FFT Spectrometer Research Articles

A Real-Time Digital Receiver for Correlation Measurements in Atomic Systems

We present the development and characterization of a generic, reconfigurable, low-cost ($<$ 350 USD) software-defined digital receiver system (DRS) for temporal correlation measurements in atomic spin ensembles. We demonstrate the use of the DRS as a component of a high resolution magnetometer. Digital receiver based fast Fourier transform spectrometers (FFTS) are generally superior in performance in terms of signal-to-noise ratio (SNR) compared to traditional swept-frequency spectrum analyzers (SFSA). In applications where the signals being analyzed are very narrow band in frequency domain, recording them at high speeds over a reduced bandwidth provides flexibility to study them for longer periods. We have built the DRS on the STEMLab 125-14 FPGA platform and it has two different modes of operation: FFT Spectrometer and real time raw voltage recording mode. We evaluate its performance by using it in atomic spin noise spectroscopy (SNS). We demonstrate that the SNR is improved by more than one order of magnitude with the FFTS as compared to that of the commercial SFSA. We also highlight that with this DRS operating in the triggered data acquisition mode one can achieve spin noise (SN) signal with high SNR in a recording time window as low as 100 msec. We make use of this feature to perform time resolved high-resolution magnetometry. While the receiver was initially developed for SNS experiments, it can be easily used for other atomic, molecular and optical (AMO) physics experiments as well.

Frequency-Agile FFT Spectrometer for Microwave Remote Sensing Applications

We report on a Fast Fourier Transform Spectrometer (FFTS) that provides larger bandwidth by fast local oscillator switching of the base-band converter. We demonstrate that this frequency scanning technique is suited for atmospheric remote sensing and conduct measurements of atmospheric ozone using the WIRA-C (WInd RAdiometer for Campaigns) Doppler wind radiometer. The comparison of our measurements to an adjusted atmospheric and instrumental model exposes no systematic biases due to the switching procedure in the measured spectra. It further shows that the combination of high spectral resolution with large bandwidth yields good measurement response to stratospheric and mesospheric ozone from approximately a 20 km to 70 km altitude with a resolution of 7 km in the lower stratosphere to 20 km in the mesosphere. We conclude that low-cost, low-power software-defined radio hardware designed for communications applications is very well suited for a variety of spectroscopic applications, including ozone monitoring. This allows the design of low-cost, multi-purpose instruments for atmospheric remote sensing and thus has a direct impact on future radiometer developments and their adoption in remote sensing campaigns and networks.

Performance evaluation of THz Atmospheric Limb Sounder (TALIS) of China

Abstract. THz Atmospheric Limb Sounder (TALIS) is a microwave limb sounder being developed for atmospheric vertically resolved profile observations by the National Space Science Center, Chinese Academy of Sciences (NSSC, CAS). It is designed to measure temperature and chemical species such as O3, HCl, ClO, N2O, NO, NO2, HOCl, H2O, HNO3, HCN, CO, SO2, BrO, HO2, H2CO, CH3Cl, CH3OH, and CH3CN with a high vertical resolution from about 10 to 100 km to improve our comprehension of atmospheric chemistry and dynamics and to monitor the man-made pollution in the atmosphere. Four heterodyne radiometers including several FFT spectrometers of 2 GHz bandwidth with 2 MHz resolution are employed to obtain the atmospheric thermal emission in broad spectral regions centred near 118, 190, 240, and 643 GHz. A theoretical simulation is performed to estimate the retrieval precision of the main targets and to compare them with that of Aura MLS standard spectrometers. Single scan measurement and averaged measurement are considered in the simulation, respectively. The temperature profile can be obtained with a precision of &lt;2 K for a single scan from 10 to 60 km by using the 118 GHz radiometer, and the 240 and 643 GHz radiometers can provide temperature information in the upper troposphere. Chemical species such as H2O, O3, and HCl show a relatively good single scan retrieval precision of &lt;20 % over most of the useful range and ClO, N2O, and HNO3 can be retrieved with a precision of &lt;50 %. The other species should be retrieved by using averaged measurements because of the weak intensity and/or low abundance.

Piggyback search for fast radio bursts using Nanshan 26 m and Kunming 40 m radio telescopes – I. Observing and data analysis systems, discovery of a mysterious peryton

ABSTRACT We present our piggyback search for fast radio bursts using the Nanshan 26 m Radio Telescope and the Kunming 40 m Radio Telescope. The observations are performed in the L band from 1380 to 1700 MHz at Nanshan and the Sband from 2170 to 2310 MHz at Kunming. We built the roach2-based FFT spectrometer and developed the real-time transient search software. We introduce a new radio interference mitigation technique named zero-DM matched filter and give the formula of the signal-to-noise ratio loss in the transient search. Though we have no positive detection of bursts in about 1600 and 2400 h data at Nanshan and Kunming, respectively, an intriguing peryton was detected at Nanshan, from which hundreds of bursts were recorded. Perytons are terrestrial radio signals that mimic celestial fast radio bursts. They were first reported at Parkes and identified as microwave oven interferences later. The bursts detected at Nanshan show similar frequency swept emission and have double-peaked profiles. They appeared in different sky regions in about tens of minutes observations and the dispersion measure index is not exactly 2, which indicates the terrestrial origin. The peryton differs drastically from the known perytons detected at Parkes, because it appeared in a precise period of p = 1.712 87 ± 0.000 04 s. Its origin remains unknown.

DEVELOPMENT OF 230 GHZ RADIO RECEIVER SYSTEM FOR SRAO

We develop a radio receiver system operating at <TEX>${\lambda}{\sim}1.3$</TEX> mm for the 6 m telescope of Seoul Radio Astronomy Observatory. It consists of a dual polarization receiver, a couple of IF processing units, two FFT spectrometers, and associated software. By adopting sideband-separating superconductor mixers with image band terminated to waveguide load at 4.2 K, we achieve <TEX>$T_{RX}{\leq}100$</TEX> K and <TEX>$T_{sys}$</TEX> less than 150 K at best weather condition over 210-250 GHz frequency range. The intermediate frequency signal of 3.5-4.5 GHz is down converted to 0-1 GHz and fed into the FFT spectrometers. The spectrometer covers 1 GHz bandwidth with a spectral resolution of 61 KHz. Test observations are conducted toward several radio sources to evaluate the performance of the system. Aperture and beam efficiencies measured by observing planets are found to be typically 44 ~ 59% and 47 ~ 61%, respectively over the RF band, which are consistent with those measured at 3 mm band previously.

Callisto radio spectrometer for observing the sun—Metsähovi Radio Observatory joins the worldwide observing network

The radio spectrum at MRO suffers from local electronics, broadband television, DBV-T, FM-radio, and some local data-links of nearby Schengen police communication and hence Metsähovi in the present situation is not ideal as a host site for a solar frequency agile or even to have an FFT spectrometer used at low frequencies. The general situation will be notably improved by installing the whole system at a more remote location or by improving the shielding of nearby electronic equipment. Also during the weekends, RFI-levels seem to be notably lower than during the office hours. Nevertheless it will be possible to detect strong flares with more than 50 sfu. A larger antenna with more gain would improve the situation drastically due to the fact that the signal-to-noise ratio of the solar flares would improve. Also a pre-amplifier with better noise figure and larger gain could improve the whole system's noise temperature. The most harmful interference frequencies are generated by the local electronics. However, during the observations the antenna is pointed away from the laboratory buildings. Thus the spectra is reasonable during observations, especially at higher frequencies (>300 MHz). The first results have shown that the measurement system is operating reliably and it can detect some weak solar radio bursts.

A Calibrated Digital Sideband Separating Spectrometer for Radio Astronomy Applications

ABSTRACTDual sideband (2SB) receivers are well suited for the spectral observation of complex astronomical signals over a wide frequency range. They are extensively used in radio astronomy, their main advantages being to avoid spectral confusion and to diminish effective system temperature by a factor 2 with respect to double sideband (DSB) receivers. Using available millimeter-wave analog technology, wideband 2SB receivers generally obtain sideband rejection ratios (SRR) of 10–15 dB, insufficient for a number of astronomical applications. We report here the design and implementation of an FPGA-based sideband separating FFT spectrometer. A 4 GHz analog front end was built to test the design and measure sideband rejection. The setup uses a 2SB front end architecture, except that the mixer outputs are directly digitized before the IF hybrid, using two 8 bit ADCs sampling at 1 GSPS. The IF hybrid is implemented on the FPGA together with a set of calibration vectors that, properly chosen, compensate for the analog front end amplitude and phase imbalances. The calibrated receiver exhibits a sideband rejection ratio in excess of 40 dB for the entire 2 GHz RF bandwidth.

Development of a 22 GHz ground-based spectrometer for middle atmospheric water vapour monitoring

The water Vapour Emission SPectrometer for Antarctica at 22 GHz (VESPA-22) has been designed for long-term middle atmospheric climate change monitoring and satellite data validation. It observes the water vapour spectral line at 22.235 GHz using the balanced beam-switching technique. The receiver antenna has been characterized, showing an HPBW of 3.5° and a sidelobe level 40 dB below the main lobe. The receiver front-end has a total gain of 105 dB and a LNA noise temperature of 125 K. A FFT spectrometer (bandwidth 1 GHz, resolution 63 kHz) will be used as back-end, allowing the retrieval of H2O concentration profiles in the 20 to 80 km altitude range. The control I/O interface is based on reconfigurable hardware (USB-CPLD).

A wideband multirate FFT spectrometer with highly uniform response

An integrating spectrometer based on a two-stage polyphase digital filter bank (PDFB) algorithm is described. The first PDFB operates on a time multiplexed wideband signal, with a bandwidth up to 1 GHz. The second PDFB is performed in parallel over couples of overlapping channels from the first filterbank. The design automatically deletes the unused portions of the first filterbank channels, providing a seamless, uniform channelization of the input band. The design has been implemented and tested on the VLBI DBBC platform, using a single Xilinx XC5VLX220 FPGA.

First measurements of lower mesospheric wind by airborne microwave radiometry

The Institute of Applied Physics operates an airborne microwave radiometer that measures the rotational transition line of water vapor at 183.3 GHz. A narrowband digital FFT spectrometer is used for the detection of the atmospheric signal with a channel resolution of 12 kHz. For the first time we measure lower mesospheric wind using the wind induced Doppler shift of the spectral line at its center. The Doppler frequency shift is calculated by the center of mass method allowing to retrieve the wind speed in the layer 55–70 km with a precision of 14 m/s. A good agreement with ECMWF operational reanalysis is found. We show that microwave remote sensing is an appropriate technique to fill the current experimental data gap in lower mesospheric winds. Precise wind measurements are of need to improve atmosphere circulation models and medium range weather forecasts.

Search for radio recombination lines at 408 MHz with the northern cross

Abstract Spectral line search was carried out for the first time with the Northern Cross radiotelescope at Medicina (Italy). The total bandpass of the antenna was sampled and processed in real time with the high resolution digital FFT spectrometer. The radio recombination line of carbon C252α was reliably detected toward Cassiopeia A. The observed line profile is composed of 3 emission components at positions -47 km s−1, -38 km s−1 and 0 km s−1 and relative intensities as 3:2:1. The detection of the component at zero velocity is of special importance since there has been only one measurement of such an emission made at 325 MHz.

Magneto-optical measurements with a hybrid magnet

An FFT spectrometer has been constructed to measure magneto-optical spectra in the far-infrared region (3 to 40 cm−1) with a hybrid magnet system. The magneto-absorption was studied in the spin-Peierls cuprate CuGeO3 at various magnetic fields. Coil constants of superconducting magnet and resistive one, of which a hybrid magnet consists, were calibrated by ESR spectra at the constant field.

The minimization of ac phase noise in interferometric systems

A simple step-by-step procedure, including several novel techniques discussed in the Appendices, is given for minimizing ac phase noise in typical interferometric systems such as two-beam interferometers, holographic setups, four-wave mixers, etc. Special attention is given to index of refraction fluctuations, direct mechanical coupling, and acoustic coupling, whose importance in determining ac phase noise in interferometric systems has not been adequately treated. The minimization procedure must be carried out while continuously monitoring the phase noise which can be done very simply by using a photodiode measurement of the interferometer output. Supplementary measurements using a microphone and accelerometer will also be helpful in identifying the sources of phase noise. Emphasis is placed on new techniques or new modifications of older techniques which will not usually be familiar to most workers in optics. Thus, the necessity of eliminating the effects of index of refraction fluctuations which degrade the performance of all interferometers is pointed out as the first priority. A substantial decrease of the effects of all vibrating, rotating, or flowing masses (e.g., cooling lines) in direct contact with the optical table will also have to be carefully carried out regardless of the type of interferometric system employed. It is recommended that this be followed by a simple, inexpensive change to a novel type of interferometer discussed in Appendix A which is inherently less sensitive to mechanical vibration. Such a change will lead to a reduction of both low-frequency and high-frequency ac phase noise by more than an order of magnitude and can be carried out for all interferometers with the exception of multiple pass optical systems and high-resolution FFT spectrometers. It is pointed out that most homemade air bladder vibration isolators are used incorrectly and do not provide sufficient reduction in the contribution of floor vibrations to phase noise. Several simple trampoline-type air bladder vibration isolator systems are described which are comparable in performance to commercial systems. With the exception of very nonrigid or undamped optical tables, the dominant source of ac phase noise at this point will usually be due to acoustic coupling to the optical components and mounts themselves. This means not only that the optical components and mounts must be rigid but that the mechanical coupling between the table and the mounts, as well as the coupling between the mounts and components themselves, be as rigid as possible. An additional damping of optical mounts beyond that generally found in commercial mountings will also have to be carried out to obtain a further reduction of phase noise. A simple damping technique employing an additional mass and an intermediate damping layer is described which will significantly improve the performance of both homemade and commercial optical mounts. Similar damping techniques which are especially suitable for homemade optical tables and breadboards are also considered.

High resolution infrared spectroscopy of the sun and the earth's atmosphere from space

The atmospheric trace molecule spectroscopy experiment (ATMOS) was designed to obtain high-resolution absorption spectra of the atmosphere from earth orbit, from which the vertical distributions of a large number of minor and trace molecular constituents could be retrieved. The ATMOS instrument is an FFT spectrometer covering the 600 to 5000 cm−1 frequency range and uses a double-passed, tilt-compensated optical configuration, with the two retroreflectors moving reciprocally. The scan time of 2 s gives spectra with an unapodized resolution of 0.01 cm−1, spaced 4 km apart, vertically.