- Email: [email protected]
Advanced digital self-triggering of radio emission of cosmic rays
Nuclear Instruments and Methods in Physics Research A 662 (2012) S146–S149
Contents lists available at ScienceDirect
Nuclear Instruments and Methods in Physics Research A journal homepage: www.elsevier.com/locate/nima
Advanced digital self-triggering of radio emission of cosmic rays Christoph Ruehle Karlsruhe Institute of Technology (KIT), Institute for Data Processing and Electronics, D-76344 Eggenstein-Leopoldshafen, Germany
For the Pierre Auger Collaboration1 a r t i c l e i n f o
a b s t r a c t
Available online 11 November 2010
Radio detection provides information about the electromagnetic part of an air shower in the atmosphere complementary to that obtained by water-Cherenkov detectors predominantly sensitive to the muonic content of an air shower at ground. For the measurement of ultra-high-energy cosmic rays (UHECR) by the detection of their coherent radio emission, several test setups have been developed and deployed at the Pierre Auger Observatory in Argentina. However, these UHECR radio pulses are significantly polluted by man-made radio frequency interferences (RFI). This requires a special design of antennas, analog, data acquisition (DAQ), and communication electronics, which are under investigation at the Pierre Auger Observatory. In large-scale detector arrays sophisticated self-triggering methods are necessary, to use the limited available communication data rate efficiently. This paper gives an overview of the electronics and self-triggering methods used in the test setups at the Pierre Auger Observatory and describes the experiences gained so far. & 2010 Elsevier B.V. All rights reserved.
Keywords: Radio detection Pierre Auger Observatory Signal processing Cosmic ray RF electronics Self-trigger
1. Introduction Results from the southern Pierre Auger Observatory [1], as well as the baseline design of the northern Observatory, point to the need for very large aperture detection systems for UHECRs. With its nearly 100% duty cycle, its high angular resolution, and its sensitivity to the longitudinal air-shower evolution, the radio technique is particularly well-suited for detection of UHECR in large-scale arrays. The present challenges are to understand the emission mechanisms and the features of the radio signal, and to develop an adequate measuring instrument. Electrons and positrons in the shower emit radio signals due to their deflection by the Earth’s magnetic field [2]. During shower development, charged particles are concentrated in a shower disk of a few meters thickness. This results in a coherent radio emission up to about 100 MHz as predicted by REAS3 [3] and MGMR [4]. Short radio pulses of 10 ns up to a few 100 ns duration are generated with an electric field strength increasing approximately linearly with the energy of the primary cosmic particle inducing a quadratic dependence of the radio pulse energy vs. primary particle energy. Using antennas, it is possible to receive these pulses. Radio detector experiments like LOPES [5] and CODALEMA [6], have already produced promising results at energies beyond 1017 eV.
E-mail address: [email protected] Av. San Martin Norte 304, (5613) Malargue, Prov. de Mendoza, Argentina.
1
0168-9002/$ - see front matter & 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.nima.2010.11.017
To establish radio detection as an independent detection method usable in large-scale experiments like the Pierre Auger Observatory, the deployed antenna stations must be able to measure pulses in self-triggering mode. To make this possible, the signals have to be cleaned from various man-made noise, using several preprocessing methods, before they are fed into the trigger system. This noise can be caused by radio stations and can be seen as sharp peaks in the frequency spectra.
2. Requirements and signal preprocessing Before a self-trigger can be used, the incoming signals are preprocessed in order to get a better separation between cosmic ray induced radio pulses and noise. In the first step, this is done by analog band filters and by using antennas with appropriate characteristics.
2.1. Idealized bandwidth considerations In a first approximation we assume Dirac-like radio pulses and white noise background. Then it would be the best to look at the full frequency range up to 100 MHz, in order to achieve the maximum possible signal to noise ratio (SNR), since the amplitude related SNR then increases with the square root of the bandwidth. But if realistic pulseshapes and galactic noise background are taken into account, things are looking quite different.
C. Ruehle / Nuclear Instruments and Methods in Physics Research A 662 (2012) S146–S149
S147
To show this, nine 5 1017 eV REAS3 events at four different azimuth directions (north, west, south, east) and three different zenith angles (01, 301, 601) were simulated. For the noise generation, the parameterization in Ref. [7] was used, together with an isotropic radiator as antenna model. The antenna signal was filtered by a blackman windowed band filter with a constant upper cutoff frequency of 80 MHz and a variable lower cutoff frequency running from 1 to 70 MHz. Fig. 1 shows the SNR over cutoff frequency averaged over all events for antennas at different distances to the shower core. The resulting behavior can be understood by looking at the frequency dependence of the radio emission’s energy spectrum and the intensity spectrum of the galactic background radiation, depicted in Fig. 2. Towards lower frequencies the energy spectrum of the shower emissions decreases while the background spectrum increases, which leads to a maximum of SNR at cutoff frequencies of about 20–25 MHz. It seems that, for getting the best trigger efficiency, the cutoff frequency should be set to a frequency in this range. 2.2. Bandwidth considerations including RFI background If background measurements as shown in Fig. 3 from the Pierre Auger Observatory are taken into account, it becomes obvious that cosmic radio signals can only be measured in the frequency range between the short wave and FM radio band from 30 to 80 MHz. Here, the background is mainly made up of galactic noise, with sporadic RFI contributions. Compared with Fig. 1 this means, that
Fig. 3. Power spectral density (PSD) of background noise measured at the Pierre Auger Observatory with a logarithmic periodic dipole antenna (LPDA), sensitive from 40 to 80 MHz [8]. The striped line shows the average theoretical galactic noise background and its uncertainty.
Signal to noise ratio [dB]
30 Fig. 4. General schematic of readout electronics. The dashed lines depict the slightly different readout chain used at the RAuger 2 stations.
62m 82m
25
with 30–80 MHz bandwidth the SNR is still close to the theoretical maximum.
125m
20
184m
2.3. General readout principle with RFI background 15
Thus, these spectra lead to a general block schematic of the readout chains used in radio detection setups as shown in Fig. 4.
10 0
10
20
30
40
50
60
70
Lower cutoff frequency [MHz] Fig. 1. Averaged SNR of REAS3 pulses in east–west polarization after band filtering at variable lower cutoff frequency.
Fig. 2. Averaged energy density spectrum of REAS3 pulses at 125 m distance to the shower core and intensity of galactic noise background radiation integrated over 4p solid angle.
2.3.1. Antenna and low noise amplifier The antennas which receive the radio pulses are an essential part of the setup. They need to be mechanically robust in harsh environments, broadband so the acceptance of the system is not limited and should have only a small sensitivity to different soil conditions. Directly attached to the antenna there is a low noise amplifier (LNA), which does a first amplification of the signal so its amplitude sticks out of the noise of the following electronics. The LNA must also be designed to withstand intermodulation caused by RFI outside of the observation range. 2.3.2. Bandpass filter The amplified signal is then fed into a bandpass filter which suppresses the unwanted RFI background and thus prevents electronics from being overdriven. Because the RFI peaks can be several tens of dB above the noise floor, the bandpass filter needs very steep filter flanks with a high stop band attenuation to suppress the RFI outside the passband completely. A side effect of this steep filters is that they also disperse the signal, what leads to a decreasing SNR, however, this behavior can be deconvoluted by digital signal processing.
S148
C. Ruehle / Nuclear Instruments and Methods in Physics Research A 662 (2012) S146–S149
2.3.3. DAQ digitizer At the end of the chain, the preprocessed, band filtered signal is acquired by the DAQ electronics, which can be seen as a smart, low power oscilloscope. The DAQ electronics has also an interface to the communication system. If a trigger occurs, the according signal is saved into a memory, from where it can be requested by a central DAQ system, e.g. if a coincidence with other stations occurred. To avoid the communication system from getting overloaded, signal processing and self-triggering systems have to be implemented in the digitizers.
3. The setups at the Pierre Auger Observatory Currently there are two test setups at the Pierre Auger Observatory, namely MAXIMA, with four stations near the Balloon Launching Station, and RAuger 2, with three stations around the infill tank Apollinario. A new radio detection array, the Auger Engineering Radio Array (AERA) [9], is currently being deployed. 3.1. RAuger 2 The RAuger 2 setup is a further development of the former RAuger setup, described in Ref. [10], and placed with three stations near the Central Laser Facility of the Pierre Auger Observatory. The setup uses horizontally aligned, broadband butterfly dipole antennas to receive radio signals. Consisting of just two wires bended into butterfly shape, this type of antenna can be constructed and deployed easily. The LNA used here is a specially developed low noise integrated circuit (IC) with 30 dB gain [11], which can also be fine tuned electronically to different frequency responses for RFI suppression. Signal acquisition and self-triggering at RAuger 2 is done in a slightly different way compared to the other setups. The LNA output signal is split up according to Fig. 4 (dashed lines) and one part is fed into a so-called ‘filter and trigger board’. On the board, an exchangeable band filter with a bandwidth from 45 to 55 MHz module suppresses RFI. Then, on the filtered signal a straightforward threshold trigger is applied. Limiting the bandwidth to this band leads to a loss of SNR compared to the 30–80 MHz solution, if just galactic noise is taken into account. But here the advantage is, that the probability of narrowband RFI carriers falling into the passband is much lower, compared to the broadband solution. To get the maximum possible information from the signal, the other, unfiltered part of the signal is fed directly into an analog ring buffer memory, sampling at 500 MS/s. If a trigger strobe from the trigger board occurs, the acquisition stops, and a trigger signal is sent via wireless communication system to the central DAQ. In the central DAQ coincident triggers from the other stations are searched. In the case of a coincidence, it sends a readout request to the stations, which then send the content of their analog memory to the central DAQ. 3.2. MAXIMA The MAXIMA setup is placed with four stations near the Balloon Launching Station of the Pierre Auger Observatory. At MAXIMA, LPDAs are used to receive signals, which have a broadband, upward directed characteristic. A first amplification is done by a broadband LNA with traditional monolithic microwave IC amplifiers giving 20 dB of gain. Currently, each MAXIMA station uses commercially available filter and amplifier modules from Mini-Circuits for band filtering from 50 to 70 MHz. These filters can be exchanged easily, making it possible to observe different frequency bands, and to choose the optimal band for self-triggering.
T1 T2 baseline TcMax Tprev
Tper
Fig. 5. Pulse parameters used to discriminate man-made radio pulses.
After filtering, the signals are acquired by a digitizer, sampling at 200 MS/s with 12 bit resolution. An Altera Cyclone III Field Programmable Gate Array (FPGA) directly connected to the analog to digital converters (ADC) is used to implement a time-domain self-trigger algorithm with adjustable parameters for pulse-shape discrimination. After dynamic baseline subtraction, the signal is compared against two independent thresholds: A primary or ‘signal’ threshold T1, and a secondary or ‘noise’ threshold T2, also illustrated in Fig. 5. The trigger can veto RFI pulses using several conditions: ensuring no signal threshold crossings during a quiet period Tprev; allowing a limited number of noise crossings in a trigger period Tper; and allowing a maximum time between noise threshold crossings TcMax. Tuning these parameters allows one to select the isolated bandwidth-limited pulses expected from air showers. Additionally, digital infinite impulse response (IIR) filters can be implemented to suppress RFI. If a trigger decision was done, the corresponding signal trace is saved into the random access memory (RAM) of a Voipac ARM processor module and can be requested by the central DAQ. The transmission of the data is done using a fiber-optical network.
3.3. AERA The knowledge gained with the formerly described setups led to the construction of AERA. AERA is currently being established in three deployment stages, beginning with 24 stations this year, and summing up to 161 stations in the last stage. The LNA is integrated into a so-called Small Black Spider LPDA [12], and has an additional band filter at its input [13]. This way, the band filter suppresses RFI carriers outside the observation band of 30–80 MHz thus preventing intermodulation caused by these carriers. A combined filter-amplifier module with a high stop band attenuation and very steep filter flanks assures a complete suppression of RFI outside the observation band. It also acts as an antialiasing filter for the following DAQ digitizers. Different electronics will be used and tested for digitization, the ones shown before, and a third one especially developed for AERA by KIT and University of Wuppertal. It samples at 180 MS/s at 12 bit resolution, with ADCs directly connected to an Altera Cyclone 3 FPGA, in which algorithms for signal preprocessing are implemented. With an additional RAM module of 2 GByte size, it is possible to buffer about 3 s of ADC data. The buffer bridges the latency of the communication system to allow for an external trigger. In the FPGA, a signal processing chain, as shown in Fig. 6, is implemented, to enhance the SNR for triggering. In this chain, the incoming ADC raw data are Fourier-transformed by a Fast-Fourier-Transformation (FFT). In frequency domain, it is easy to correct the dispersion of the analog band
C. Ruehle / Nuclear Instruments and Methods in Physics Research A 662 (2012) S146–S149
Optimal filter to suppress RFI
Deconvolution offilter response
F (wn) FFT
the SNR increases by about 25–30% after the deconvolution was applied and the signal was transformed back into time domain. Because of the broad observed frequency range, it is more probable, that RFI carriers fall into the passband. To suppress them, an optimal filter is applied to the signal, which does a lower weighting of frequency components polluted with RFI. After an inverse FFT, the cleaned, deconvoluted signal is fed into the trigger logic block, where pulse parameters are calculated, from which a trigger decision is derived.
f (tn) FFT-1
to DAQ Envelope
Threshold
Pulseparameter
S149
Trigger decision
Transient supression Fig. 6. Signal processing and triggering in the FPGA.
arbitrary units
4. Conclusion 600 500 400 300 200 100 0 −100 −200 −300 −400 −500
(a) pulse from analog filter (b) deconvoluted pulse
60
70
80
90
100
110
120
130
samples Fig. 7. Pulse filtered by the filter-amplifier, before and after the deconvolution.
60
number of entries
50
References
40 30 20 10 0 1.05
Different solutions for receiving and triggering on radio pulses have been developed, and deployed in the test setups MAXIMA and RAuger 2 at the Pierre Auger Observatory. One of the main requirements for a system for self-triggered radio detection is to design it for a high attenuation of man-made RFI, to enhance the SNR for triggering. In the first step, this attenuation is done using analog band filters, which limit the observed spectrum to frequencies likely to be untroubled from RFI. Here, MAXIMA and RAuger 2 have relatively narrow band filters. This way, the possibility to catch sporadic RFI carriers is lower, but this results also in a loss of SNR compared to a broadband observation of radio pulses. The experiences gained with these test setups led to the construction of AERA, which is currently being deployed. With a broad bandwidth ranging from 30 to 80 MHz also a suppression of leftover RFI is necessary. This can be implemented by FPGAs, where RFI filtering is done using IIR, respectively, optimal filters, making it possible, to get the maximum SNR for radio pulses.
1.1
1.15
1.2
1.25
1.3
1.35
1.4
1.45
1.5
1.55
S/N ratio increase Fig. 8. Increase of SNR caused by the deconvolution of the analog filter’s dispersion, simulated for several REAS3 pulses.
filters in front of the digitizer as depicted in Fig. 7. The increase of SNR caused by the deconvolution was simulated using several pulses from REAS3, polluted with galactic noise. As shown in Fig. 8,
[1] J. Abraham, et al., Pierre Auger Collaboration, Nucl. Instr. and Meth. A 523 (2004) 50. [2] H. Falcke, P.W. Gorham, Astropart. Phys. 19 (2003) 477. [3] T. Huege, R. Ulrich, R. Engel, Astropart. Phys. 30 (2008) 96. [4] O. Scholten, K. Werner, F. Rusydi, Astropart. Phys. 29 (2008) 94. [5] H. Falcke, et al., LOPES Collaboration, Nature 435 (2005) 435. [6] O. Ravel, et al., CODALEMA Collaboration, Nucl. Instr. and Meth. A 518 (2004) 213. [7] G.A. Dulk, W.C. Erickson, R. Manning, J.-L. Bougeret, Astron. Astrophys. 365 (2001) 294. [8] T. Winchen, Diploma Thesis, RWTH Aachen Unversity, 2007. [9] S. Fliescher, forthe Pierre Auger Collaboration, Nucl. Instr. and Meth. A, this issue, doi:10.1016/j.nima.2010.11.045. [10] B. Revenu, for the Pierre Auger Collaboration, Nucl. Instr. and Meth. A, this issue. [11] D. Charrier, for the Pierre Auger Collaboration, Nucl. Instr. and Meth. A, this issue, doi:10.1016/j.nima.2010.10.141. [12] O. Seeger, for the Pierre Auger Collaboration, Nucl. Instr. and Meth. A, this issue, doi:10.1016/j.nima.2010.10.140. [13] M. Stephan, for the Pierre Auger Collaboration, in: Proceedings of APMC 2010, to appear.
Contents lists available at ScienceDirect
Nuclear Instruments and Methods in Physics Research A journal homepage: www.elsevier.com/locate/nima
Advanced digital self-triggering of radio emission of cosmic rays Christoph Ruehle Karlsruhe Institute of Technology (KIT), Institute for Data Processing and Electronics, D-76344 Eggenstein-Leopoldshafen, Germany
For the Pierre Auger Collaboration1 a r t i c l e i n f o
a b s t r a c t
Available online 11 November 2010
Radio detection provides information about the electromagnetic part of an air shower in the atmosphere complementary to that obtained by water-Cherenkov detectors predominantly sensitive to the muonic content of an air shower at ground. For the measurement of ultra-high-energy cosmic rays (UHECR) by the detection of their coherent radio emission, several test setups have been developed and deployed at the Pierre Auger Observatory in Argentina. However, these UHECR radio pulses are significantly polluted by man-made radio frequency interferences (RFI). This requires a special design of antennas, analog, data acquisition (DAQ), and communication electronics, which are under investigation at the Pierre Auger Observatory. In large-scale detector arrays sophisticated self-triggering methods are necessary, to use the limited available communication data rate efficiently. This paper gives an overview of the electronics and self-triggering methods used in the test setups at the Pierre Auger Observatory and describes the experiences gained so far. & 2010 Elsevier B.V. All rights reserved.
Keywords: Radio detection Pierre Auger Observatory Signal processing Cosmic ray RF electronics Self-trigger
1. Introduction Results from the southern Pierre Auger Observatory [1], as well as the baseline design of the northern Observatory, point to the need for very large aperture detection systems for UHECRs. With its nearly 100% duty cycle, its high angular resolution, and its sensitivity to the longitudinal air-shower evolution, the radio technique is particularly well-suited for detection of UHECR in large-scale arrays. The present challenges are to understand the emission mechanisms and the features of the radio signal, and to develop an adequate measuring instrument. Electrons and positrons in the shower emit radio signals due to their deflection by the Earth’s magnetic field [2]. During shower development, charged particles are concentrated in a shower disk of a few meters thickness. This results in a coherent radio emission up to about 100 MHz as predicted by REAS3 [3] and MGMR [4]. Short radio pulses of 10 ns up to a few 100 ns duration are generated with an electric field strength increasing approximately linearly with the energy of the primary cosmic particle inducing a quadratic dependence of the radio pulse energy vs. primary particle energy. Using antennas, it is possible to receive these pulses. Radio detector experiments like LOPES [5] and CODALEMA [6], have already produced promising results at energies beyond 1017 eV.
E-mail address: [email protected] Av. San Martin Norte 304, (5613) Malargue, Prov. de Mendoza, Argentina.
1
0168-9002/$ - see front matter & 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.nima.2010.11.017
To establish radio detection as an independent detection method usable in large-scale experiments like the Pierre Auger Observatory, the deployed antenna stations must be able to measure pulses in self-triggering mode. To make this possible, the signals have to be cleaned from various man-made noise, using several preprocessing methods, before they are fed into the trigger system. This noise can be caused by radio stations and can be seen as sharp peaks in the frequency spectra.
2. Requirements and signal preprocessing Before a self-trigger can be used, the incoming signals are preprocessed in order to get a better separation between cosmic ray induced radio pulses and noise. In the first step, this is done by analog band filters and by using antennas with appropriate characteristics.
2.1. Idealized bandwidth considerations In a first approximation we assume Dirac-like radio pulses and white noise background. Then it would be the best to look at the full frequency range up to 100 MHz, in order to achieve the maximum possible signal to noise ratio (SNR), since the amplitude related SNR then increases with the square root of the bandwidth. But if realistic pulseshapes and galactic noise background are taken into account, things are looking quite different.
C. Ruehle / Nuclear Instruments and Methods in Physics Research A 662 (2012) S146–S149
S147
To show this, nine 5 1017 eV REAS3 events at four different azimuth directions (north, west, south, east) and three different zenith angles (01, 301, 601) were simulated. For the noise generation, the parameterization in Ref. [7] was used, together with an isotropic radiator as antenna model. The antenna signal was filtered by a blackman windowed band filter with a constant upper cutoff frequency of 80 MHz and a variable lower cutoff frequency running from 1 to 70 MHz. Fig. 1 shows the SNR over cutoff frequency averaged over all events for antennas at different distances to the shower core. The resulting behavior can be understood by looking at the frequency dependence of the radio emission’s energy spectrum and the intensity spectrum of the galactic background radiation, depicted in Fig. 2. Towards lower frequencies the energy spectrum of the shower emissions decreases while the background spectrum increases, which leads to a maximum of SNR at cutoff frequencies of about 20–25 MHz. It seems that, for getting the best trigger efficiency, the cutoff frequency should be set to a frequency in this range. 2.2. Bandwidth considerations including RFI background If background measurements as shown in Fig. 3 from the Pierre Auger Observatory are taken into account, it becomes obvious that cosmic radio signals can only be measured in the frequency range between the short wave and FM radio band from 30 to 80 MHz. Here, the background is mainly made up of galactic noise, with sporadic RFI contributions. Compared with Fig. 1 this means, that
Fig. 3. Power spectral density (PSD) of background noise measured at the Pierre Auger Observatory with a logarithmic periodic dipole antenna (LPDA), sensitive from 40 to 80 MHz [8]. The striped line shows the average theoretical galactic noise background and its uncertainty.
Signal to noise ratio [dB]
30 Fig. 4. General schematic of readout electronics. The dashed lines depict the slightly different readout chain used at the RAuger 2 stations.
62m 82m
25
with 30–80 MHz bandwidth the SNR is still close to the theoretical maximum.
125m
20
184m
2.3. General readout principle with RFI background 15
Thus, these spectra lead to a general block schematic of the readout chains used in radio detection setups as shown in Fig. 4.
10 0
10
20
30
40
50
60
70
Lower cutoff frequency [MHz] Fig. 1. Averaged SNR of REAS3 pulses in east–west polarization after band filtering at variable lower cutoff frequency.
Fig. 2. Averaged energy density spectrum of REAS3 pulses at 125 m distance to the shower core and intensity of galactic noise background radiation integrated over 4p solid angle.
2.3.1. Antenna and low noise amplifier The antennas which receive the radio pulses are an essential part of the setup. They need to be mechanically robust in harsh environments, broadband so the acceptance of the system is not limited and should have only a small sensitivity to different soil conditions. Directly attached to the antenna there is a low noise amplifier (LNA), which does a first amplification of the signal so its amplitude sticks out of the noise of the following electronics. The LNA must also be designed to withstand intermodulation caused by RFI outside of the observation range. 2.3.2. Bandpass filter The amplified signal is then fed into a bandpass filter which suppresses the unwanted RFI background and thus prevents electronics from being overdriven. Because the RFI peaks can be several tens of dB above the noise floor, the bandpass filter needs very steep filter flanks with a high stop band attenuation to suppress the RFI outside the passband completely. A side effect of this steep filters is that they also disperse the signal, what leads to a decreasing SNR, however, this behavior can be deconvoluted by digital signal processing.
S148
C. Ruehle / Nuclear Instruments and Methods in Physics Research A 662 (2012) S146–S149
2.3.3. DAQ digitizer At the end of the chain, the preprocessed, band filtered signal is acquired by the DAQ electronics, which can be seen as a smart, low power oscilloscope. The DAQ electronics has also an interface to the communication system. If a trigger occurs, the according signal is saved into a memory, from where it can be requested by a central DAQ system, e.g. if a coincidence with other stations occurred. To avoid the communication system from getting overloaded, signal processing and self-triggering systems have to be implemented in the digitizers.
3. The setups at the Pierre Auger Observatory Currently there are two test setups at the Pierre Auger Observatory, namely MAXIMA, with four stations near the Balloon Launching Station, and RAuger 2, with three stations around the infill tank Apollinario. A new radio detection array, the Auger Engineering Radio Array (AERA) [9], is currently being deployed. 3.1. RAuger 2 The RAuger 2 setup is a further development of the former RAuger setup, described in Ref. [10], and placed with three stations near the Central Laser Facility of the Pierre Auger Observatory. The setup uses horizontally aligned, broadband butterfly dipole antennas to receive radio signals. Consisting of just two wires bended into butterfly shape, this type of antenna can be constructed and deployed easily. The LNA used here is a specially developed low noise integrated circuit (IC) with 30 dB gain [11], which can also be fine tuned electronically to different frequency responses for RFI suppression. Signal acquisition and self-triggering at RAuger 2 is done in a slightly different way compared to the other setups. The LNA output signal is split up according to Fig. 4 (dashed lines) and one part is fed into a so-called ‘filter and trigger board’. On the board, an exchangeable band filter with a bandwidth from 45 to 55 MHz module suppresses RFI. Then, on the filtered signal a straightforward threshold trigger is applied. Limiting the bandwidth to this band leads to a loss of SNR compared to the 30–80 MHz solution, if just galactic noise is taken into account. But here the advantage is, that the probability of narrowband RFI carriers falling into the passband is much lower, compared to the broadband solution. To get the maximum possible information from the signal, the other, unfiltered part of the signal is fed directly into an analog ring buffer memory, sampling at 500 MS/s. If a trigger strobe from the trigger board occurs, the acquisition stops, and a trigger signal is sent via wireless communication system to the central DAQ. In the central DAQ coincident triggers from the other stations are searched. In the case of a coincidence, it sends a readout request to the stations, which then send the content of their analog memory to the central DAQ. 3.2. MAXIMA The MAXIMA setup is placed with four stations near the Balloon Launching Station of the Pierre Auger Observatory. At MAXIMA, LPDAs are used to receive signals, which have a broadband, upward directed characteristic. A first amplification is done by a broadband LNA with traditional monolithic microwave IC amplifiers giving 20 dB of gain. Currently, each MAXIMA station uses commercially available filter and amplifier modules from Mini-Circuits for band filtering from 50 to 70 MHz. These filters can be exchanged easily, making it possible to observe different frequency bands, and to choose the optimal band for self-triggering.
T1 T2 baseline TcMax Tprev
Tper
Fig. 5. Pulse parameters used to discriminate man-made radio pulses.
After filtering, the signals are acquired by a digitizer, sampling at 200 MS/s with 12 bit resolution. An Altera Cyclone III Field Programmable Gate Array (FPGA) directly connected to the analog to digital converters (ADC) is used to implement a time-domain self-trigger algorithm with adjustable parameters for pulse-shape discrimination. After dynamic baseline subtraction, the signal is compared against two independent thresholds: A primary or ‘signal’ threshold T1, and a secondary or ‘noise’ threshold T2, also illustrated in Fig. 5. The trigger can veto RFI pulses using several conditions: ensuring no signal threshold crossings during a quiet period Tprev; allowing a limited number of noise crossings in a trigger period Tper; and allowing a maximum time between noise threshold crossings TcMax. Tuning these parameters allows one to select the isolated bandwidth-limited pulses expected from air showers. Additionally, digital infinite impulse response (IIR) filters can be implemented to suppress RFI. If a trigger decision was done, the corresponding signal trace is saved into the random access memory (RAM) of a Voipac ARM processor module and can be requested by the central DAQ. The transmission of the data is done using a fiber-optical network.
3.3. AERA The knowledge gained with the formerly described setups led to the construction of AERA. AERA is currently being established in three deployment stages, beginning with 24 stations this year, and summing up to 161 stations in the last stage. The LNA is integrated into a so-called Small Black Spider LPDA [12], and has an additional band filter at its input [13]. This way, the band filter suppresses RFI carriers outside the observation band of 30–80 MHz thus preventing intermodulation caused by these carriers. A combined filter-amplifier module with a high stop band attenuation and very steep filter flanks assures a complete suppression of RFI outside the observation band. It also acts as an antialiasing filter for the following DAQ digitizers. Different electronics will be used and tested for digitization, the ones shown before, and a third one especially developed for AERA by KIT and University of Wuppertal. It samples at 180 MS/s at 12 bit resolution, with ADCs directly connected to an Altera Cyclone 3 FPGA, in which algorithms for signal preprocessing are implemented. With an additional RAM module of 2 GByte size, it is possible to buffer about 3 s of ADC data. The buffer bridges the latency of the communication system to allow for an external trigger. In the FPGA, a signal processing chain, as shown in Fig. 6, is implemented, to enhance the SNR for triggering. In this chain, the incoming ADC raw data are Fourier-transformed by a Fast-Fourier-Transformation (FFT). In frequency domain, it is easy to correct the dispersion of the analog band
C. Ruehle / Nuclear Instruments and Methods in Physics Research A 662 (2012) S146–S149
Optimal filter to suppress RFI
Deconvolution offilter response
F (wn) FFT
the SNR increases by about 25–30% after the deconvolution was applied and the signal was transformed back into time domain. Because of the broad observed frequency range, it is more probable, that RFI carriers fall into the passband. To suppress them, an optimal filter is applied to the signal, which does a lower weighting of frequency components polluted with RFI. After an inverse FFT, the cleaned, deconvoluted signal is fed into the trigger logic block, where pulse parameters are calculated, from which a trigger decision is derived.
f (tn) FFT-1
to DAQ Envelope
Threshold
Pulseparameter
S149
Trigger decision
Transient supression Fig. 6. Signal processing and triggering in the FPGA.
arbitrary units
4. Conclusion 600 500 400 300 200 100 0 −100 −200 −300 −400 −500
(a) pulse from analog filter (b) deconvoluted pulse
60
70
80
90
100
110
120
130
samples Fig. 7. Pulse filtered by the filter-amplifier, before and after the deconvolution.
60
number of entries
50
References
40 30 20 10 0 1.05
Different solutions for receiving and triggering on radio pulses have been developed, and deployed in the test setups MAXIMA and RAuger 2 at the Pierre Auger Observatory. One of the main requirements for a system for self-triggered radio detection is to design it for a high attenuation of man-made RFI, to enhance the SNR for triggering. In the first step, this attenuation is done using analog band filters, which limit the observed spectrum to frequencies likely to be untroubled from RFI. Here, MAXIMA and RAuger 2 have relatively narrow band filters. This way, the possibility to catch sporadic RFI carriers is lower, but this results also in a loss of SNR compared to a broadband observation of radio pulses. The experiences gained with these test setups led to the construction of AERA, which is currently being deployed. With a broad bandwidth ranging from 30 to 80 MHz also a suppression of leftover RFI is necessary. This can be implemented by FPGAs, where RFI filtering is done using IIR, respectively, optimal filters, making it possible, to get the maximum SNR for radio pulses.
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S/N ratio increase Fig. 8. Increase of SNR caused by the deconvolution of the analog filter’s dispersion, simulated for several REAS3 pulses.
filters in front of the digitizer as depicted in Fig. 7. The increase of SNR caused by the deconvolution was simulated using several pulses from REAS3, polluted with galactic noise. As shown in Fig. 8,
[1] J. Abraham, et al., Pierre Auger Collaboration, Nucl. Instr. and Meth. A 523 (2004) 50. [2] H. Falcke, P.W. Gorham, Astropart. Phys. 19 (2003) 477. [3] T. Huege, R. Ulrich, R. Engel, Astropart. Phys. 30 (2008) 96. [4] O. Scholten, K. Werner, F. Rusydi, Astropart. Phys. 29 (2008) 94. [5] H. Falcke, et al., LOPES Collaboration, Nature 435 (2005) 435. [6] O. Ravel, et al., CODALEMA Collaboration, Nucl. Instr. and Meth. A 518 (2004) 213. [7] G.A. Dulk, W.C. Erickson, R. Manning, J.-L. Bougeret, Astron. Astrophys. 365 (2001) 294. [8] T. Winchen, Diploma Thesis, RWTH Aachen Unversity, 2007. [9] S. Fliescher, forthe Pierre Auger Collaboration, Nucl. Instr. and Meth. A, this issue, doi:10.1016/j.nima.2010.11.045. [10] B. Revenu, for the Pierre Auger Collaboration, Nucl. Instr. and Meth. A, this issue. [11] D. Charrier, for the Pierre Auger Collaboration, Nucl. Instr. and Meth. A, this issue, doi:10.1016/j.nima.2010.10.141. [12] O. Seeger, for the Pierre Auger Collaboration, Nucl. Instr. and Meth. A, this issue, doi:10.1016/j.nima.2010.10.140. [13] M. Stephan, for the Pierre Auger Collaboration, in: Proceedings of APMC 2010, to appear.