’s performances, an eventcollector and synchronization module is currently under devel-opment. It will distribute a common clock and time stamp ref-erence through a 2 gigabits serial link, as well as some globalcommands, such as start acquisition and reset settings.

The acquisition software TUC will be expanded and dis-patched over the network allowing remote slow-control andmonitoring. This will improve the whole system throughputsince less computation will be done on the computers actuallyconnected to the cards. This is in prevision of the use of morethan 40 channels for experiments in Jyvaskyla and Dubna sites.

ACKNOWLEDGMENT

The authors would like to thank to thank the Grace groupof IPHC, especially Dr. Gerard Rudolf and Dr. Strahinja Lukicfor their helpful contributions in the development of algorithms.Tests for the future Jyvaskyla experiments were performed with

728 IEEE TRANSACTIONS ON NUCLEAR SCIENCE, VOL. 53, NO. 3, JUNE 2006

the help of Dr. Benoit Gall and Dr. Peter Jones. Our final thanksgo to Dr. Gilbert Duchene for useful discussions and his supportto this project.

REFERENCES

[1] AD6645 flash analog to digital converter website, [Online]. Available:www.analog.com/en/prod/0,2877,AD6645,00.html

[2] Virtex II field programmable gate array website, [Online]. Available:www.xilinx.com/products/silicon_solutions/fpgas/virtex/virtex_ii_platform_fpgas/index.htm

[3] Cypress semiconductor FX2 website, [Online]. Available: www.cy-press.com/portal/server.pt?space=CommunityPage&control=SetCommunity&CommunityID=209&PageID=259&fid=14&rpn=CY7C68013

[4] V. T. Jordanov and G. F. Knoll, “Digital synthesis of pulse shapes inreal time for high resolution radiation spectroscopy,” Nucl. Instrum.Methods Phys. Res. A., pp. 337–345, 1994.

[5] V. Georgiev and W. Gast, “Digital pulse processing in high resolu-tion, high throughput, gamma-ray spectroscopy,” IEEE Trans. Nucl.Sci., vol. 40, no. 4, pp. 770–779, Aug. 1993.

[6] S. Lukic, “Mesure de sections efficaces de reactions (n, xn) par spec-troscopie prompte aupres d’un faisceau atres haut flux instantane,”Ph.D. dissertation, Strasbourg, France, 2004 [Online]. Available:http://eprints-scd-ulp.u-strasbg.fr:8080/archive/00000245

[7] M. Lauer, “Digital Signal Processing for Segmented HPGe Detectors.Preprocessing Algorithms and Pulse Shape Analysis,” Ph.D. disser-tation, Heidelberg, Germany, 2004 [Online]. Available: http://www.mpi-hd.mpg.de/cb/theses.html

[8] L. Mihailescu, “Principles and Methods for A Ray Tracking With LargeVolume Germanium Detectors,” Ph.D. dissertation, Bonn, Germany,2000 [Online]. Available: http://www.fz-juelich.de/ikp/kernspek-troskopie/luke/

[9] AD9852 Analog Devices DDS. [Online]. Available: http://www.analog.com/en/prod/0,C2877,CAD9852,C00.html

[10] JUROGAM experiment website. [Online]. Available: http://www.phys.jyu.fi/research/gamma/jurogam/index.html

[11] The Advanced Gamma-Tracking Array (AGATA). [Online]. Available:http://www-w2k.gsi.de/agata