Even though it is one of the oldest scientific disciplines astronomy remains at the cutting edge of technology and engineering. Once limited to the the observation of visible light, mostly in the night time sky, the 20th century saw the development of a host of new methods for exploring the cosmos. Using instrumentation sensitive to an ever expanding part of the electromagnetic spectrum scientists have been able to detect a host of previously unknown phenomena and look further into space than ever before. Radio telescopes were developed revealing the remnants of exploding stars while even higher energy instruments, like those using X-rays and gamma rays, made it possible to study the behavior of matter in extreme states, such as those found in pulsars and blazars.
Radio telescopes and interferometers typically capture and analyze millimeter wavelength molecular spectra using a spectrometer. Historically this was done using analog technology but recent developments have seen the use of high speed digitizers coupled with FFT based data analysis. For radio astronomy Spectrum digitizers with high sampling rates and wide bandwidth can offer advantages in dynamic range, frequency stability, size and cost.
In gamma ray astronomy scientists observe Cherenkov radiation that is produced when gamma rays are absorbed in the earth’s atmosphere. The Cherenkov radiation detected is fast, lasting only a few nanoseconds, and low level. As such fast and sensitive electronics is required to detect the phenomena. Typically banks of detectors are arranged in arrays so that the arrival times of each event can be be used determine the direction from which the gamma rays originated.
For radio and gamma ray astronomy Spectrum offers digitizers with a range of bandwidths, sampling rates, and dynamic range so that they can best match the requirements of the application. When large dynamic range and maximum sensitivity is needed high-resolution 14 and 16 bit digitizers are available for the capture and analysis of signals that go as high as 250 MHz in frequency. These high-resolution cards deliver outstanding signal-to-noise ratio's (up to 72 dB) and spurious free dynamic range (of up to 90 dB) so that low level signals can be detected and analyzed. For even higher frequencies 8 bit digitizers are available that offer up to 5 GS/s sampling rates and 1.5 GHz bandwidth.
Each digitizer card can have from one to four channels and up to eight cards can be linked together with Spectrum's StarHub system to create instruments with up to 32 fully synchronous channels. This makes them ideally suited to applications where multiple receivers or detectors are used. With large on-board memories (up to 4 Gsamples/card) and advanced streaming and readout modes the digitizers are ideal for capturing long and complex signals. Streaming data over the cards fast PCIe bus to a RAID based storage array can even allow the capture and storage of hours of information. Spectrum's S Bench 6 software can also be used to view and qualify signals as well as perform FFT calculations for frequency domain analysis.
Spectrum Product Features
- 12, 14 and 16 Bit Resolution
- Sampling rates up to 10 GS/s and Bandwidth over 1.5 GHz
- Segmented Memory with FIFO Readout
- Streaming data to RAID disc array at up to 3 GByte/s continuously
- GPU-based fast data processing with SCAPP
- Star-Hub for creating systems with up to 32 fully synchronous channels
Matching Card Families
6.40 GS/s - 3.20 GS/s
1.25 GS/s - 5 GS/s
130 MS/s - 400 MS/s
14 Bit 16 Bit
RF Measurements Using a Modular Digitizer
Modern modular digitizers, like the Spectrum M4i series PCIe digitizers, offer greater bandwidth and higher resolution at any given bandwidth than ever before. Although they are in the class of general purpose measuring instruments they are capable of many RF and lower microwave frequency measurements. This article focuses on some examples of common RF measurements that can be performed with these modular digitizers.
Case Study: Groundbreaking microwave spectrometer for research of interstellar materials at Smithonian Institute
Microwave spectroscopy is a very powerful tool for discovering molecular structures and operates at very low temperatures near absolute zero (1 to 5 Kelvin). The spectrometers generally either operate with high sensitivity over a very narrow bandwidth or a wide frequency with reduced sensitivity. Researchers at the Harvard Smithsonian Center for Astrophysics have used a Spectrum Instrumentation digitizer card to create a next generation molecular spectrometer with both high resolution and high sensitivity that is capable of capturing sample data substantially faster.
Signal Processing for Digitizers
Modular digitizers allow accurate, high resolution data acquisition that can be quickly transferred to a host computer. Signal processing functions, applied in the digitizer or in the host computer, permit the enhancement of the acquired data or the extraction of extremely useful information from a simple measurement.
Case Study: Max Planck Institute uses Spectrum's digitizer cards to measure diameters of distant stars
The MAGIC telescopes on the Canary Island of La Palma were built to observe cosmic objects that emit high-energy gamma rays, i.e. supernovae or black holes. Astronomers also use the twin telescope to measure the diameter of stars to investigate the processes throughout their life cycle. This is a challenging task for earthbound telescopes, since the angular diameter of stars is extremely small: only a few milli-arc-seconds.
High Energy Stereoscopic System
At the Erlangen Centre for Astroparticle Physics, Friedrich-Alexander-Universität Erlangen-Nürnberg, Germany, the calibration of photomultiplier tubes for intensity interferometry used at H.E.S.S. (High Energy Stereoscopic System) has been the basis of a Master’s Thesis. The study examined a calibration process for two types of PMT used at the H.E.S.S. telescopes under various voltages. Pulse shape was also investigated. To acquire and analyse the PMT pulses an M4i.2212-x8 1.25 GS/s, 8-bit Digitizer was used. The thesis document discussing the setup and results can be found here:Master Thesis
Space Debris Tracking
At the Polytechnic University of Madrid’s, Technical School of Telecommunications Engineering, in Spain, they are studying how radar systems can be improved to allow better tracking of space debris orbiting earth. A masters paper discussing how signal generation can be adapted and optimized is below (in Spanish) with data acquired using an M4i.4420-x8, 250 MS/s, 16 bit, digitizerMasters Paper (Spanish)
Search for Axion-like Dark Matter
In a search for axion-like dark matter the Department of Physics, Boston University, USA is using an M4i.4421-x8 250 MS/s, 16 bit, digitizer to help monitor and analyse the behaviour of oscillating magnetic fields. A paper discussing their findings and experimental results that explore the coupling strengths and masses of axion-like particles can be downloaded here:Research Paper
The Department of Physics, Keio University, Japan is using an M4i.2230-x8 5 GS/s Digitizer for high speed data acquisition with fast time domain averaging to perform scan-free high-resolution direct-comb spectroscopy where the mode spacing of an optical frequency comb is reduced down to 260 kHz by phase modulation. Details on the development can be found here:Research Paper
Optical Frequency Combs in Spectroscopy
The optical frequency comb (OFC) technique enables high-precision optical metrology which has found application in many scientific areas, such as molecular spectroscopy, astronomy, and particle physics. At Keio University, Yokohama, Japan they have demonstrated a measurement protocol that enables determination of the absolute mode numbers of both OFCs in a dual-comb spectroscopy system. The system uses an M2p.5962-x4 125 MS/s, 16-bit Digitizer for signal acquisition. A research paper on the development can be found hereResearch Paper
Astronomical Intensity Interferometry
At the Erlangen Centre for Astroparticle Physics, Friedrich–Alexander University, Germany they are studying astronomical intensity interferometry methods that enable quantitative measurements of the source geometry by measuring the photon fluxes in individual telescopes and correlating them, instead of correlating the electromagnetic waves' amplitudes. Laboratory simulation uses an M4i.2212-x8 1.25 GS/s, 8-bit Digitizer for signal acquisition and a paper describing the research is available here:Research Paper
Searching for Axion Dark Matter
At the Department of Physics, Korea Advanced Institute of Science and Technology (KAIST), in the Republic of Korea, they have developed a fast data acquisition system for axion dark matter searches. The system is based on a microwave resonant cavity and uses an M4i.4470-x8 180 MS/s, 16-bit, Digitizer for signal acquisition. The aim is to achieve very high scanning rates. A paper discussing the development can be found here:Research Paper