Time-periodic modulations

Module time period Number of participants students f m Number of participants pupils f m Total number of participants of the module... 

The CDF can be controlled by controlling the period of conduction, in other words, the pulse widths (periodic time period, T remaining the same). Thus the a.c. output voltage in an IGBT inverter can be controlled with the help of modulation. The modulation in the inverter circuit is acliieved by superposing a cairier voltage waveform... 

Figure 4.10 Representation of the Uansforniation of data from a single-column data suing to a mauix form, based on the sampling frequency and modulation time. The data points acquired for each modulation period are placed in a separate row of the mauix. The matrix data are then in a suitable format to read into an appropriate plotting package such as the Transfoim program.

Regime II Defect Diff ision the zigzag pattern is modulated quasi-periodically in time. The chaos in the domain boiindary is still localized but it can now move about in space. For r 0.1, the phase change Regime I —> Regime II occurs at about a 3.82. 

Figure 3.1 The various time periods in a two-dimensional NMR experiment. Nuclei are allowed to approach a state of thermal equilibrium during the preparation period before the first pulse is applied. This pulse disturbs the equilibrium ptolariza-tion state established during the preparation period, and during the subsequent evolution period the nuclei may be subjected to the influence of other, neighboring spins. If the amplitudes of the nuclei are modulated by the chemical shifts of the nuclei to which they are coupled, 2D-shift-correlated spectra are obtained. On the other hand, if their amplitudes are modulated by the coupling frequencies, then 2D /-resolved spectra result. The evolution period may be followed by a mixing period A, as in Nuclear Overhauser Enhancement Spectroscopy (NOESY) or 2D exchange spectra. The mixing period is followed by the second evolution (detection) period) ij.

There are actually two independent time periods involved, t and t. The time period ti after the application of the first pulse is incremented systematically, and separate FIDs are obtained at each value of t. The second time period, represents the detection period and it is kept constant. The first set of Fourier transformations (of rows) yields frequency-domain spectra, as in the ID experiment. When these frequency-domain spectra are stacked together (data transposition), a new data matrix, or pseudo-FID, is obtained, S(absorption-mode signals are modulated in amplitude as a function of t. It is therefore necessary to carry out second Fourier transformation to convert this pseudo FID to frequency domain spectra. The second set of Fourier transformations (across columns) on S (/j, F. produces a two-dimensional spectrum S F, F ). This represents a general procedure for obtaining 2D spectra. 

The thermodynamic solubility of a drug is the concentration of the compound that is dissolved in aqueous solution in equilibrium with the undissolved amount, when measured at 25°C after an appropriate time period. Aqueous solubility has long been recognized as a key molecular property in pharmaceutical science. Drug delivery, transport and distribution phenomena depend on solubility thus, it is of considerable value to possess information of the solubility value of a drug candidate, to be able to predict the solubility for unknown compounds and, finally, to be able to modify the structure of a compound in order to modulate its solubility value in an appropriate manner. 

A peristaltic pump pulls the sample stream to each of the module detectors for a selected time period this is the Pump Time . TTie peristaltic pump then stops and the detectors are allowed to come to equilibrium this is the Measure Time . At the end of... 

Impulsive phase modulation.— Let the phase of the modulation function periodically jump by an amount (p at times t, 2t,. .. Such modulation can be achieved by a train of n identical, equidistant, narrow pulses of nonresonant radiation, which produce pulsed frequency shifts Now... 

Figure 12. Sample modulation data of chemical amplifier of Cantrell et al. (130,), showing stability over a 1-h time period (top) and the time dependence on a shorter time scale (3 min).

Preparation, evolution and detection are the time periods of pulse sequences described for. /-modulated spin-echo and polarization transfer experiments. The basic FT NMR technique operates without any evolution period tt immediately after generation of transverse magnetization (preparation) its free induction decay S(t2) is detected. Subsequent Fourier transformation provides the FT NMR spectrum S (f5). 

A constant evolution period t, is the new feature of. /-modulated spin-echo, DEPT, and INADEQUATE sequences. During this time period, a 7-modulation or a polarization transfer may evolve. Such pulse sequences provide FID signals S(t2) which are still functions of one variable time t2. The Fourier transforms, however, are NMR spectra with specific information, depending on the constant evolution period ti. One simple example is the generation of the quaternary carbon-13 subspectrum by means of a. /-modulated spin-echo experiment with an evolution time of tj2 = x = as in... 

It also should be noted that the sensitivities of excitable cells to electric fields decrease rapidly as the electric stimulus is applied for time periods decreasingly short in comparison to the refractory period of the order of 1 msec. Hence, quotation of reported low frequency membrane sensitivities as done by Frey (35) carries no implication with regard to sensitivities claimed at microwave frequencies correspond to time periods of the order of 1 nsec, which is a million times smaller than the refractory period. More recently, it has been postulated (36) that microwave fields may well be perceived, provided they are modulated with frequencies below 10 or 20 Hz. This would be possible in principle if induced in situ fields and currents could be rectified with some degree of efficiency so that microwave fields would generate detectable low frequency currents. No evidence for such a mechanism has been demonstrated so far at the membrane level. 

D. T. Schwartz, T. J. Rehg, P. Stroeve, and B. G. Higgins, "Fluctuating Flow with Mass-Transfer Induced by a Rotating-Disk Electrode with a Superimposed Time-Periodic Modulation," Physics of Fluids A-Fluid Dynamics, 2 (1990) 167-177. 

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