scholarly journals Cassiopeia A, Cygnus A, Taurus A, and Virgo A at ultra-low radio frequencies

2020 ◽  
Vol 635 ◽  
pp. A150 ◽  
Author(s):  
F. de Gasperin ◽  
J. Vink ◽  
J. P. McKean ◽  
A. Asgekar ◽  
I. Avruch ◽  
...  
Keyword(s):  
Dynamic Range ◽  
Low Frequency ◽  
High Dynamic Range ◽  
Source Model ◽  
Antenna System ◽  
Low Frequencies ◽  
Cassiopeia A ◽  
Cygnus A ◽  
Long Baselines ◽  
Better Than

Context. The four persistent radio sources in the northern sky with the highest flux density at metre wavelengths are Cassiopeia A, Cygnus A, Taurus A, and Virgo A; collectively they are called the A-team. Their flux densities at ultra-low frequencies (< 100 MHz) can reach several thousands of janskys, and they often contaminate observations of the low-frequency sky by interfering with image processing. Furthermore, these sources are foreground objects for all-sky observations hampering the study of faint signals, such as the cosmological 21 cm line from the epoch of reionisation. Aims. We aim to produce robust models for the surface brightness emission as a function of frequency for the A-team sources at ultra-low frequencies. These models are needed for the calibration and imaging of wide-area surveys of the sky with low-frequency interferometers. This requires obtaining images at an angular resolution better than 15″ with a high dynamic range and good image fidelity. Methods. We observed the A-team with the Low Frequency Array (LOFAR) at frequencies between 30 MHz and 77 MHz using the Low Band Antenna system. We reduced the datasets and obtained an image for each A-team source. Results. The paper presents the best models to date for the sources Cassiopeia A, Cygnus A, Taurus A, and Virgo A between 30 MHz and 77 MHz. We were able to obtain the aimed resolution and dynamic range in all cases. Owing to its compactness and complexity, observations with the long baselines of the International LOFAR Telescope will be required to improve the source model for Cygnus A further.

Preamplifier With Ultra Low Frequency Cutoff for Infrasonic Condenser Microphone

2012 ◽  
Author(s):  
Rasmus Trock Kinnerup ◽  
Arnold Knott ◽  
Ole Cornelius Thomsen ◽  
Kresten Marbjerg ◽  
Per Rasmussen
Keyword(s):  
Input Impedance ◽  
Dynamic Range ◽  
Cutoff Frequency ◽  
Low Frequency ◽  
Input Resistance ◽  
Bias Current ◽  
Microphone Signal ◽  
Low Frequencies ◽  
Pass Filter ◽  
Condenser Microphone

Measuring infrasonic sound sets high requirements on the instruments used. Typically the measurement chain consists of a microphone and a preamplifier. As the input resistance of the preamplifier forms a high pass filter with the capacitance of the microphone in the picofarad range, measuring ultra low frequencies becomes a challenge. The electric preamplifier presented in this paper together with a prepolarized condenser microphone form a measurement system. The developed preamplifier connects the microphone signal directly to the input of an operational amplifier with ultra high input impedance. The bias current for the preamplifier further complicates the signal amplification. A configuration of two diode-connected FETs provide the input bias current. The resulting input impedance of nearly 1 TΩ yields a total lower limiting −3 dB cutoff frequency of 8 mHz and a dynamic range of 95 dB. Being able to measure down to ultra low frequencies in the infrasonic frequency range will aid actors in the debate on wind turbine noise. Sonic booms from supersonic flights include frequencies down to 10 mHz and the preamplifier proposed in this paper will aid scientists trying to modify the N-shaped shock wave at high level which prohibits flights in land zones.

scholarly journals A Large Decametric Wavelength Antenna Array for IPS Observations of Radio Sources

1980 ◽  
Vol 91 ◽  
pp. 403-403
Author(s):  
Ch. V. Sastry
Keyword(s):  
Solar Wind ◽  
Antenna Array ◽  
Low Frequency ◽  
Antenna System ◽  
Radio Sources ◽  
The Sun ◽  
Low Frequencies ◽  
Interplanetary Scintillations

Most observations of interplanetary scintillations of radio sources are made at frequencies around 80 MHz. These observations are limited to regions close to the sun, where the scintillations are maximum at this frequency. It is possible to extend these observations to the weakly scattering regions beyond 1 A.U. by making measurements at low frequencies. We have built a low frequency antenna system at Gauribidanur, India (Lat. 13° 36′ N and Long. 5 hrs. 10 min.), which can be used for this purpose. Although this system will not be dedicated to IPS, we intend to use it exclusively for solar wind observations during periods of interest.

scholarly journals Galactic neutral hydrogen and the magnetic ISM foreground

2017 ◽  
Vol 12 (S333) ◽  
pp. 146-150
Author(s):  
S.E. Clark
Keyword(s):  
Interstellar Medium ◽  
Dynamic Range ◽  
Low Frequency ◽  
Neutral Hydrogen ◽  
Depth Resolution ◽  
High Dynamic Range ◽  
Neutral Gas ◽  
High Dynamic ◽  
New Generation ◽  
The Relationship

AbstractThe interstellar medium is suffused with magnetic fields, which inform the shape of structures in the diffuse gas. Recent high-dynamic range observations of Galactic neutral hydrogen, combined with novel data analysis techniques, have revealed a deep link between the morphology of neutral gas and the ambient magnetic field. At the same time, an observational revolution is underway in low-frequency radio polarimetry, driven in part by the need to characterize foregrounds to the cosmological 21-cm signal. A new generation of experiments, capable of high angular and Faraday depth resolution, are revealing complex filamentary structures in diffuse polarization. The relationship between filamentary structures observed in radio-polarimetric data and those observed in atomic hydrogen is not yet well understood. Multiwavelength observations will enable new insights into the magnetic interstellar medium across phases.

scholarly journals Computer simulation of low‐frequency electromagnetic data acquisition

Geophysics ◽  
1983 ◽  
Vol 48 (9) ◽  
pp. 1219-1232 ◽  
Author(s):  
William A. San Filipo ◽  
Gerald W. Hohmann
Keyword(s):  
Computer Simulation ◽  
Data Acquisition ◽  
Dynamic Range ◽  
Low Frequency ◽  
Noise Removal ◽  
Digital Data ◽  
Low Frequencies ◽  
Pass Filter ◽  
Natural Field ◽  
Field Noise

Computer simulation of low‐frequency electromagnetic (EM) digital data acquisition in the presence of natural field noise demonstrates several important limitations and considerations. Without a remote reference noise removal scheme, it is difficult to obtain an adequate ratio of signal to noise below 0.1 Hz for frequency‐domain processing and below 0.3 Hz base frequency for time‐domain processing for a typical source‐receiver configuration. A digital high‐pass filter substantially facilitates rejection of natural field noise above these frequencies; however, at lower frequencies where much longer stacking times are required, it becomes ineffective. Use of a remote reference to subtract natural field noise extends these low‐frequency limits by one decade, but the remote reference technique is limited by the resolution and dynamic range of the instrumentation. Gathering data in short segments so that natural field drift can be offset for each segment allows a higher gain setting to minimize dynamic range problems. The analysis is also applicable to the induced polarization technique in which similar problems arise at low frequencies in the presence of telluric noise.

Towards a microchannel-based X-ray detector with two-dimensional spatial and time resolution and high dynamic range

2015 ◽  
Vol 22 (5) ◽  
pp. 1202-1206 ◽  
Author(s):  
Bernhard W. Adams ◽  
Anil U. Mane ◽  
Jeffrey W. Elam ◽  
Razib Obaid ◽  
Matthew Wetstein ◽  
...  
Keyword(s):  
Spatial Resolution ◽  
Time Resolution ◽  
Dynamic Range ◽  
High Dynamic Range ◽  
High Time Resolution ◽  
Two Dimensional ◽  
X Ray ◽  
High Dynamic ◽  
Time Gating ◽  
Better Than

X-ray detectors that combine two-dimensional spatial resolution with a high time resolution are needed in numerous applications of synchrotron radiation. Most detectors with this combination of capabilities are based on semiconductor technology and are therefore limited in size. Furthermore, the time resolution is often realised through rapid time-gating of the acquisition, followed by a slower readout. Here, a detector technology is realised based on relatively inexpensive microchannel plates that uses GHz waveform sampling for a millimeter-scale spatial resolution and better than 100 ps time resolution. The technology is capable of continuous streaming of time- and location-tagged events at rates greater than 107events per cm2. Time-gating can be used for improved dynamic range.

scholarly journals Digital signal processsing functions for ultra-low frequency calibrations

ACTA IMEKO ◽  
2020 ◽  
Vol 9 (5) ◽  
pp. 374
Author(s):  
Henrik Ingerslev ◽  
Soren Andresen ◽  
Jacob Holm Winther
Keyword(s):  
Signal Processing ◽  
Low Frequency ◽  
Digital Signal ◽  
Low Noise ◽  
Key Comparison ◽  
Low Frequencies ◽  
Dual Channel ◽  
Ultra Low Noise ◽  
Lower Noise ◽  
Better Than

The demand from industry to produce accurate acceleration measurements down to ever lower frequencies and with ever lower noise is increasing. Different vibration transducers are used today for many different purposes within this area, like detection and warning for earthquakes, detection of nuclear testing, and monitoring of the environment. Accelerometers for such purposes must be calibrated in order to yield trustworthy results and provide traceability to the SI-system accordingly. For these calibrations to be feasible, suitable ultra low-noise accelerometers and/or signal processing functions are needed. <br />Here we present two digital signal processing (DSP) functions designed to measure ultra low-noise acceleration in calibration systems. The DSP functions use dual channel signal analysis on signals from two accelerometers measuring the same stimuli and use the coherence between the two signals to reduce noise. Simulations show that the two DSP functions are estimating calibration signals better than the standard analysis. <br />The results presented here are intended to be used in key comparison studies of accelerometer calibration systems, and may help extend current general low frequency range from e.g. 100 mHz down to ultra-low frequencies of around 10mHz, possibly using somewhat same instrumentation.

scholarly journals 21. The low-frequency spectrum of Cygnus A and Cassiopeia A

1957 ◽  
Vol 4 ◽  
pp. 145-147
Author(s):  
R. J. Lamden ◽  
A. C. B. Lovell
Keyword(s):  
Lower Limit ◽  
Radio Telescope ◽  
Low Frequency ◽  
Radio Sources ◽  
Pencil Beam ◽  
Frequency Range ◽  
Present Communication ◽  
Cassiopeia A ◽  
Cygnus A ◽  
Radio Transmitters

The published measurements of the intensity of the radio sources cover a frequency range down to a lower limit of 22·6 Mc./s., at which measurements have been made on Cygnus and Cassiopeia by Hey and Hughes (1954)[1]. Information about the spectrum at still lower frequencies is difficult to obtain because of interference arising from ionospheric reflexion of distant radio transmitters. Some of this trouble can be alleviated by using a narrow pencil-beam radio telescope for reception and the present communication describes measurements made on frequencies of 16·5, 19·0, 22·6 and 30·0 Mc./s. using the 218 ft. transit radio telescope at Jodrell Bank.

scholarly journals High dynamic range, Interferences Tolerant, Digital Receivers for Radioastronomy: Results and Projects at Paris and Nanay Observatory

2002 ◽  
Vol 199 ◽  
pp. 506-507
Author(s):  
Carlo Rosolen ◽  
Alain Lecacheux ◽  
Eric Gerard ◽  
Vincent Clerc ◽  
Laurent Denis
Keyword(s):  
Real Time ◽  
Dynamic Range ◽  
Block Diagram ◽  
Low Frequency ◽  
Parallel Architecture ◽  
Digital Signal ◽  
High Dynamic Range ◽  
Dsp System ◽  
High Dynamic ◽  
Digital Receivers

Radio astronomy in the decameter to centimeter wavelength range is facing new challenges because of man made interferences due to increasing needs in telecommunications. At the Radioastronomy department of Paris Meudon Observatory, we have been working since four years on high dynamic range digital receivers based on Digital Signal Processors (DSP). The first achievement is a digital spectro- polarimeter devoted to spectroscopy of astrophysical radiation in decameter range, now in operation at the Nancay Decameter array. The block diagram of the receiver includes a high dynamic range analogue section followed by a 12 bits analogue to digital converter. The digital part makes use of high power, programmable digital circuits for signal processing, arranged in a dedicated parallel architecture, able to compute in real time the power spectrum and the correlation of the input signals. This receiver was also used, as spectrometer backend, at Nancay decimetric radiotelescope and has performed very well in the presence of very strong interferences. We are presently working on a new digital receiver with broader bandwidth. The objective is 2 × 25 MHz band with at least 60 dB dynamic range. This new receiver will use additional computation power in order to recognise and avoid man made interferences which corrupt the radio astronomical signal. At the Nancay Radioastronomy Observatory, we have started to develop a new digital configurable receiver with 8 times 25 MHz band and ten thousand channels. For low frequency radioastronomy, direct spectrum computation technique is really powerful and offers new capabilities for real time interferences excision. Fig. 1 shows pulsar observations in the presence of interference made with the DSP receiver on the UTR-2 radiotelescope. Fig. 2 shows the effect of satellite interfernce on OH observations made with the Nancay telescope. Fig. 3 shows the block diagram of the DSP system and demonstrates how offline excision of interference in the frequency time-domain enables recovery of the signal. The final spectrum had 960 minutes integration on and off source and took 8045 minutes of procession on a 450 MHz Pentium II.

scholarly journals Adaptive-scale wide-field reconstruction for radio synthesis imaging

2020 ◽  
Vol 640 ◽  
pp. A80
Author(s):  
L. Zhang ◽  
L. G. Mi ◽  
M. Zhang ◽  
X. Liu ◽  
C. L. He
Keyword(s):  
Dynamic Range ◽  
High Dynamic Range ◽  
Wide Field ◽  
Observation Data ◽  
Field Observations ◽  
High Dynamic Range Imaging ◽  
Range Imaging ◽  
Low Frequencies ◽  
Very Large Array ◽  
High Dynamic

Sky curvature and non-coplanar effects, caused by low frequencies, long baselines, or small apertures in wide field-of-view instruments such as the Square Kilometre Array (SKA), significantly limit the imaging performance of an interferometric array. High dynamic range imaging essentially requires both an excellent sky model and the correction of imaging factors such as non-coplanar effects. New CLEAN deconvolution with adaptive-scale modeling already has the ability to construct significantly better narrow-band sky models. However, the application of wide-field observations based on modern arrays has not yet been jointly explored. We present a new wide-field imager that can model the sky on an adaptive-scale basis, and the sky curvature and the effects of non-coplanar observations with the w-projection method. The degradation caused by the dirty beam due to incomplete spatial frequency sampling is eliminated during sky model construction by our new method, while the w-projection mainly removes distortion of sources far from the image phase center. Applying our imager to simulated SKA data and the real observation data of the Karl G. Jansky Very Large Array (an SKA pathfinder) suggested that our imager can handle the effects of wide-field observations well and can reconstruct more accurate images. This provides a route for high dynamic range imaging of SKA wide-field observations, which is an important step forward in the development of the SKA imaging pipeline.

A calibration and imaging strategy at 300 MHz with the Murchison Widefield Array (MWA)

2021 ◽  
Vol 38 ◽  
Author(s):  
Jaiden H. Cook ◽  
Nicholas Seymour ◽  
Marcin Sokolowski
Keyword(s):  
Dynamic Range ◽  
Low Frequency ◽  
High Dynamic Range ◽  
Primary Beam ◽  
Main Lobe ◽  
Beam Model ◽  
Highly Sensitive ◽  
Imaging Strategy ◽  
Murchison Widefield Array ◽  
Beam Patterns

Abstract At relatively high frequencies, highly sensitive grating sidelobes occur in the primary beam patterns of low frequency aperture arrays (LFAA) such as the Murchison Widefield Array (MWA). This occurs when the observing wavelength becomes comparable to the dipole separation for LFAA tiles, which for the MWA occurs at ${\sim}300$ MHz. The presence of these grating sidelobes has made calibration and image processing for 300 MHz MWA observations difficult. This work presents a new calibration and imaging strategy which employs existing techniques to process two example 300 MHz MWA observations. Observations are initially calibrated using a new 300 MHz sky-model which has been interpolated from low frequency and high frequency all-sky surveys. Using this 300 MHz model in conjunction with the accurate MWA tile primary beam model, we perform sky-model calibration for the two example observations. After initial calibration a self-calibration loop is performed by all-sky imaging each observation. We mask the main lobe of the all-sky image, and perform a sky-subtraction by estimating the masked image visibilities. We then image the main lobe of the sky-subtracted visibilities, which results in high dynamic range images of the two example observations. These images have been convolved with a Gaussian to a resolution of $2.4$ arcminutes, with a maximum sensitivity of ${{\sim}}31\,\textrm{mJy/beam}$ . The calibration and imaging strategy demonstrated in this work opens the door to performing science at 300 MHz with the MWA, which was previously an inaccessible domain. With this paper we release the code described below and the cross-matched catalogue along with the code to produce a sky-model in the range 70–1 400 MHz.

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