Unmodulated Components Research Articles

Mismatches analysis based on channel response and an amplitude correction method for time interleaved photonic analog-to-digital converters.

A channel mismatch model for time-interleaved photonic analog-to-digital converters is built by decomposing both optical time division multiplexing channels and the wavelength division multiplexing channels into time channels with uniform and equal-amplitude sampling clock. Based on the model, the influence of power and timing mismatch of optical sampling pulse trains, optical sampling pulse temporal shape mismatches, photodetection bandwidth mismatches on the sampling results are analyzed theoretically. Depending on the found relationship between unmodulated components and modulated components, an effective online amplitude mismatch correction method applicable to wideband signals is proposed. The theoretical results are verified on a 4-channel TIPADC system. The proposed amplitude mismatch correction method can suppress the spurs caused by unmodulated components mismatch and modulated components mismatch by ~50 dB and ~30 dB for a two-tone signal, respectively.

Atomic structure of GdBa [formula omitted]Cu [formula omitted]O [formula omitted] superconductor thin films on NdGaO [formula omitted](0 0 1)Probed by X-ray standing waves and photoelectron spectroscopy

We characterized the structure of GdBa 2 Cu 3 O 7 - δ thin films grown on NdGaO 3 (0 0 1) substrates using X-ray standing waves combined with photoelectron spectroscopy. The O 1 s and Cu 2 p core-level spectra display components that exhibit distinct X-ray standing wave modulations, which we identify as arising from the well-ordered (intrinsic) parts of the oxide films, but also contain nearly unmodulated components arising from reacted, degraded phases (non-intrinsic). The large mean-free paths of high kinetic-energy photoelectrons permits, in addition, the detection of the Nd and Ga atoms near the buried interface below a 3.5-nm film. Our measurements reveal an essentially bulk-terminated NdGaO 3 surface at the interface. By considering a pseudomorphically strained, orthorhombic Y123 structure for the 3.5-nm film, we find that our X-ray standing wave data are consistent with an interface composed of CuO–NdO planes, where the Cu–Nd distances are determined to be 0.18 nm. The analysis rules out the possibility for the film to start its stacking with a BaO plane. The X-ray standing wave measurement for the GdBa 2 Cu 3 O 7 - δ valence band yields results nearly identical to the intrinsic part of the Cu 2 p core level and therefore confirms the major contribution of Cu 3 d to the valence band in high-temperature cuprates. For thicker films, the standing wave analysis on a 17.5-nm sample indicates that the film is partially relaxed, in good agreement with the previous X-ray diffraction results. Repeating the measurements at 60 K suggests that the in-plane lattice mismatch between the film and substrate increases with decreasing temperature.

The contribution of spike threshold to the dichotomy of cortical simple and complex cells.

The existence of two classes of cells, simple and complex, discovered by Hubel and Wiesel in 1962, is one of the fundamental features of cat primary visual cortex. A quantitative measure used to distinguish simple and complex cells is the ratio between modulated and unmodulated components of spike responses to drifting gratings, an index that forms a bimodal distribution. We have found that the modulation ratio, when derived from the subthreshold membrane potential instead of from spike rate, is unimodally distributed, but highly skewed. The distribution of the modulation ratio as derived from spike rate can, in turn, be predicted quantitatively by the nonlinear properties of spike threshold applied to the skewed distribution of the subthreshold modulation ratio. Threshold also increases the spatial segregation of ON and OFF regions of the receptive field, a defining attribute of simple cells. The distinction between simple and complex cells is therefore enhanced by threshold, much like the selectivity for stimulus features such as orientation and direction. In this case, however, a continuous distribution in the spatial organization of synaptic inputs is transformed into two distinct classes of cells.

Inelastic dark matter at DAMA, CDMS and future experiments

The DAMA annual modulation signature, interpreted as evidence for a spin-independent WIMP coupling, seems in conflict with null results from CDMS. However, in models of “inelastic dark matter”, the experiments are compatible. Inelastic dark matter can arise in supersymmetric theories as the real component of a sneutrino mixed with a singlet scalar. In contrast with ordinary sneutrino dark matter, such particles can satisfy all experimental constraints while giving the appropriate relic abundance. We discuss the modifications to the signal seen at DAMA, in particular noting the strong suppression of low energy events in both modulated and unmodulated components. We discuss future experiments, with emphasis on distinguishing inelastic dark matter from ordinary dark matter, and stressing the significance of experiments with heavy target nuclei, such as xenon and tungsten.

Generalized quadratic receivers for orthogonal signals over the Gaussian channel with unknown phase/fading

The orthogonal signal structure has been shown to be the superposition of an antipodal signal set and an unmodulated (pilot tone) component which can be used for channel measurement. Starting from this point of view, the quadratic receiver for orthogonal signals over the Gaussian channel with unknown phase/fading has been shown to be equivalent to a detector-estimator receiver. The estimator makes an optimum estimate of the unknown complex channel gain based on the channel measurement provided by the unmodulated component of the received signal. This channel estimate then forms a (partially) coherent reference for the detector in detecting the data carried by the antipodal signaling component of the received signal. This paper exploits this detector-estimator structure of the quadratic receiver, and generalizes it to a receiver in which the estimator makes an estimate of the channel gain in each signaling interval based on the totality of signals received over all the signaling intervals or a subset of these intervals. The generalized quadratic receiver is just as simple to implement as the conventional quadratic receiver, and theoretical and simulation results show that it can achieve substantial performance gains over the conventional receiver. A theory is presented to show that the generalized quadratic receiver is an implementable approximation to the optimum symbol-by-symbol receiver for uncoded orthogonal signals over the Gaussian channel with unknown phase/fading. The theory shows that the structure provides a unified and systematic approach to the design of coherent symbol-by-symbol receivers, and shows that the conventional carrier-loop-type receivers are ad hoc.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Maximum likelihood carrier phase recovery for coherently orthogonal CPFSK signals

ML estimation of carrier phase for coherently orthogonal continuous-phase frequency-shift-keying (COCPFSK) signals is considered. Although the estimator, in general is nonimplementable, its high and low signal-to-noise-ratio approximations both lead to linear readily implementable receiver structures. The high SNR approximation yields a DA receiver, whereas the low SNR approximation yields an NDA receiver. The performance of both receivers in term of bit error probability is analyzed. The existence of an unmodulated component in the sufficient statistical representation of a COCPFSK signal is pointed out, and it is shown how this component enters directly into maximum-like carrier recovery. This leads to interpretation of the NDA receiver as a generalization of the conventional matched-filter envelope-detector receiver. The insights gained here are useful to the problem of ML carrier recovery for Viterbi decoding of continuous phase modulation signals.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Hyperfine interaction constants and lifetime of the 62D3/2and 62D5/2states of39K measured by the quantum beam method

The hyperfine structure and lifetime of the 62D3/2 and 62D5/2 states in 39K are studied experimentally. The states were excited from the ground state by two-photon absorption of the laser light pulses. The hyperfine-structure intervals, and hence the hyperfine-structure interaction constants, were obtained from the numerical analysis of the zero-field quantum beat signals. The unmodulated component of the fluorescence decay curve served to determine the lifetime. In a separate experiment a magnetic field was applied to induce partial decoupling of the electronic and nuclear angular momenta in the 62D states. The sign of the magnetic dipole interaction constants was deduced from the resulting difference between the intensities of sigma + and sigma - polarised fluorescence.

Structure of a complex receptive field in the cat visual cortex

The most common type of complex receptive field, whose response to the passage of sinusoidal gratings across it consisted of modulated and unmodulated components, was analyzed. The use of a mask to cover half the field, according to the filter theory, led to widening of the transmission band of the field as a spatial frequency filter, due to the appearance or enhancement of the response at lateral low and high frequencies. Modulated components of responses from the left and right halves of the field were out of phase. Analysis of this fact, and also of responses of the field to thin light and dark bars enabled the field structure to be described. It consists of linear and nonlinear subsystems, converging on the output neuron of the complex field. The former is composed of several pairs of on- and off-subfields of the lateral geniculate body. The on- and off-subfields in the pair overlap spatially and converge on the output neuron of the linear subsystem with opposite signs. The nonlinear subsystem is composed of either on- or off-subfields. Other types of complex fields may include different combinations of subsystems. The results indicate that complex fields are spatiotemporal grating filters.

Heteronuclear J spectroscopy

In a study of heteronuclear J NMR spectroscopy, the radio frequency phase of the initial 90 Deg pulse was shifted together with the phase of the refocusing pulse. In heteronuclear spectra, the phase cycling method shifts all resonances by DF1 = (4Dt-1)-1 corresponding to 1/2 of the spectral width in the F1 frequency domain. Thus the proton transmitter can be set in the middle of the proton spectrum without introducing aliasing problems, and the technique does not interfere with ordinary quadrature detection, which is routinely used in the F-2 or P dimension. The phase-cycling scheme represents a simple alternative to the quadrupole phase detection proposed by R. R Ernst et al. (1978) since it does not require addnl. software for a hypercomplex Fourier transform. To eliminate the unmodulated component in the P spectra, the phase of the last 90 Deg proton pulse is alternated while the P free-induction decays are added and subtracted in alternate transients. Results are presented for Me3PO4 and 2'-guanosine monophosphate. [on SciFinder (R)]

Kinetics of Photochemical Systems with Modulated Optical Excitation

AbstractThe kinetic equations of a photochemical system are discussed in which the transitions from the ground to the first excited state are produced by a sinusoidally modulated photon source. The system includes irreversible reactions starting from the first excited singlet state S1 and the first triplet state T1 (cf. Fig. 1). Explicit solutions are given for a linear approximation of the equations of motion for the parametrically modulated system. The solutions apply only when there is weak optical pumping, low quantum yields of the photochemical end‐products and for certain conditions, of the modulation frequency. The concentrations of all particles occurring are shown to consist of modulated and unmodulated components where the amplitudes of the modulated components are slowly decaying exponentials. Thc phases of the modulated components are shown to be non‐decreasing along a sequence of irreversible steps. Formulae are given which permit determination of the photochemical system rate constants and location of a particle in a sequence of irreversible reactions.

Effect of Transverse Pumping in Optical Pumping

A theoretical and experimental study has been made of the effect of transverse optical pumping in the optical-rf double resonance of cesium atoms. The solution of Bloch type equations shows that the Hanle effect and new type resonances become notable in the weak magnetic field or for strong rf field, as the effect of the transverse pumping. The new type resonances appear in the unmodulated component and the components modulated at the first and the second harmonics of the applied rf frequency in the pumping light beam transmitted through the sample cell. It is predicted that the new type resonances are not only shifted but also broadened as the intensity of rf field increases. The shift, broadening and the intensity for observed new type resonances are in good agreement with the theoretical prediction.