Complex Demodulation Technique Research Articles

Multiplexed optical fiber cell temperature sensing system with high sensitivity and accuracy.

A multiplexed fiber laser sensing system for cell temperature is proposed. To the best of the authors' knowledge, this is the first multilongitudinal mode (MLM) optical fiber laser sensor array designed for cell temperature sensing. A two-channel cell temperature sensing system with high sensitivity and real-time sensing capability is achieved. The temperature change of human hepatoellular carcinomas (HepG2) cells under the influence of exogenous chemical aflatoxin B1 (AFB1) can be monitored in real time. A fiber laser cavity consists of a pair of fiber Bragg gratings (FBGs) with matched central wavelengths and a piece of erbium-doped fiber (EDF). The static FBG is utilized for design of fiber laser cavity and laser modes selection. In comparison, the sensing FBG is used for cell temperature sensing. The sensing FBG has a length of 10mm and a diameter of . Beat frequency signals (BFS) are generated by MLM lasers after optical-to-electrical conversion at a photodetector. Frequency change of a BFS is closely related to the reflected wavelength change of the sensing FBG. Through frequency division multiplexing, two fiber laser cavities are designed in the sensing system for two-channel temperature sensing. Frequency shift of a BFS that represents temperature change of cells can be automatically recorded in seconds. A two-channel cell temperature sensing system is designed with high sensitivities of 101.62 and , respectively. The temperature change of HepG2 cells under the influence of exogenous chemical AFB1 is monitored in real time. The proposed system has the advantages of simple structure, high sensitivity, and two-channel sensing capability. Our study provides a simple and effective method to design a fiber laser sensor system without complex demodulation techniques and expensive optical components.

The Onset of Interictal Spike-Related Ripples Facilitates Detection of the Epileptogenic Zone.

Objective: Accurate estimation of the epileptogenic zone (EZ) is essential for favorable outcomes in epilepsy surgery. Conventional ictal electrocorticography (ECoG) onset is generally used to detect the EZ but is insufficient in achieving seizure-free outcomes. By contrast, high-frequency oscillations (HFOs) could be useful markers of the EZ. Hence, we aimed to detect the EZ using interictal spikes and investigated whether the onset area of interictal spike-related HFOs was within the EZ.Methods: The EZ is considered to be included in the resection area among patients with seizure-free outcomes after surgery. Using a complex demodulation technique, we developed a method to determine the onset channels of interictal spike-related ripples (HFOs of 80–200 Hz) and investigated whether they are within the resection area.Results: We retrospectively examined 12 serial patients who achieved seizure-free status after focal resection surgery. Using the method that we developed, we determined the onset channels of interictal spike-related ripples and found that for all 12 patients, they were among the resection channels. The onset frequencies of ripples were in the range of 80–150 Hz. However, the ictal onset channels (evaluated based on ictal ECoG patterns) and ripple onset channels coincided in only 3 of 12 patients.Conclusions: Determining the onset area of interictal spike-related ripples could facilitate EZ estimation. This simple method that utilizes interictal ECoG may aid in preoperative evaluation and improve epilepsy surgery outcomes.

Temperature-insensitive pressure transducer based on reflected broadened spectrum with enhanced sensitivity

Pasting of half the grating of an FBG sensor on a natural rubber diaphragm was used to sense the pressure measurement. The pressure-induced strain on half of the grating resulted in bandwidth broadening. From the pressure range of 0 kPa to 20 kPa, the pressure sensitivity at 15 dB bandwidth changed was obtained at 77.7 pm/kPa with 99.94% of linearity. Through total reflected power monitoring via photodetector, the pressure transducer retrieved a sensitivity of 0.1686 dB m/kPa and evaded a complex demodulation technique. As a result of the temperature-independent bandwidth change, the pressure transducer was totally temperature insensitive.

An investigation of the nature of a Pc5 pulsation event using SuperDARN and magnetometer data

Pc5 pulsations are global magnetohydrodynamic events in the magnetosphere. We employed an Automated Pulsation Finder program to identify significant Pc5 pulsation events in SuperDARN radar data. The event presented here was visible in the Goose Bay, Saskatoon and Þykkvibaer high-frequency radars, located in the northern polar region. These observations were coordinated with magnetometers within their field of view. These two instrument types – radars and magnetometers – complement each other. These observations represent a significant fraction of the globe in longitude. Pulsation studies of this nature are rare in the literature. Combining these two instrument types, we investigated the nature of the pulsation and determined its qualitative polarisation characteristics. A complex demodulation technique was employed to determine amplitude and phase relationships between field components observed by the radars and magnetometers, which, in turn, afforded resolution of other characteristics of pulsations, such as wave number and phase velocity. The results are discussed in the context of the magnetohydrodynamic theory of magnetic pulsations, speculating on its generation mechanism. Investigation of this mechanism will be the subject of a future publication.

Sympathetic Activation and Baroreflex Function during Intradialytic Hypertensive Episodes

BackgroundThe mechanisms of intradialytic increases in blood pressure are not well defined. The present study was undertaken to assess the role of autonomic nervous system activation during intradialytic hypertensive episodes.Methodology/Principal FindingsContinuous interbeat intervals (IBI) and systolic blood pressure (SBP) were monitored during hemodialysis in 108 chronic patients. Intradialytic hypertensive episodes defined as a period of at least 10 mmHg increase in SBP between the beginning and the end of a dialysis session or hypertension resistant to ultrafiltration occurring during or immediately after the dialysis procedure, were detected in 62 out of 113 hemodialysis sessions. SBP variability, IBI variability and baroreceptor sensitivity (BRS) in the low (LF) and high (HF) frequency ranges were assessed using the complex demodulation technique (CDM). Intradialytic hypertensive episodes were associated with an increased (n = 45) or decreased (n = 17) heart rate. The maximal blood pressure was similar in both groups. In patients with increased heart rate the increase in blood pressure was associated with marked increases in SBP and IBI variability, with suppressed BRS indices and enhanced sympatho-vagal balance. In contrast, in those with decreased heart rate, there were no significant changes in the above parameters. End-of- dialysis blood pressure in all sessions associated with hypertensive episode was significantly higher than in those without such episodes. In logistic regression analysis, predialysis BRS in the low frequency range was found to be the main predictor of intradialytic hypertension.Conclusion/SignificanceOur data point to sympathetic overactivity with feed-forward blood pressure enhancement as an important mechanism of intradialytic hypertension in a significant proportion of patients. The triggers of increased sympathetic activity during hemodialysis remain to be determined. Intradialytic hypertensive episodes are associated with higher end-of- dialysis blood pressure, suggesting that intradialytic hypertension may play a role in generation of interdialytic hypertension.

A statistical approach for accurate detection of atrial fibrillation and flutter

We have developed an algorithm for real-time detection of atrial fibrillation (AF) and flutter (AFL) that relies on 4 statistical techniques (root mean square of successive differences, turning point ratio and Shannon entropy). For AFL detection we use a time-frequency based method via the complex demodulation technique to recognize a characteristic rhythm of AFL. We used the MIT-BIH Atrial Fibrillation database to train the algorithm and then tested it on the MIT-BIH Arrhythmia database. We achieved sensitivity (specificity) of 94% (95%) on the training set and 90 % (91%) on the test database for AF detection. We further tested the AF performance on 36 actual Holter files obtained from The ScottCare Corporation and achieved sensitivity and specificity of 95% and 95% respectively. AFL detection was tested on 2 files of the AFIB database and we achieved 97% sensitivity and 95% specificity. The ease of implementation and minimal memory and speed requirements make this algorithm ideal for on-chip or similar usage.

Measurement of coefficients of the Ginzburg-Landau equation for patterns of Taylor spirals

Patterns of Taylor spirals observed in the counter-rotating Couette-Taylor system are described by complex Ginzburg-Landau equations (CGLE) and have been investigated using spatiotemporal diagrams and complex demodulation technique. We have determined the real coefficients of the CGLE and their variations versus the control parameters, i.e., the rotation frequency of cylinders.

Baroreflex sensitivity and sympatho-vagal balance during intradialytic hypotensive episodes

The role of the baroreflex function in the pathogenesis of hemodialysis-associated hypotension is controversial. Complex demodulation technique (CDM), providing continuous assessment of the amplitude of cardiovascular oscillation over time, is particularly suitable to assess dynamic changes in autonomic nervous system and baroreceptor sensitivity (BRS) during dialysis. In the present study, CDM was used to determine the effects of dialysis treatment on BRS and to characterize BRS changes during acute intradialytic hypotension. Continuous beat-to-beat blood pressure and interbeat intervals (IBIs) were monitored in 93 chronic patients without (n = 70) and with (n = 26) hypotension during 96 dialysis sessions. The amplitudes of SBP and DBP, IBIs, and BRS change in the low-frequency (around center frequency of 0.09 Hz) and high-frequency (around center frequency of 0.30 Hz) ranges were followed during the whole dialysis session. Hemodialysis treatment was associated with increased low-frequency BRS, especially in sessions without hypotension. Hypotensive episodes were associated with significant increases in both low-frequency BRS and high-frequency BRS, mainly in patients with severe hypotension. The magnitude of the increase in baroreflex indices was proportional to the decrease in blood pressure. Low-frequency IBI/high-frequency IBI ratio, a marker of sympatho-vagal balance, did not significantly change during hypotension. Our study shows that the baroreflex mechanism is preserved and adequately activated during intradialytic hypotension. Other factors, such as ischemic heart disease, left ventricular dysfunction, and inadequate arteriolar tone, rather than failure of baroreflex function, are more likely to be responsible for dialysis-induced hypotension.

Slow head-up tilt causes lower activation of muscle sympathetic nerve activity: loading speed dependence of orthostatic sympathetic activation in humans

Many earlier human studies have reported that increasing the tilt angle of head-up tilt (HUT) results in greater muscle sympathetic nerve activity (MSNA) response, indicating the amplitude dependence of sympathetic activation in response to orthostatic stress. However, little is known about whether and how the inclining speed of HUT influences the MSNA response to HUT, independent of the magnitude of HUT. Twelve healthy subjects participated in passive 30 degrees HUT tests at inclining speeds of 1 degrees (control), 0.1 degrees (slow), and 0.0167 degrees (very slow) per second. We recorded MSNA (tibial nerve) by microneurography and assessed nonstationary time-dependent changes of R-R interval variability using a complex demodulation technique. MSNA averaged over every 10 degrees tilt angle increased during inclination from 0 degrees to 30 degrees , with smaller increases in the slow and very slow tests than in the control test. Although a 3-min MSNA overshoot after reaching 30 degrees HUT was observed in the control test, no overshoot was detected in the slow and very slow tests. In contrast with MSNA, increases in heart rate during the inclination and after reaching 30 degrees were similar in these tests, probably because when compared with the control test, greater increases in plasma epinephrine counteracted smaller autonomic responses in the very slow test. These results indicate that slower HUT results in lower activation of MSNA, suggesting that HUT-induced sympathetic activation depends partially on the speed of inclination during HUT in humans.

Subcritical instabilities in a convective fluid layer under a quasi-one-dimensional heating

The study and characterization of the diversity of spatiotemporal patterns generated when a rectangular layer of fluid is locally heated beneath its free surface is presented. We focus on the instability of a stationary cellular pattern of wave number ks which undergoes a globally subcritical transition to traveling waves by parity-breaking symmetry. The experimental results show how the emerging traveling mode (2ks/3) switches on a resonant triad (ks, ks/2, 2ks/3) within the cellular pattern yielding a "mixed" pattern. The nature of this transition is described quantitatively in terms of the evolution of the fundamental modes by complex demodulation techniques. The Bénard-Marangoni convection accounts for the different dynamics depending on the depth of the fluid layer and on the vertical temperature difference. The existence of a hysteresis cycle has been evaluated quantitatively. When the bifurcation to traveling waves is measured in the vicinity of the codimension-2 bifurcation point, we measure a decrease of the subcritical interval in which the traveling mode becomes unstable. From the traveling wave state the system undergoes a global secondary bifurcation to an alternating pattern which doubles the wavelength (ks/2) of the primary cellular pattern; this result compares well with theoretical predictions [P. Coullet and G. Iooss, Phys. Rev. Lett. 64, 866 (1990)]. In this cascade of bifurcations towards a defect dynamics, bistability due to the subcritical behavior of our system is the reason for the coexistence of two different modulated patterns connected by a front. These fronts are stationary for a finite interval of the control parameters.

Quantifying the nonlinear mode competition in the flow over an open cavity at medium Reynolds number

Our purpose is to quantify the rate of intermittency of nonlinearly competing modes, in a dominantly mode-switching scenario. What is the rate of presence of each mode? Can they simultaneously appear in, or disappear from the signal? The study is done in the context of open flows, exhibiting self-sustained oscillations, where air is here flowing over an open cavity. Reynolds numbers are of the order of 14,000. Velocity measurements downstream of the cavity are based on a laser Doppler velocimetry technique. We propose two methods to estimate the rate of presence of each mode: one based on a complex demodulation technique, the other on the distribution of the state vectors in the phase portrait of the signal.

The Time‐Varying Phillips Correlation

We use complex demodulation techniques to investigate changes in the correlation between real activity and inflation at the business‐cycle frequencies in the United States, the United Kingdom, the Eurozone, and 10 other Organization for Economic Cooperation and Development (OECD) countries over the post‐WWII era. Consistent with the analysis of Ball, Mankiw, and Romer (1988) we document a positive correlation between the time‐varying average gain of real activity onto inflation at the business‐cycle frequencies and inflation's Hodrick‐Prescott trend, which is compatible with New Keynesian theories emphasizing the link between trend inflation, the frequency of price adjustments, and the slope of the Phillips trade‐off.

MODULATIONAL INSTABILITY IN NONLINEAR BI-INDUCTANCE TRANSMISSION LINE

A nonlinear dissipative transmission line is considered and by performing the complex demodulation technique of the signal which allows, in particularly, to separate the right traveling and left traveling waves, we show that the amplitudes of these waves can be described by a complex coupled Ginzburg–Landau equations (CG-LE). The so-called phase winding solutions of the constructed CG-LE is analyzed. We also study the coherent structures in the obtained complex Ginzburg–Landau system. We show that the constructed CG-LE possesses nonlinear plane wave solutions and the modulational instability of these solutions is analyzed. The condition of the modulational instability is given in term of the coefficients of the constructed CG-LE and then in term of the wavenumber of the two right traveling and left traveling waves in the considered transmission line. The results obtained here show that the nonlinear plane wave solutions of the CG-LE under perturbation with zero wavenumber cannot be stable under modulation.

A statistical correlation of Pc5 pulsations and solar wind pressure oscillations

The SHARE high frequency (HF) radar in Antarctica is used to compare ionospheric plasma flow oscillations in the Pc5 frequency range with low-frequency oscillations in the solar wind pressure measured by the ACE spacecraft. Ten different days in 2000 and 2001 are analysed with respect to different frequencies and geomagnetic latitudes. Both data sets are bandpass filtered and a complex demodulation technique is used to calculate the correlation in each band. On a number of occasions the wave packet structure of the Pc5 pulsations is in good or excellent agreement with the wave packet structure of the solar wind pressure oscillations. This strongly suggests that the oscillations were directly driven by the solar wind. Particularly good correlation is found in the frequency band 0.8–1.2 mHz. Pulsations in this frequency range are hard to reconcile with the magnetospheric cavity mode model. We conclude that, at least on some occasions, Pc5 pulsations may be directly driven and the magnetosphere cavity/waveguide then assumes a more passive role.

Nonlinear evolution of Mack modes in a hypersonic boundary layer

In hypersonic boundary layer flows the nonlinear disturbance evolution occurs relatively slowly over a very long length scale and has a profound effect on boundary layer transition. In the case of low-level freestream disturbances and negligible surface roughness, the transition is due to the modal growth of exponentially growing Mack modes that are destabilized by wall cooling. Cross-bicoherence measurements, derived from hot-wire data acquired in a quiet hypersonic tunnel, are used to identify and quantify phase-locked, quadratic sum and difference interactions involving the Mack modes. In the early stages of the nonlinear disturbance evolution, cross-bicoherence measurements indicate that the energy exchange between the Mack mode and the mean flow first occurs to broaden the sidebands; this is immediately followed by a sum interaction of the Mack mode to generate the first harmonic. In the next stages of the nonlinear disturbance evolution, there is a difference interaction of the first harmonic, which is also thought to contribute to the mean flow distortion. This difference interaction, in the latter stages, is also accompanied by a difference interaction between Mack mode and first harmonic, and a sum interaction, which forces the second harmonic. Analysis using the digital complex demodulation technique, shows that the low-frequency, phase-locked interaction that is identified in the cross bicoherence when the Mack mode and first harmonic have large amplitudes, arises due to the amplitude modulation of Mack mode and first harmonic.

Temperature-insensitive fiber Bragg grating accelerometer

A cantilever beam and fiber Bragg grating is used to measure acceleration. The cantilever induces strain on the grating, resulting in a Bragg wavelength modification that is subsequently detected. The output signal is insensitive to temperature variations and for a temperature change from -20/spl deg/C to 40/spl deg/C, the output signal fluctuated less than 5% without any temperature compensation schemes. Because the accelerometer does not utilize expensive and complex demodulation techniques, it is potentially cost effective. For the experimental system, a linear output range of 8 g could be detected.

Temperature insensitive measurements of static displacements using a fiber Bragg grating

We report a chirped fiber Bragg grating transducer for the measurement of acceleration, in which a cantilever beam and fiber Bragg grating are used. The cantilever induces strain on the grating resulting in a Bragg grating wavelength modification that is subsequently detected. The output signal is insensitive to temperature variations and for a temperature change from -20 degrees C to 40 degrees C, the output signal fluctuated less than 5 % without any temperature compensation schemes. Because the accelerometer does not utilize the complex demodulation techniques it is potentially inexpensive. For the experimental system a linear output range of 8 g could be detected.

Extracting Envelopes of Rossby Wave Packets

Abstract Packets of Rossby waves play an important role in the transfer of kinetic energy in the extratropics. The ability to locate, track, and detect changes in the envelope of these wave packets is vital to detecting baroclinic downstream development, tracking the impact of the analysis errors in numerical weather forecasts, and analyzing the forecast effects of targeted weather observations. In this note, it is argued that a well-known technique of digital signal processing, which is based on the Hilbert transform, should be used for extracting the envelope of atmospheric wave packets. This technique is robust, simple, and computationally inexpensive. The superiority of the proposed algorithm over the complex demodulation technique (the only technique previously used for this purpose in atmospheric studies) is demonstrated by examples. The skill of the proposed algorithm is also demonstrated by tracking wave packets in operational weather analyses from the National Centers for Environmental Prediction...

HF radar observations of Pc5 ULF pulsations driven by the solar wind

We present HF radar observations of Pc5 field line resonances in the post‐midnight Antarctic ionosphere on April 28/29 1997. Simultaneous oscillations were seen in the solar wind parameters observed by WIND. The spectra of both sets of data show peaks near 1.3 mHz, 1.9 mHz, 2.7 mHz and 3.3 mHz. The data were bandpass filtered and a complex demodulation technique applied to calculate the instantaneous energy flux in the solar wind in each band and hence the power incident on the front of the magnetosphere 70 minutes later. The magnitude of the instantaneous power deposited in the ionosphere at each frequency through Joule heating followed this closely and was an order of magnitude smaller. We conclude that, at least on this occasion, the field line resonances could have been directly driven by the solar wind oscillations.

Geo-electrical structure of the mantle beneath the Indian region derived from the 27-day variation and its harmonics

Estimates of the C-response function were determined by Z: H method to obtain conductivity and depth values for a substitute perfect conductor (conductosphere) beneath the Indian region, utilizing geomagnetic variations at periods of 27-day and its harmonics. Two and half years of continuous geomagnetic data were utilized. These data were recorded during 1975–77, at a chain of 13 stations confined to the 150° geomagnetic longitude band, which extended from the dip-equator at the southern tip of India, to the northern parts of Russia. Complex demodulation technique was employed to determine the electromagnetic (EM) responses. Taking advantage of the dense latitudinal distribution of the observatories, the demodulates of all the stations were tested statistically to check the validity of the P 1 0 approximation for the inducing field. Single-station response estimates, for a 27-day period, computed by a robust method have shown that reliable EM responses (consistent with P 1 0 source dependence and with local 1-D Earth structure) could be obtained for only 6 stations, all situated in the mid-latitude region. The depth estimates at all 6 stations are consistent, including Sabhawala (SAB) which is situated close to the Himalayan collision zone. The negligible differences in the depth estimates of these mid-latitude stations do not show any latitudinal dependence, as against such an observation reported for the European and the North American regions. The mean depth of the conductosphere is found to be 1200 (±200) km, with an average conductivity of 0.7 (±0.3) S/m. Comparison of the mean geo-electrical structure with those of other regional models shows that the presence of a mid-mantle conductor at 850 km depth could be considered to be a global phenomenon.