A synchronization system of very low-frequency interferometers

A solution to the problem of synchronization of spaced autonomous data-acquisition systems and obtaining stable reference-frequency signals for measuring phases of very low-frequency (VLF) radio signals is described. The expediency of using GPS receivers and tunable crystal oscillators in the synchronization system of VLF interferometers is grounded. Measurement results of the phase stability of the reference signal, received from the designed device, are presented. The short-term frequency stability of a 200-kHz pulse signal at the output of the designed unit is no more than 1.5 × 10^?7, allowing one to measure the phase of VLF signals with an accuracy of no worse than ±0.81°.     

A data acquisition system of the S/X-wave range of the radio interferometer for the universal time on-line monitoring

A system is intended for equipping radio telescopes with antennas of small (12–13 m) diameters, used in the very long baseline interferometers (VLBIs), including universal time on-line monitoring interferometers. The system operates in S- and X-wavelength ranges and process up to 8 signals with spectrum widths of up to 512 MHz, converts them into digital sequences with a 1024-MHz clock frequency, and forms data streams in the VLBI Data Interchange Format (VDIF) with a 2048-Mbit/s rate per channel. Data from each channel of the system via the 10G Ethernet interface are transmitted to the data buffering unit for their sending to the correlation processing center.     

A Data Stream Converter for a Radio Telescope with a Wideband-Signal Digital Recording System

A data-stream converter makes it possible to connect a radio telescope with wideband (512 MHz) signal-recording channels to active very-long-baseline interferometers using multichannel recording systems for narrowband (16 or 8 MHz) signals. A digital converter based on a field programmed gate array (FPGA) selects and formats up to 16 narrowband signals.     

A Radiometric System for the РТФ-32 Radio Telescope

A radiometric system developed by the Russian Academy of Sciences’ Institute of Applied Astronomy was mounted on the three РТФ-32 radio telescopes constructed as part of the Kvazar project. The system is composed of cooled high-sensitivity receivers for wave bands of 18–21, 13, 6, 3.5, and 1.35 cm, and a four-channel measuring radiometric module, operating in the intermediate-frequency base band of 100–1000 MHz. The radiometric system achieves its maximum sensitivity of 1.9–2 mK in the wave bands of 6 and 3.5 cm and 3.7–4.8 mK in the other ranges. Observations were completely automated on the basis of the Mark IV Field Systems software. The receivers were developed in cooperation with NPO Saturn (Kiev, Ukraine). The measuring radiometric module was designed by ZAO Radioelektronnaya tekhnologicheskaya apparatura (St. Petersburg) with the support of the Foundation for Assistance to Small Innovative Enterprises and was manufactured in cooperation with the OAO Mikrotekhnika (St. Petersburg). The supply of such modules for the РТФ-32 radio telescopes is supported by the Russian Federal Agency for Science and Innovations.__________Translated from Pribory i Tekhnika Eksperimenta, No. 4, 2005, pp. 66–75.Original Russian Text Copyright © 2005 by Ipatov, Kol’tsov, Krokhalev.     

NON-STATIONARY SIGNALS: PHASE-ENERGY APPROACH—THEORY AND SIMULATIONS

Modern time–frequency methods are intended to deal with a variety of non-stationary signals. One specific class, prevalent in the area of rotating machines, is that of harmonic signals of varying frequencies and amplitude.This paper presents a new adaptive phase-energy (APE) approach for time–frequency representation of varying harmonic signals. It is based on the concept of phase (frequency) paths and the instantaneous power spectral density (PSD). It is this path which represents the dynamic behaviour of the system generating the observed signal.The proposed method utilises dynamic filters based on an extended Nyquist theorem, enabling to extract signal components with optimal signal-to-noise ratio. The APE detects the most energetic harmonic components (frequency paths) in the analysed signal.Tests on simulated signals show the superiority of the APE in resolution and resolving power as compared to STFT and wavelets wave-packet decomposition. The dynamic filters also enable the reconstruction of the signal components (paths) from the noisy signal. A quantitative comparison was performed both for the detected path in the time–frequency plane as well as for the reconstructed signal, demonstrating the performance of the APE.     

A Functional Generator

A synchronizing generator of triangular, sine-wave, and rectangular signals with variable frequencies and amplitudes is described. The main generator of triangular signals is complemented with a functional converter to obtain a sine function using the piecewise-linear approximation. The frequency of generated signals is continuously regulated within the bands of 10–100 Hz, 100–1000 Hz, 1–10 kHz, and 10–100 kHz. The signal amplitudes are continuously varied between 0 and 10 V. The nonlinear distortion factor of the sine signal is within 0.5%.     

A 32-Channel High-Speed Simultaneously Sampling Data Acquisition System

The system is intended for recording data in the multichannel diagnostic sections of experimental plasma facilities. It contains eight four-channel modules that record the shapes of single pulse signals, a controller module of the system bus of the crate, a fiber-optic communication line, and an interface card for connection to the ISA bus of a personal computer. The recording modules are based on 12-bit analog-to-digital converters (ADCs) with a sampling frequency of up to 50 MHz, ensuring a conversion accuracy equal to the least significant bit in a band of the input signal of up to 20 MHz. The ADC samples are fixed in 32-Kword/channel buffer storage units with a page organization. The current values of the amplitude of the input signals in all of the recording channels are measured simultaneously with a time jitter of no more than 0.2 ns. The software selects an amplitude conversion scale and a zero offset voltage value for each recording channel, as well as the current value of the sampling frequency for all the channels.     

Spectrum occupancy investigation: Measurements in South Africa

Regulatory bodies have predicted an impending shortage in commercial radio frequency spectrum in the near future. However, due to outdated regulatory practices, many of these bands are in fact inefficiently underutilised. Spectrum measurement campaigns have be carried out around the world to determine the extent to which this is true. However, there still seems to be a lack of knowledge regarding spectrum occupancy in South Africa. A spectrum measurement system was thus designed and employed to measure the spectrum occupancy of the ultra-high frequency (UHF), global system for mobile communications (GSM) 900 MHz and GSM 1800 MHz bands through a measurement campaign in the Hatfield area of Pretoria, South Africa. A method for determining spectrum occupancy, from raw spectrum measurements, has been described and used to calculate the average spectrum occupancy of these bands. Occupancy in the UHF band was found to be fairly constant at approximately only 20%. While the maximum occupancy of the GSM 900 MHz band was calculated to be much higher at approximately 92% and that of the GSM 1800 band to be approximately 40%. However, the GSM 900 MHz and 1800 MHz bands did exhibit fluctuations in occupancy of between 10% and 20% respectively according to the time of day. Slight variations in occupancy of between 1% and 3% were also evident over the days of the week. These results are placed into context by a comparison with the findings of various other measurement campaigns from around the world. When compared, occupancy was generally found to be lower in the UHF bands but higher in the mobile bands.     

A 128-Channel Simultaneously Sampling Data Acquisition System for the Diagnostic Complex of Experimental Plasma Facilities

The simultaneously sampling data acquisition system contains 16 eight-channel modules, which record the shapes of single pulse signals, and the controller module of the system bus of the Eurocrate, which interacts with the basic computer of the diagnostic complex via an Ethernet-FX communication line supported by TCP/IP protocols. The recording modules are based on 12-bit analog-to-digital converters (ADC). The ADC samples are fixed in buffer storage units with a capacity of up to 1 Mword/channel. The current amplitudes of the input signals are measured simultaneously in all recording channels with a time jitter of 3 ns. The maximum sampling frequency is below 4 MHz.     

A soft and hardware complex for recording and processing of acoustic emission signals and for location and identification of their sources

A soft and hardware complex designed for recording and processing acoustic emission (AE) signals in real time is described. The hardware part of the complex includes a computer with PCI connectors, four-channel parallel-reading analog-to-digital converters (ADCs), preamplifiers, and receiving converters. The software part of the complex is used to record and save in all recording channels the full shape of signals with a preset sampling frequency of up to 20 MHz, locate, identify, and classify AE sources in real time. It differs from analogs in the capability of saving the full shape of AE signals for further analysis and processing, in use of commercially produced ADCs, and in real-time data processing and classifying of AE sources.     

Spectral-selective radiometers with bandwidths of up to 1 GHz

Spectral-selective radiometers, ensuring high-accuracy measurements in continuum under exposure to narrow-band radio-interferences and detection of radio-wave radiations in spectral lines, are described. Two parallel channels of the radiometer simultaneously receive signals of two polarizations or signals of two wave bands. The channels of the radiometer installed at the Badary observatory operate in frequency bands of up to 32 MHz and have a resolution of 62.5 kHz, if the reception band is at maximum. The channels of the radiometer at the Svetloe observatory have reception bands of 1024 or 512 MHz and a resolution of 1.0 or 0.5 MHz, respectively.     

A digital signal converter for radio astronomical systems

A digital signal converter operating in a video frequency band (up to 32 MHz) and intended for radio astronomical equipment is based on an LTC2288 analog-to-digital converter and an XC4VSX25 field programmed gate array. A two-band base band converter improving parameters of the radio interferometric terminal, a two-channel spectrometer minimizing times of observation of radio-frequency sources in spectral lines, and a radiometer for recording signals in a continuum operating under the action of radio interference have been designed using the converter. The advantages of this equipment are confirmed by results of tests at the Svetloe radio astronomical observatory.     

Use of a pressure-sensitive radiotelemetering capsule to study the canine gastric interdigestive complex

An inexpensive pressure-sensitive radiotelemetering capsule, transmitting at 100 MHz, was used to study gastric interdigestive complexes. The oscillator was frequency modulated; pressure changes on a rubber diaphragm caused displacements of a ferromagnetic core altering the frequency of oscillation by modifying the inductance in the circuit. The system was linear for applied pressures up to 40 mm of Hg; for 40 mm Hg the frequency decreased by 22 kHz. Three fasted dogs were used; an encapsulated radio pill picked up the presence fluctuations associated with gastric contractions and transmitted these signals as radio waves. An FM receiver intercepted, amplified, and relayed the signals to a chart recorder, which produced a permanent record. During the first part of each 24-hour recording period, cyclical interdigestive patterns consisting of a series of high-amplitude contractions followed by long-lasting quiescence were recorded. Regular outbursts showed mean durations ranging from 11.6 to 17.6 minutes, mean periods from 27 to 51 minutes, and mean amplitudes from 9.9 to 11.1 mm Hg. Noncyclical, irregular recordings were obtained during the last part of each recording period. These results are similar to ones obtained using force transducers.     

A Power Meter for a Radio Frequency Cathode Sputtering Installation

A meter of the radio frequency power deposited in a gas discharge of a magnetron sputtering system of an installation for depositing thin films is described. The device is based on a KP5252 integral analog multiplier. The power measurement range is 10–1000 W, the operating frequency is 13.56 MHz, and the measurement error is within ±3%.     

Polarimetric Borehole Radar System for Fracture Measurement

Radar polarimetry is a technology that overcomes the limitation between the radar resolution and the penetration depth of borehole radar. We have developed a stepped-frequency polarimetric borehole radar system. This is a polarimetric borehole radar system which measures the full-radar polarimetry in a borehole by changing the antenna arrangements. By using a network analyzer and an optical analog signal link, this system has a frequency bandwidth of 2–500MHz, which is suitable for two different antennas for radar polarimetry. We also propose a technique for polarimetric antenna calibration. In order to understand the potential of polarimetric borehole radar, field measurement were carried out at the Mirror Lake fractured-rock research site (NH, USA). The radar penetration depth from the borehole in the reflection measurement was over 10m, at the frequency range of 2–402MHz. We observed many clear reflections from fractures in each polarization status. Even in the raw data, we found the differences in the radar profile for different polarization status. We believe the polarimetric feature is closely related to the roughness of each fracture, and it is also related to the physical properties of the fracture such as water permeability.     

The Prospects for Using High-Speed Analog-to-Digital Converters to Analyze the Signals of Ionizing-Radiation Detectors

The potential advantages of recording digital signals from ionizing-radiation detectors are considered. State-of-the-art high-speed analog-to-digital converters operating at sampling frequencies of 400 MHz (2.5 ns) and higher with a buffer storage of up to 4 Gbyte and a data-input rate of up to 400 Mbyte/s are capable of recording continuous pulse sequences with a throughput of >10⁷ pulse/s in less than 10 s. In contrast to standard recording methods in which overlapping pulses are truncated, the recording of complete signal flows allows one to detect and resolve overlapping pulses quite well, provided they arrive within 20–40 ns of one another.     

Precision Phase Discriminator for the Heterodyne Interferometric Plasma-Density Measurement Technique

The discriminator is intended for the plasma density recording systems based on optical heterodyne interferometry. The discriminator has a high phase resolution (10^–3 rad) and a virtually unlimited dynamic range in a modulation band of up to one-half of the carrier frequency (1 MHz). These features allow one to use the discriminator for recording phase shifts caused by both slow and fast plasma-density fluctuations on setups with high levels of thermal and mechanical influences on the interferometer.     

Results of testing a radio interferometer with digital signal processing in a 400-MHz band

Results of testing the astronomical radio interferometer at the Svetloe-Zelenchukskaya observatory using digital signal processing channels with 400-MHz bands are presented. Observation data were recorded in the VDIF format with a 2048-Mbit/s rate by the Mark 5C terminal. Simultaneously, data obtained using the standard equipment of the Quasar complex were recorded. The observation results were processed by both the DiFX experimental software correlator and APK standard correlator, which is a part of the Quasar complex. Due to the used wideband data acquisition system, the sensitivity of the radio interferometer went up by a factor of 4.7–4.8, and the accuracy of determining the time position of the correlation response increased by a factor of 1.6. The experiment confirmed the efficiency of the new wideband digital data acquisition system and possibility of its applications in astronomical radio interferometers with small (～13 m) antennas.     

A Bandpass Power Amplifier with Enhanced Frequency-Response Linearity

A band-pass power amplifier with a frequency-response corrector is described. The controlled nonlinear collector–emitter capacitance of a disabled low-frequency transistor is used as the correcting element. The technical specifications of the amplifier are as follows: the maximum output power is 147 W; the output power limited by the compression of sync pulses of the television-picture radio signal by 2% is no less than 125 W; the operating band is 72–86 MHz; and the gain is 22 dB.