Swept Frequency Technique Research Articles

Laser Linear Sweep Frequency Technique Based on Delay Self-Heterodyne Optical Phase Locking and Its Application

Laser Linear Sweep Frequency Technique Based on Delay Self-Heterodyne Optical Phase Locking and Its Application

OFDM-based electrical impedance spectroscopy technique for pacemaker-induced fibrosis detection implemented in an ARM microprocessor

Fibrosis represents an open issue for medium to long-term active implants, such as pacemakers, given that this biological medium surrounds the stimulation electrodes and can impact or modify the performances of the system. For this reason, Embedded Impedance Spectroscopy (EIS) techniques have been investigated these last years to sense the fibrosis. The following article introduces a new technique for EIS derived from multi-carrier digital communication methods. Due to its properties of flat spectrum and fast generation the Orthogonal-Frequency Division Multiplexing (OFDM) technique for EIS represents an efficient and a low foot-print alternative compared to the classical sweep frequency technique. This article focuses on this approach and also proposes a solution that reduces the effect of high crest factor typically found in OFDM systems. An embedded implementation is also presented. This designed prototype is used here to characterize the impedance spectrum of a pacemaker's electrode achieving an accuracy of 99% when measuring with 64 OFDM subcarriers and with a sampling frequency of 12 kHz.

Noninvasive detection of artificial heart valve outlet strut fracture using sweep frequency eddy current testing

Thepaper proposes utilizationof eddy current inspection fordetection ofdefects intheartificialheart valve. Artificial heart valves are prosthetic replacements which are used if the function of the natural heart valve is limited. A special type of prosthetic heart valve replacements is the Bj¨ ork Shiley convexo-concave (BSCC) heart valve. The fractures of the BSCC artificial heart valve usually affect the outlet strut. If the fracture of the strut is not detected in time the patient can die of heart attack. Detection of the fracture in the outlet strut is the concern here. Experimental measurements are performed in order to evaluate the detection capability of the technique and the sensitivity of response signal to dimensional parameters of a fracture. A BSCC heart valve prototype having the electromagnetic characteristics of the Co-Cr (Hayness 25) alloy is inspected with a probe. A non-conductive defect is positioned in the outlet strut and its dimensions are varied. The probe is positioned above the centre of the crack and the frequency of exciting signal is changed in a wide range. The presented results demonstrate that the sweep frequency technique can enhance eddy current inspection of a defect in BSCC heart valve.

Ionospheric absorption and planetary wave activity in East Asia sector

In this paper, we focus on ionospheric absorption in the East Asia sector, and look for manifestations of atmospheric influences in this area. First, a 4-year historical record of absorption measurement at Beijing is presented. This record was obtained by a sweep frequency technique, in which 27-days periodic variation of the absorption level was found to be dominant, appearing in most seasons except winters. Instead, unusual enhancements of the absorption level appeared in winters (winter anomaly), at the meantime the level varied with periods mainly in the range of 8–12 days. Comparing to 27-days period from the Sun, the shorter period oscillations should be related to planetary wave activities in lower atmosphere. Second, fmin data from 5 mid-latitude ionosondes in Japan were used as an indirect but long-term measurement. With the fmin data covering two solar cycles, disturbances with various periods were found to be active around solar maximum years, but the 8–12 days oscillations always existed in winter, showing seasonal dependence instead of connection to solar activity. These results given in this paper demonstrate seasonal and solar cycle-dependent features of the ionospheric absorption in East Asia sector, and confirm the existence of influence from atmosphere-ionosphere coupling in this area, as well as the relationship between ionospheric winter anomaly and planetary wave activity.

Magnetic moment relaxation studies by swept and pulsed microwave technique in LaSrCuO epitaxial thin film

We report magnetic relaxation experiments in a LaSrCuO epitaxial thin film under the action of crossed rf and dc magnetic fields. By applying a swept-frequency technique we found discontinuities in the relaxation curves in the form of a single jump at the lowest temperatures and dc magnetic fields, and sharp peaks either near Tc or for the largest dc magnetic fields. Maxima in the temperature and dc magnetic field dependences of the maximum output voltage signal of the SQUID magnetometer used for the magnetic measurements are observed in a defined temperature range when a pulsed microwave technique is used.

An ultrasonic time-delay spectrometry system employing digital processing

Time-delay spectrometry (TDS) is a swept-frequency technique that has proven useful in several ultrasonic applications. Commercial TDS systems are available, but only in the audio frequency range. Several ultrasonic research TDS systems have been constructed, and they have been used effectively for substitution calibration of hydrophones and for measurement of attenuation and sound velocity in materials. Unfortunately these systems depend on features of commercial equipment no longer manufactured, so a new system has been designed using modern equipment and straightforward signal processing. This system requires a frequency source with a reasonably linear sweep of frequency versus time, audio frequency filters, a standard double-balanced mixer, a power splitter, a waveform digitizer capable of handling audio frequency signals, and a personal computer. An optional implementation that shifts the signal to a lower frequency for more convenient digitization and easier velocity measurements additionally requires an audio frequency oscillator and an audio-range analog multiplier. The processing steps are performed with standard signal processing software. To demonstrate the operation of the system, substitution calibration measurements of hydrophones as well as attenuation measurements on a tissue mimicking material were obtained and compared to a custom TDS system previously described by the authors. The data from these two TDS systems agree to within +/- 0.5 dB in the 1-10 MHz frequency range used. Higher frequency source transducers could be used to extend this range.

Comparison of density fluctuation measurements between O-mode and X-mode reflectometry on Tore Supra

Reflectometry is a versatile diagnostic which allows both electronic density profile and density fluctuation measurements. Fast sweep heterodyne technique is particularly suitable for precise measurement of the phase of the reflected signal, which records the story of the wave propagation through the plasma up to the cutoff layer, including the density fluctuations. The present article exhibits a comparison of the density fluctuation radial profile measurements between fast sweep frequency technique, both using O-mode and X-mode polarizations, and fixed frequency technique. The correct agreement between all measurements of the relative values of the density fluctuation profiles reinforces the validity of the approximations used.

1-3 piezoelectric composite transducers for swept-frequency calibration of hydrophones from 100 kHz to 2 MHz.

Rapid calibration of hydrophones used in biomedical ultrasound is possible with swept frequency techniques such as time delay spectrometry. However, calibrations below 2 MHz largely have been neglected because of insufficient transmitting transducer bandwidth, even though important medical applications operate in this range. To address this deficiency, several transmitting transducer designs were developed and tested, and two 1-3 piezoelectric composite designs were found to have the requisite bandwidth and uniformity of response. In one the element has a plane front face and spherically concave back face (plano-concave), and in the second both faces are concave, but with different radii of curvature (biconcave). The nonuniform thickness disperses the thickness resonance, and the composite structure suppresses radial-mode resonances. Also, the composite's lower acoustic impedance provides a more efficient match to water. The piezoelectric composite transducers were found to have transmitting pressure sensitivities superior to ceramic single-element and segmented designs having similar dimensions, and their responses were significantly more uniform (< 25 dB variation from 0.1-2 MHz, with < 1 dB fine structure variation), likely due to decreased contributions from radial modes.

Interlaboratory evaluation of hydrophone sensitivity calibration from 0.1 to 2 MHz via time delay spectrometry

Knowing the low-frequency response of hydrophones, down to 100 kHz at least, is important for accurate biomedical ultrasound measurements. However, current international standards do not extend below 500 kHz. Furthermore, commercial hydrophone sources typically do not supply sensitivity data below 1–2 MHz. Therefore, to help identify and validate practical calibration methods below 2 MHz, the authors have extended their previous individual efforts in an interlaboratory evaluation of sensitivity calibration using the swept-frequency technique, time delay spectrometry (TDS). Calibrations were performed for needle and membrane PVDF hydrophones using each laboratory's TDS system. Each site employed the same purpose-built broadband source transducers, comprising both plano-concave and biconcave 1–3 piezocomposite elements 4 cm in diameter, with maximum and minimum thicknesses of approximately 1.5 and 0.1 cm. Agreement between laboratories was within the estimated measurement precision of ±0.6 dB. The results demonstrated that a TDS system employing such transducers constitutes a viable method for hydrophone calibrations in this frequency range.

Permittivity and Conductivity Measurements of Building Materials at 5.8 GHz and 41.5 GHz

The results of various experiments performed to characterize electromagnetic properties of typical building materials at BRAN (Broadband Radio Access Networks) and MVDS (Multipoint Video Distribution System) bands are presented and compared. Transmission and reflection coefficients were measured as functions of the angle of incidence, using a wideband sounder based on the swept frequency technique. A high precision angular positioning system was built and used in co-ordination with the measurement equipment. The dielectric constants (er) were estimated by comparing values predicted with a multiple successive internal reflection model and measured transmission and reflection coefficients. It has been found that the variation of the dielectric constant value with the frequency does not necessarily follow the same trend for different materials. For example, the measured er for plasterboard presents an increment with frequency (2.02 at 5.8 GHz, 2.5 at 41.5 GHz – measured by the authors –, 2.58 at 59.5 GHz, and 2.81 at 60.2 GHz – given by literature –), while the values for glass follow a wandering pattern (6.06 at 5.8 GHz, 5–10 at 10 GHz, 3.41 at 41.5 GHz, 7.51 at 57.6 GHz, and 5.29 at 60.2 GHz). Moreover, the measured er for a brick wall at 5.8 GHz (3.58) shows differences with values reported at 2 GHz (4.44).

A microwave broadband technique to measure the complex resistivity of HTS thin films

We propose a new broadband swept frequency technique to measure the complex resistivity of high T/sub c/ superconducting thin films in the microwave range (from a few GHz up to 50 GHz). A coaxial cable connects the sample to a vector network analyzer which performs reflection measurements; the film and the coaxial end are separated by a short section of circular waveguide. The absence of direct electrical contact between the sample and the coaxial core simplifies realization and avoids contact instability with large temperature variations. We provide an electromagnetic analysis of the coaxial to cylindrical transition (CCT) and find a closed form relation between the measured quantities and the film resistivity. Finally, we present some experimental results obtained for an YBa/sub 2/Cu/sub 3/O/sub 7-/spl delta// thin film using the described method.

The change in the acoustic admittance phase angle: a study in children suffering from acute otitis media.

Middle ear mechanics, in normal and in pathological conditions, is the subject of this research, with acoustic impedance measurements as the cornerstone. Previous studies have established the importance of admittance-phase tympanograms, mainly in frequencies higher than the conventional 226 Hz. The purpose of the present study was to record how acute otitis media (AOM) affects the middle ear system and function by evaluating the recordings of the change in phase angle parameter (deltatheta) provided by an automated tympanometer using the sweep-frequency technique. Multifrequency and conventional tympanograms were obtained from 70 children suffering from AOM on consecutive visits. Values of deltatheta from these subjects were compared to normative data previously acquired in our Department. It appears that changes in the mechanical status of the middle ear after AOM are reflected in abnormal deltatheta values, despite the normal findings of conventional tympanometry. A positive history of AOM did not seem to influence the behaviour of the middle ear. In most cases, abnormal deltatheta values coexisted with abnormal values of resonance frequency (RF), i.e. the frequency at which mass and stiffness of the middle ear are in balance, and total susceptance (deltaB) reaches 0mmhos and the converse. The deltatheta seems to be an important indicator of middle ear mechanical status that can record changes occurring in the system after AOM and undetected by low probe-tone tympanometry.

Wide-Band UHF Radio Propagation Characteristics in a Tunnel Environment

This paper presents the measured wide-band characteristics of 900 and 1800 MHz band radio propagation channels in a tunnel environment for microcellular and personal communications. The measurements were made using the swept frequency technique. The analysis of the average measured power delay profiles reveals that the rms time delay spread is less than 25 ns when the tunnel is empty and can be as large as 103 ns when the tunnel is blocked by vehicles.

Resonance acoustoelasticity measurement of stress in thin plates

Abstract A computer-driven, swept-frequency measurement technique is developed on the basis of resonance birefringence acoustoelasticity to evaluate the stresses in thin plates. The resonance frequency depends on the thickness and the elastic wave velocity; they change with stress because of the Poisson effect and the acoustoelastic effect. The resonance frequency is obtained from the spectral response curve in the electric impedance of the piezoelectric transducer. The frequency displacement induced by acoustically coupling the transducer can be minimized by employing the resonance peak closest to the transducer fundamental frequency. To illustrate the method, the residual stress is measured in butt-welded aluminum alloy plates and is compared with the results of conventional methods.

Network Analysis on Radio Frequency Prototype Industrial Applications

AbstractNetwork Analysers provide a powerful diagnostic tool involving swept frequency techniques for the design of radio frequency applicators. Such prototype applicators have been used with a number of industrial materials to assess the feasibility of drying or heating with radio frequency power. Essentially, two aspects are considered in this paper. Firstly, with the prototype applicators not coupled to an oscillatory (tank) circuit of a radio frequency source, the resonance characteristics and Q-factors are measured leading to the establishment of the utilisation efficiency. Secondly, the interaction at lower power between the loaded applicator and the tank oscillatory circuit which is coupled to it, is considered in terms of the return loss and the impedance loci. A critical electrode separation, dc, is observed which is dependent on the material loading and at which the impedance loci and return losses of the loaded applicator and tank oscillatory circuits are identical. Such an observation may be a...

Traveling-wave electro-optic modulator with maximum bandwidth-length product

A traveling-wave Ti-indiffused LiNbO3 guided-wave Mach–Zehnder interferometric modulator has been fabricated and characterized at λ=0.85 μm. The modulator achieves the theoretical 3-dB linear small-signal bandwidth limit of 16 GHz for a 4-mm-long interaction length. With a ±2.25-V drive signal, an extinction ratio of 22 dB is achieved. Linear small-signal measurements made with a swept-frequency technique are compared to previously reported measurements.

Broad-band guided-wave electrooptic modulators

Lumped-element Mach-Zehnder interferometric modulators have been designed, fabricated in LiNbO 3 , tested and analyzed. These modulators had 3 dB bandwidths from 280 MHz to 2.75 GHz and V π 's from 1 V to 4 V, respectively. A simple RLC equivalent circuit is utilized to model the packaged modulator performance and results are compared with those measured using a swept-frequency technique. The model is seen to break down at the higher frequencies due to a frequency-dependent resistance and the electrode and parasitic inductances are seen to limit the overall modulator performance. The effects of varying the electrode capacitance, resistance, or inductance on modulator performance are demonstrated.

17-GHz bandwidth electro-optic modulator

A high-speed integrated-optic Ti:LiNbO3 Mach–Zehnder interferometric modulator for 0.83-μm wavelength operation has been fabricated and characterized. The modulator exhibits smooth, resonance-free frequency response with a 17-GHz 3-dB bandwidth. The modulator has a built-in phase bias of π/2 for maximum linearity. Complete intensity modulation can be achieved with 120-mW drive power. Optical modulation was measured up to 18 GHz directly by using a very high-speed photodiode and indirectly using the swept frequency technique.

Mid-Film for Microwave Integrated Circuits

Mid-filmr is a process which has been developed to combine the high resolution capability of thin film technology and the nonvacuum low cost of thick film technology. In this paper the performance of mid-film microstrip transmission lines is described for applications in microwave integrated circuits (MIC's) in the frequency range of 3-12 GHz. The experiments were carried out on a number of ring resonators with characteristic impedances of 35, 50, and 71 <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">\Omeg</tex> . The resonators were fabricated using mid-film gold conductors on 96 percent and 99.6 percent alumina substrates. Theoretical calculations of microstrip losses were carried out and compared with microstrip attenuation measurements using a swept frequency technique. The results showed that the attenuation in midfilm microstrip transmission lines is esentially identical to the attenuation in thick film lines using the same conductor materials with the added advantages of fine line resolution capability (50-µm line widths and spaces) and lower cost of mid-films. Typically, the attenuation constant of 50 <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">\Omeg</tex> lines on 99.6 percent alumina was 0.06 dB/cm at 7 GHz while the theoretical lower bound is 0.05 dB/cm. In addition a Wilkinson 3-dB in-phase power divider has been fabricated using mid-film gold microstrip branches which were isolated by a 100- <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">\Omeg</tex> thick film resistor (EMCA5512). The mid-film Conductor was found to be compatible with the thick film resistor and the Wilkinson 3-dB hybrid behaved as expected with a loss of 0.3 dB and isolation of greater than 30 dB over a 500-MHz band centered at 6 GHz.

The Use of Ultrasonic Spectroscopy to Characterize Calcified Lesions

A quantitative basis is developed for the medical application of ultrasonic spectroscopy, particularly in the characterization of calcified lesions associated with atherosclerotic conditions. A theory of the field radiation patterns of disk and ring sources is presented which can be used to predict minima in the reflection spectra of a target as a function of frequency and angle relative to the acoustic axis. The theory is then tested experimentally for objects of known geometry by the use of a time delay spectroscopy system using a swept frequency technique, and results are obtained which demonstrate the usefulness of the technique. Finally, the theory is verified in the determination of lesion diameter and orientation from spectra received from calcified lesions on in vitro arterial specimens as a function of angle.