DC Amplitude Research Articles

Rectification and phase locking for particles on symmetric two-dimensional periodic substrates.

We demonstrate a rectification phenomenon for overdamped particles interacting with a 2D symmetric periodic substrate when driven with a dc and a circular ac drive. As a function of longitudinal dc amplitude, the longitudinal velocity increases in a series of quantized steps distinct from Shapiro steps with transverse rectification occurring near these transitions. The rectification phenomenon is explained using symmetry arguments and a simple model.

Signal processing by means of noise

Noisy integrate-and-fire neurons encode periodic signals best at a preferred noise-frequency combination. We show that the preferred frequency, optimum noise and optimum signal-to-noise ratio are linearly related to the signal AC amplitude with the signal DC amplitude serving as tuning parameter. We argue that this may facilitate selective signal processing in neuronal networks.

Post-synchronization of dynamic images of periodically moving organs.

The motion of periodically moving organs can be studied by acquisition of dynamic image series. When time is short, it is necessary either to find a sync signal to sum synchronous images in real time or to acquire a regular time series and to synchronize a posteriori. Dynamic acquisitions were performed (gastric and lung studies). The activity in each pixel of the moving organ can be expressed as h(t)=a0 + a1 cos(omegao0t - phi). The time-activity curve u(t) over a region of interest (ROI) of the considered organ was computed. When the ROI is well chosen, the power spectrum of u(t) exhibits a sharp peak near the characteristic frequency of the periodic motion. The DC component, amplitude and phase in each pixel can be then estimated by minimizing the following function: J=sigma[h(t) - g(t)]2, where g(t) is a noisy measurement of h(t). It is then easy to reconstruct an a posteriori gated time series by computing h(t) for various times over a single period. This approach was successful in characterizing lung and gastric motions. Dynamic series were acquired as for gastric emptying studies. The characteristic frequency of antral motility was easily and unambiguously estimated and DC, amplitude and phase images were computed. Dynamic pulmonary functional imaging was performed with 81Krm. The characteristic frequency was also easily estimated from the time-activity curve power spectrum using a ROI drawn over the lower part of the lungs. The DC, amplitude and phase images were then computed from the dynamic series and the characteristic frequency. In conclusion, a posteriori gating of dynamic series of periodically moving organs can be achieved in a simple fashion. This approach overcomes the difficulty of direct analysis of time-activity curves and provides amplitude and phase images.

Terahertz bandwidth prediction for amplitude modulation response of unipolar intersubband semiconductor lasers

An analysis is performed of the direct-current (dc) amplitude modulation response of unipolar intersubband semiconductor lasers. It is shown that unipolar lasers have the potential for achieving terahertz dc modulation bandwidths.

What is the time scale of magnetic field interaction in biological systems?

An experimental test constraining the intrinsic time scale of a primary physical mechanism that detects extremely-low-frequency (ELF) magnetic fields in biological systems is proposed. The suggested test postulates that a transductive mechanism operating on time scales much shorter than the period of an applied magnetic field cannot obtain any information about the exposure conditions other than the absolute magnitude of the field. By generating field exposure that differ in their vector properties but are equivalent in their time-varying absolute amplitude, it is possible to differentiate between two broad classes of mechanisms: 1) those with intrinsic time scales comparable with or longer than those of the external influence, and 2) those that are much faster than the period of the applied field. The hypothesis assumes an experimental model proven to respond to magnetic fields and sensitive to a change of about a factor of two in one of the field parameters (AC, DC amplitude or frequency). The case of general linearly polarized fields is discussed, and an analytical solution for the case of perpendicular AC/DC fields is given.

Flux Pinning and Vortex Dynamics in K3C60 Crystals

AC susceptibility measurements were carried out on K3C60 crystals. The complex susceptibility χ=χ′+χ′′ shows dependence on dc magnetic-field amplitude H dc and ac amplitude h ac. The temperature where χ′′ is maximal, T p, exhibits a weak dependence on ac frequency f: T p rises with increasing f. Increasing H dc or h ac enhances the effect. The inverse temperature, T p-1, roughly obeys the linear relationship T p-1≈-5×10-5H dcln f+b(H dc), where b is a constant and SI units are used. This is in qualitative agreement with the predictions of the thermally activated flux creep model. This suggests that in the experiment flux lines are pinned at pinning sites and ac losses are mainly due to flux pinning losses. The temperature T p is found to obey an empirical scaling law (1-T p/T c)3/2≈4×10-4H dc[ln (f0/f)+25]+1.4×10-2. The scaling relationship is in good agreement with the predicted equation based on the model of thermally activated flux motion.

Implementation of a gallium arsenide multichip digital circuit operating at 500-1000 MHz clock rates using a Si/Cu/SiO/sub 2/ MCM-D technology

Two different deposited multichip modules (MCMs) were fabricated in nCHIP's nC3000 Si/Cu/SiO/sub 2/ process. The first of these MCMs was a passive test coupon containing a variety of microstrip and stripline transmission line structures, allowing the measurement of dc and ac signal amplitude losses in long conductors, as well as assessments of crosstalk and reflections as functions of line dimensions and spacings. The second MCM incorporated sixteen Gallium Arsenide (GaAs) integrated circuits, all designed to work together at clock rates in the hundreds of MHz; all components were attached, face up, with an aluminum wire bonding process. The design, fabrication, assembly and test processes for these modules will be described, as well as the lessons learned about this MCM process for the design of subsystems up to the high hundreds of MHz clock rates.

Logarithmic AC Response in Rb3C60 Crystals

AC susceptibility measurements were performed on Rb3C60 crystals. Two peaks are observed in the imaginary part of the susceptibility χ′′(T) and the peak near the transition temperature T c is more pronounced. This is interpreted in terms of weak links in the fullerene superconductors. The temperature associated with the intragranular χ′′ maximum T m exhibits extremely weak dependence on ac frequency f; T m shifts to higher temperature with increasing f and the effect is enhanced by increasing dc magnetic-field amplitude H dc or ac amplitude h ac. The inverse temperature 1/T m is found to depend roughly linearly on logarithmic frequency. This result is consistent with the prediction of the thermally activated flux creep model. This implies that the ac power dissipation in the Rb3C60 superconductors is predominantly due to hysteretic losses and therefore that magnetic flux is in the irreversible regime of the magnetic phase diagram. The thermal activation energy for flux lines U0 is estimated to be ≈6.1 eV at H dc=1 kOe and h ac=1 Oe. Finally, (1-T m/T c)3/2 is found to depend linearly on ln f.

A chips-first multichip module implementation of passive and active test coupons utilizing Texas Instruments' high density interconnect technology

In this paper, we describe the development and measurement of two separate test structures, or "coupons" to assess the performance of Texas Instruments' "chips-first" multichip module technology at high operating clock rates, A "passive" coupon containing a variety of microstrip and stripline transmission line structures allowed the measurement of DC and AC signal amplitude losses in long conductors, as well as assessments of crosstalk and reflections as functions of line dimensions and spacings. An "active" coupon containing 16 interconnected gallium arsenide (GaAs) chips of two separate designs allowed the assessment of this MCM technology's ability to support the propagation of digital signals at clock and pulse rates above 500 MHz, and the ability of the power and ground plane structures to deliver clean power to the operating components. These comprehensive tests have allowed the development of design rules for developing future high performance systems using this unique packaging technology. Finally, comments are presented regarding future directions for this technology to lower manufacturing costs while preserving the high levels of operational performance demonstrated by these tests.

Direct measurements of the true vibrational amplitudes in shear force microscopy

A new method to measure the tip’s true vibrational amplitude in the shear force microscopy is demonstrated. The measurements are based on modifications of the beam diffraction method. In addition to the dither vibration, the equilibrium position of the tip is set to move along the direction of the vibration. The ratio of ac amplitude to the derivative of dc amplitude gives the tip’s vibration. The sensitivity of this method is determined by the size of the laser focal spot times the ratio of the ac to dc signals of the diffracted light.

High-frequency flux dynamics in single-crystal Nd1.85Ce0.15CuO4.

Flux dynamics in a ${\mathrm{Nd}}_{1.85}$${\mathrm{Ce}}_{0.15}$${\mathrm{CuO}}_{4}$ single crystal is investigated in the 20 kHz--3 MHz frequency range by using a mutual-inductance technique. We show that the superconducting ac response is due to thermally activated flux motion; from the dc field, ac amplitude, and frequency dependencies of this ac response, we extract the effective energy barriers U(T,B,J)=${\mathit{U}}_{0}$(1-T/${\mathit{T}}_{\mathit{c}}$)${\mathit{B}}^{\mathrm{\ensuremath{-}}0.66}$ln(${\mathit{J}}_{\mathit{c}}$/J). Significantly, these energies account for the time-dependent response of these superconductors over ten decades of time. As a consequence of them, the ac response is found to be strongly nonlinear, i.e., the ac susceptibility depends on the driving-field amplitude for ac fields as small as ${10}^{\mathrm{\ensuremath{-}}6}$ T superposed to dc fields up to 2 T. The ac transition is well described by a critical-state model with a frequency-dependent effective critical current ${\mathit{J}}_{\mathit{c}}$(T,B,f) stemming from the thermally activated flux motion. Our analysis allows us to obtain the temperature and frequency dependencies of ${\mathit{J}}_{\mathit{c}}$(T,B,f). ${\mathit{J}}_{\mathit{c}}$(f) follows a power law which is shown to be in agreement with the observed logarithmic U(J) dependence. Finally, isothermal cuts carried out for different driving-field amplitudes are used to explore the onset and limits of the nonlinear response.

Amplitude-modulated tone encoding behavior of cochlear nucleus neurons: Modeling study

A recent study of amplitude-modulated (AM) tone encoding behavior of dorsal and posteroventral cochlear nucleus (DCN and PVCN) neurons by Kim et al. [Hear. Res. 45, 95–113, 1990] observed that certain neurons (e.g., pause/build type-III neurons and chop-S neurons) tended to exhibit band-pass modulation transfer functions (MTFs) and intrinsic oscillations (IOs) whereas certain other neurons (e.g., chop-T neurons) tended to exhibit low-pass MTFs and no IOs. The goal of the present study develop models of these response characteristics in an attempt to understand the underlying neuronal mechanisms. We hypothesized that chopper neurons corresponded to stellate cells and pause/build neurons corresponded to fusiform cells. We also hypothesized that, with right input combination, appropriate models of a single stellate and fusiform cell could account for band-pass and low-pass MTFs as well as the associated IOs. The neuron models developed by Arle and Kim [Biol. Cybern. 64, 273–283, 1991] for the stellate and fusiform models were used in this study. The models are modified versions of MacGregor type neuron model incorporating cell-specific nonlinear voltage-dependent conductances. The AM tone excitation via the auditory nerve fibers was represented by a current at the soma of the neuron model, which consisted of dc, ac and aero-mean Gaussian noise. The dc, ac and noise represent a high-frequency carrier beyond the neuron's phase-locking limit, an envelope, and randomness of the system, respectively. With systematic variation of dc, ac and noise amplitudes, we observed the following: the band-pass MTF behaviors of pause/build and chop-S neurons were reproduced by the fusiform cell model and the stellate cell model with a strong dc/noise ratio, respectively. The low-pass MTF behavior of a chop-T neuron was reproduced by the stellate cell model with a weak dc/noise ratio. It was observed that the stellate cell model was more susceptible to the noise, in the sense that an increase in noise tended to abolish the IO and change the MTF of the model from band-pass to low-pass more readily in the stellate cell model than in the fusiform cell model. Kim et al. (1990) observed a close correlation between the IO frequency and the best envelope frequency (BEF). In the models, a similar correlation was observed between the two measures for both the stellate and fusiform cell models. The present results support the hypothesis that intrinsic cellular mechanisms of fusiform and stellate cells similar to those postulated in these cell models underlie the observed MTF behavior of CN neurons in response to AM tone stimuli.

Motional Sidebands in the Microwave Spectra of Ions in an Rf Trap

Abstract The atomic motion of an ion in an rf trap adds sidebands to the ion's hyperfine transitions, shifted and broadened by the space charge. Sidebands and carrier depend differently on the microwave polarization, which is determined by observing microwave spectra with the orientation of the dc magnetic field varied. This phenomenon is caused by dc amplitude and phase modulation of the microwave radiation that is partially reflected inside the trap.

Effect of frequency on sensitivity of HTS sensors and devices

The nonlinear strong magnetic response of high temperature superconductors to alternating magnetic fields, has been employed for measurement of DC fields below one nanotesla. The sensitivity of magnetometers based on YBCO polycrystalline sensors is reported on as a function of applied DC and AC field amplitude and frequency and sensor design parameters.

Class E low dv/dt synchronous rectifier with controlled duty ratio and output voltage

A class-E high-frequency low-dv/dt synchronous rectifier/DC regulator is analyzed. The circuit is derived from a class-E rectifier by replacing a diode with a controllable switch. It can then both rectify and regulate the rectified output against variations in DC load and amplitude of the AC input voltage. The switch consists of a transistor and an antiparallel diode or a series diode. The DC output voltage of the rectifier is regulated by varying the ON duty ratio of the switch. The AC input voltage can be at a constant or varying frequency. The controllable switch turns on and off at zero voltage with low dv/dt, reducing switching losses, stresses, and noise. Equations governing the circuit operation are derived. Experimental results at 0.5 MHz were in good agreement with the theoretical predictions. The rectifier can be used in high-frequency-power-density multiple-output power supplies, all of whose outputs are regulated. The frequency need not be varied to regulate the DC output.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Nonlinear dynamical behavior of a free‐standing metglas ribbon

Dynamical oscillations of a vertical, free‐standing magnetoelastic ribbon clamped at the base are induced by parametric driving. The Young's modulus is a nonlinear function of an externally applied axial magnetic field, H = H0 + H1 cos 2πft, where H0 and H1 are dc and ac amplitudes, respectively, of the drive, and f is the applied frequency. Field parameters and ribbon length are chosen so that gravitational buckling occurs. A variety of nonlinear dynamical effects have been observed, including crisis‐induced intermittency [W. L. Ditto et al., Phys. Rev. Lett. 63, 923 (1989)] and strange (fractal attractor), nonchaotic (no positive Lyapunov exponents) behavior [W. L. Ditto et al., Phys. Rev. Lett. 65 (1990)]. The experiments, some of the results, and a simple model (if time permits) are described.

Combined optical and electronic analysis of cells with the AMAC transducers.

Two approaches to the development of a true automated multiparameter analyzer for cells, AMAC, are discussed. The AMAC IV uses a water immersion lens mounted downstream from a Coulter Orifice for optical measurements. Although this type of device has a very high numerical aperture (0.70) and optimum light scattering characteristics, it is very difficult to minimize the distance between the objective and the orifice, and increasing this distance results in trajectory irregularities of the cells, with the possibility of exchange of sequence of cells between the orifice and the optical sensing zone. The other, the AMAC III, which is the optimal solution to this problem, performs the optical and electronic measurements simultaneously. This transducer is a small square orifice fabricated out of quartz, with sensing electrodes located at both ends. This combined electro-optical transducer has been used for fluorescence, electronic cell volume and opacity (AC amplitude divided by DC amplitude) measurements. Diffraction effects due to the orifice appear minimal and should not interfere with the scattering produced by the cells. Previous studies with a round but otherwise electronically similar orifice have indicated that it was possible to determine the shift in phase produced by the presence of a particle in the orifice. The capacity to measure all of these parameters simultaneously will greatly enhance the reliability of flow cell analysis.

Doping uniformity and geometry of LSA oscillator diodes

For maximum dc to RF power conversion efficiency, the dc and RF electric-field amplitudes must be properly related throughout the volume of a limited space-charge accumulation (LSA) diode. Space-charge due to a few percent fluctuation in the relative doping can distort the electric field enough to reduce the maximum attainable efficiency. Standing-wave effects with the diode limit the width in the direction of wave propagation to about 0.04 free-space wavelengths.