Cross-polarisation method for improvement of 14N NQR signal detectability

This is a study of the cross-polarization effects in the case of 14N quadrupolar spin-system with a long spin-lattice relaxation time. Two important benefits of the cross-polarization technique were demonstrated for PETN: (i) a polarization transfer resulting in increased NQR single shot signal response and (ii) a dynamic reduction in recovery time of the NQR system allowing scan repetition on a much shorter timescale. It was proved that this technique can reduce the optimal waiting time between pulse sequences up to 60 times through a significant reduction of the relaxation time of the quadrupolar spin-system. All experiments were carried out at room temperature using spin-locking multi-pulse sequences and small external magnetic field.     

Polarization enhancement of NQR signals for explosive detection

This paper presents first experiments on using Zeeman polarization-enhanced nuclear quadrupole resonance (NQR) method in detecting explosive substance pentaerythritoltetranitrate (PETN). The NQR signals were detected on nitrogen nuclei at room temperature using a multi-pulse spin-locking sequence after the sample was kept in a very inhomogeneous magnetic field. The dependence of enhancement factor on the time of protons polarization in the magnetic field was studied.     

A method for hiding information utilizing double-random phase-encoding technique

A method for hiding information is proposed. Firstly, the image to be hidden is encoded by the doubled-random phase-encoding technique. Then, the real part of the encoded data, together with the imaginary part, is embedded into a sufficiently large host image, which has been enlarged according to some rules. The superposition weighting is adjusted to a proper value such that the composed image will not be subject to severe degradation compared to the original host image. When decryption is needed, the reconstructed hidden image can be extracted directly from the composed image without using original host image, with the result being quite satisfactory. The quality of the reconstructed hidden image and of the composed image has been analyzed as the superposition weight varies, allowing the determination of the optimum superposition weight. The optimum superposition weight is related to the average gray level value of the hiding image.     

Marked signal improvement by stochastic resonance for aperiodic signals in the double-well system

On the basis of our mixed-signal simulations we report significant stochastic resonance induced input-output signal improvement in the double-well system for aperiodic input types. We used a pulse train with randomised pulse locations and a band-limited noise with low cut-off frequency as input signals, and applied a cross-spectral measure to quantify their noise content. We also supplemented our examinations with simulations in the Schmitt trigger to show that the signal improvement we obtained is not a result of a potential filtering effect due to the limited response time of the double-well dynamics.     

Modification of SSFP technique for the effective detection of NQR signals

Modification of the steady-state free precession (SSFP) multi-pulse technique for the effective detection of the NQR signals in the presence of a strong coherent noise is proposed. This modification based on the use of additional blocks of the pulses and phase cycling technique enables the suppression of the coherent noise signals such as the magneto-acoustic and piezoelectric signals or the ringing signals from the NQR probe. Experimental results of applying the proposed technique to nitrogen-14 NQR in the sample of RDX (C₃H₆N₆O₆) are presented and convincingly demonstrate its effectiveness.     

Transient NQR signals in the effective field of multipulse trains

This paper is a theoretical analysis of time-dependent nutation and echo signals in the effective field of multipulse trains in NQR. The results of experimental investigations of the same aspects were reported in earlier papers. The various features of dipole-dipole interactions in three-level quadrupole spin systems are discussed. It is shown that, in contrast to NMR, the dipole Hamiltonian in the interaction representation determined by the quadrupole Hamiltonian contains only three diagonal components of the dipole-dipole interaction tensor. On the other hand, the strong inhomogeneous broadening characteristic of NQR hinders exact measurement of these components by ordinary methods. The theoretical analysis suggests that the decay of the echo-signal envelope in the effective field of multipulse trains is determined solely by the dipole relaxation time, which serves as justification of a new experimental method used in measuring the characteristics of the dipole-dipole interaction tensor in spin systems with an inhomogeneously broadened spectrum. Zh. éksp. Teor. Fiz. 115, 1580–1592 (May 1999)     

Effectiveness of NQR signal detection in noise in multiple-pulse sequences

A technique for signal processing in multiple-pulse experiments is examined. Results are obtained which permit selection of the NQR-signal parameters for its detection in a noise background and optimization of the signal processing parameters, in particular, minimization of the experiment time.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 1, pp. 46–50, January, 1989.     

Singularties of the shape of the nonresonance nutation spectra of NQR powders

The singularities of nutation NQR spectra for nuclei with spin J=1, 3/2 in the absence of resonance are considered for powders. It is shown that when determining the asymmetry parameter from the frequency characteristics of the nutation spectrum one must take into account the frequency shift of the spectrometer relative to exact resonance. Structural studies of single crystals by nutation spectroscopy are demonstrated to be feasible.Kaliningrad State University. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 6, pp. 66–72, June, 1995.     

A secure communication method for a high-power information signal based on chaotic masking

In this paper,we present a secure communication method for a high-power information signal based on chaotic masking.In the transmitter,an adaptive controller is adopted to pick up the change of the information signal ,and to inject the controller‘s error into the transmitting system.At the same time,the information is directly added to the chaotic signal in transmission to drive the receiving system.In the receiver,another adaptive controller is used to maintain chaotic synchronization of the transmitting and receiving systems and to recover the information signal.Since the synchronization error is independent from the information signal,the power of the information signal can be equivalent to that of the chaotic signal,and the frequency of the information signal can be set within the range of the principal frequencies of the chaotic signal,The results of theoretical analysis and numerical simulation show that the presented method not only enhances the degree of security of low-dimensional chaotic systems but also significantly improves the signal-to-noise ratio at the receiving end.     

Security analysis and improvement on a double-random phase-encoding technique based information hiding method

The security of an information hiding method using the double-random phase-encoding technique is analyzed and improved. The method encodes the hidden image with the double-random phase-encoding technique, and then embeds the real and imaginary parts of the encoded image to an enlarged host image. Recovering the hidden image from the composed image does not need the original host image, while the quality of the reconstructed image is satisfying. However, the original method is insecure in the sense of information hiding, since the composed image can be detected easily with some technical metrics. The security is improved by adding random perturbation on the encoded data. The validity of the improved method is demonstrated by numerical simulation experiments.     

Simulation of AE signals and signal analysis systems

A stochastic series is presented by which a typical AE signal detected by resonant transducer can be described. The series is then analysed numerically by a simulated system for AE amplitude distribution analysis. Comparison between the analytic distribution used in the generation of random series and the distribution obtained from the simulated system demonstrates how closely the measured parameter β corresponds to the parameter of source amplitude distribution b. The analysis shows that amplitude analysis gives better results when performed on ring-down bursts than on corresponding demodulated signals.     

Application of the spherical tensor method for Two-Frequency pure NQR of spinI=1 nuclei

The results of a theoretical investigation of two-frequency excitation in pure nuclear quadrupole resonance (NQR) for a spinI=1 nucleus with a nonaxial elecric field gradient are presented. The multipole tensor operator technique is used for the treatment of the one- and two-frequency pulse excitations. The results are applied to the characterization of the two-frequency signal of nitrogen¹⁴N nuclei. The experiments on sodium nitrite, NaNO₂, confirm the presence of additional (two-frequency) echo in the NQR signal. The effect of resonance offsets on two-frequency NQR is also considered.     

Intensity of NQR lines for integer spins

Because of the publication of contradictory information demonstrating an incorrect approach to the modeling of NQR-NMR frequency-intensity spectra of integral-spin isotopes (for example, boron-10), we study the NQR-NMR double resonance with level crossing (DRLC) in¹⁰B-¹H,¹¹B-¹H,⁵⁹K-¹H and²³Na-¹H in crystal hydrates of sodium tetraborate (Na₂B₄O₇·10H₂O) and potassium tetraborate (K₂B₄O₇·4H₂O) and the results of the measurements are compared with theoretical calculations. The fact that for a system including three elements-quadrupole (NQR), Zeeman (NMR), and dipole-dipole, which establishes the interaction of the first two- the intensities of the lines in double resonance spectra are determined both by the equations of balance of the populations of the levels of the quadrupole and Zeeman reservoirs and by the probabilities of transitions within each reservoir is taken into account in the calculations. The distortion of the energy levels of the NQR subsystem by the magnetic field in the process of level crossing with the NMR subsystem is taken into account.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 12, pp. 3–9, December, 1986.     

Using HDC to evaluate signal similarity for information masking

In the many studies done on informational masking, interfering speech reduces speech intelligibility. This effect is often used to secure privacy in public spaces. These applications require estimates of how much masking is required. In general, masking effects are estimated by using spectrum information as excitation patterns. However, estimates of informational masking can hardly be obtained by only using spectrum information. Therefore, we estimated the effects of informational masking using time-domain information. Then, we calculated the cepstra of the envelopes’ magnitude histograms. If these cepstra are different between the target and the masker, the signals are not similar in the time-domain. Furthermore, the effect of informational masking would be low. Therefore, we considered the histograms’ cepstra distances (HCD) to estimate signal similarities. The signal similarities in our first experiment were estimated using five maskers by utilizing the HCD. These maskers were random noise, music, female speech, male speech, and target speaker’s speech. Male and female speech were more similar to the target speech than music and noise. Also, the same speaker’s speech was the most similar in the set of maskers. A listening test was carried out in the second experiment to verify the HCD. A double masker was used in this experiment as an effective informational masker. It has similar characteristics to reversal speech. The listening test results suggest the double-masker’s masking effects has the same relation with HCD. This suggests informational masking can be estimated by signal similarity using the HCD.     

Highly linear transmission and EVM improvement of vector modulation signals for radio-over-fiber applications

The principle of error vector magnitude (EVM) against modulator nonlinearity for vector modulation signal (VMS) transmission in radio-over-fiber (RoF) systems is theoretically and experimentally investigated. A highly linear modulation scheme is proposed and demonstrated using a single-drive dual parallel Mach-Zehnder modulator (MZM). This method improves EVM performance and enlarges the linear input dynamic range of the VMS transmission. An index of maximum allowable input power difference (MAIPD) that reflects the difference of upper input power limits between these two schemes is measured. An EVM limitation of 5% MAIPD has 5 dB. Both 16and 64-QAM results indicate that the proposed scheme supports VMS transmission better than the MZM one.