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We review the development of our digital broadband Fast Fourier Transform Spectrometers (FFTS). In just a few years, FFTS back-ends - optimized for a wide range of radio astronomical applications - have become a new standard for heterodyne receivers, particularly in the mm and sub-mm wavelength range. They offer high instantaneous bandwidths with many thousands spectral channels on a small electronic board (100 x 160 mm). Our FFT spectrometer make use of the latest versions of GHz analog-to-digital converters (ADC) and the most complex field programmable gate array (FPGA) chips commercially available today. These state-of-the-art chips have made possible to build digital spectrometers with instantaneous bandwidths up to 1.8 GHz and 8192 spectral channels.
We present our second generation of broadband Fast Fourier Transform Spectrometer (FFTS), optimized for a wide range of radio astronomical applications. The new digitizer and analyzer boards make use of the latest versions of GHz analogto-digital converters and the most complex field programmable gate array chips commercially available today. These state-ofthe-art chips have made possible to build digital spectrometers with instantaneous bandwidths up to 1.8 GHz and 8192 spectral channels.
Known and novel techniques are described to implement a Fast Fourier Transform (FFT) in hardware, such that parallelized data can be processed. The usage of both - real and imaginary FFT-input - can help saving hardware. Based on the different techniques, flexible to use FFT-implementations have been developed by combining standard FFT-components (partly IP) and are compared, according to their hardware utilization. Finally, applicability has been demonstrated in practice by a FFTimplementation with 8192 channels as part of a FPGAspectrometer with a total bandwidth of 1.5 GHz.
Superconducting heterodyne receiver has played a vital role in the high resolution spectroscopy applications for astronomy and atmospheric research up to 2THz. NbN hot electron bolometer (HEB) mixer, as the most sensitive mixer above 1.5THz, has been used in the Herschel space telescope for 1.4-1.9THz and has also shown an ultra-high sensitivity up to 5.3THz. Combined a HEB mixer with a novel THz quantum cascade laser (QCL) as local oscillator (LO), such an all solid-state heterodyne receiver provides the technology which can be used for any balloon-, air- and space-borne heterodyne instruments above 2THz. Here we report the first high-resolution heterodyne spectroscopy measurement using a gas cell and using such a HEB-QCL receiver. The receiver employs a 2.9THz metal-metal waveguide QCL as LO and a NbN HEB as a mixer. By using a gas cell filled with methanol (CH3OH) gas in combination with hot/cold blackbody loads as signal source, we successfully recorded the methanol emission line around 2.918THz. Spectral lines at different pressures and also different frequency of the QCL are studied.
Based on our reconfigurable FPGA spectrometer technology, we have developed a read-out system, operating in the frequency domain, for arrays of Microwave Kinetic Inductance Detectors (MKIDs). The readout consists of a combination of two digital boards: A programmable DAC-/FPGA-board (tone-generator) to stimulate the MKIDs detectors and an ADC-/FPGA-unit to analyze the detectors response. Laboratory measurement show no deterioration of the noise performance compared to low noise analog mixing. Thus, this technique allows capturing several hundreds of detector signals with just one pair of coaxial cables.
Multi-epoch searches for relativistic binary pulsars and fast transients in the Galactic Centre
(2021)
We report the status of a search for pulsars in the Galactic Centre, using a completely revised and improved high-sensitivity doublehorn system at 4.85-GHz. We also present calculations about the success rate of periodicity searches for such a survey, showing that in contrast to conclusions in recent literature pulsars can be indeed detected at the chosen search frequency.
Microwave Kinetic Inductance Detectors have great potential for large very sensitive detector arrays for use in, for example, ground and spaced based sub?mm imaging. Being intrinsically readout in the frequency domain, they are particularly suited for frequency domain multiplexing allowing 1000s of devices to be readout with one pair of coaxial cables. However, this moves the complexity of the detector from the cryogenics to the warm electronics. We present the use of a readout based on a Fast Fourier transform Spectrometer, showing no deterioration of the noise performance compared to low noise analog mixing while allowing high multiplexing ratios (>100). We present use of this technique to multiplex 44 MKIDs, while this and similar setups are regularly now being used in our array development. This development will help the realization of large cameras, particularly in the short term for ground based astronomy.
The Anomalous X‐ray Pulsar 4U 0142+61 is the only neutron star where it is believed that one of the long searched‐for ‘fallback’ disks has been detected in the mid‐IR by Wang et al. [1] using Spitzer. Such a disk originates from material falling back to the NS after the supernova. We search for cold circumstellar material in the 90 GHz continuum using the Plateau de Bure Interferometer. No millimeter flux is detected at the position of 4U 0142+61, the upper flux limit is 150 μJy corresponding to the 3σ noise rms level. The re‐processed Spitzer MIPS 24μm data presented previously by Wang et al. [2] show some indication of flux enhancement at the position of the neutron star, albeit below the 3σ statistical significance limit. At far infrared wavelengths the source flux densities are probably below the Herschel confusion limits.
We report on submillimetre bolometer observations of the isolated neutron star RX J1856.5−3754 using the Large Apex Bolometer Camera bolometer array on the Atacama Pathfinder Experiment telescope. No cold dust continuum emission peak at the position of RX J1856.5−3754 was detected. The 3σ flux density upper limit of 5 mJy translates into a cold dust mass limit of a few earth masses. We use the new submillimetre limit, together with a previously obtained H-band limit, to constrain the presence of a gaseous, circumpulsar disc. Adopting a simple irradiated disc model, we obtain a mass accretion limit of Graphic and a maximum outer disc radius of ∼1014 cm. By examining the projected proper motion of RX J1856.5−3754, we speculate about a possible encounter of the neutron star with a dense fragment of the CrA molecular cloud a few thousand years ago.
Context.We present the technology and first scientific results of a new generation of very flexible and sensitive spectrometers, well-suited for the needs of spectral-line radio and (sub)millimeter astronomy: Fast Fourier Transform Spectrometers (FFTS), which are in operation at the Atacama Pathfinder EXperiment (APEX) telescope.
Aims. The FFTS for APEX is a novel high-resolution 2 x 1 GHz bandwidth digital spectrometer backend. Due to its high frequency resolution, and the demonstrated capability of operating at high altitude, the FFTS became the facility spectrometer for spectral line observations at APEX.
Methods. The FFTS is based on one of the currently most powerful digitizer/analyzer boards available from Acqiris, Switzerland. The board incorporates two 1 Gsamples/s analog-to-digital converters (ADCs) with 8-bit resolution which feed an on-board complex field programmable gate array (FPGA) chip. The enormous processing power by today's FPGAs allow for a complete real-time FFT signal processing pipeline to decompose a 1 GHz band into 16 384 spectral channels in just one chip.
Results. Since May 2005 an MPIfR FFTS has been extensively used in all regular spectroscopic science observations. The performance at APEX was demonstrated to be very reliable and as good as measured in the first laboratory tests which finally led to the request to provide a second, facility type FFTS for APEX. The unit was delivered and commissioned in March this year.
Conclusions. Using a commercially available digitizer board, it was possible to develop a complete FFTS in only a few months. Successful observations at APEX demonstrate that this new generation of FPGA-based spectrometers easily matching and superseding the performance of older technology spectrometers and can built up much more easily. Furthermore, the by now available class of new high-speed ADCs and the continuous increase of FPGA processing power makes it very likely that FFTS can be pushed to broader bandwidth and even more spectral channels in the near future.
We discuss our recent discovery of the giant radio emission from the Crab pulsar at its high frequency components (HFCs) phases and show the polarization characteristic of these pulses. This leads us to a suggestion that there is no difference in the emission mechanism of the main pulse (MP), interpulse (IP) and HFCs. We briefly review the size distributions of the Crab giant radio pulses (GRPs) and discuss general characteristics of the GRP phenomenon in the Crab and other pulsars.