Pulsed variable frequency

Pulsed variable frequency with constant current mode is considered the superior method and is used by most manufacturers of ECDs. A preselected level of current is required. When an electron-capturing solute enters the detector, the standing current decreases the electronic circuit is adjusted to a frequency of pulsing to maintain a constant current. The detector response is in the frequency... 

Precisely controllable rf pulse generation is another essential component of the spectrometer. A short, high power radio frequency pulse, referred to as the B field, is used to simultaneously excite all nuclei at the T,arm or frequencies. The B field should ideally be uniform throughout the sample region and be on the order of 10 ]ls or less for the 90° pulse. The width, in Hertz, of the irradiated spectral window is equal to the reciprocal of the 360° pulse duration. This can be used to determine the limitations of the sweep width (SW) irradiated. For example, with a 90° hard pulse of 5 ]ls, one can observe a 50-kHz window a soft pulse of 50 ms irradiates a 5-Hz window. The primary requirements for rf transmitters are high power, fast switching, sharp pulses, variable power output, and accurate control of the phase. 

Pulse fluorometry uses a short exciting pulse of light and gives the d-pulse response of the sample, convoluted by the instrument response. Phase-modulation fluorometry uses modulated light at variable frequency and gives the harmonic response of the sample, which is the Fourier transform of the d-pulse response. The first technique works in the time domain, and the second in the frequency domain. Pulse fluorometry and phase-modulation fluorometry are theoretically equivalent, but the principles of the instruments are different. Each technique will now be presented and then compared. 

A pulsed NMR spectrometer, with a variable frequency and variable temperature facility is best suited for the study of these disordered systems.26 27 For dipolar glasses and relaxor systems, a spin echo Fourier transformation NMR spectra of the system have been measured in a wide bore superconducting magnet ( typically at 9.2 T). 

AC electric drives require more sophisticated converters when they are supplied with DC sources, because electric machines requires periodic voltage and current waves with a variable frequency depending on the load requirements. In Fig. 5.8, the scheme of an example of three-phase induction motor driven by a pulse-width-modulated inverter is reported. In this scheme a three-phase bridge connection with six power modules is shown to form the so-called inverter. Each power module can be composed by a number of power switches connected in parallel to carry higher currents. Across each power switch (IGBT) a parallel diode is connected to provide a return path for the phase current when the power module is switched off. 

The control signal calculated by the PID algorithm (see Note 3) is translated into a heating profile by way of pulse width modulation. The heater receives a pulse train (frequency of 0.5 Hz) with variable on time. During the on segment of each 0.5 s pulse, the heater is on at lull power. The integrated power over the lull pulse is controlled to achieve the desired temperature. The thermal capacitance of the capillaries and the heater itself is sufficient to damp out any temperature fluctuations such that the temperature measured inside the capillaries is steady (does not show any 0.5 Hz ripple). 

This chaotic mode is also a variable frequency mode of virtually unpredictable frequency spectrum and therefore unpredictable EMI and noise characteristics too. That is why fixed-frequency operation is usually preferred in commercial applications. And fixed frequency basically means no pulse-skipping ... 

Hydrochloride salts have been popular materials to study, particularly in recent years, as evidenced by the reports of Bryce et al., Chapman and Bryce, and Hamaed et al. (see Figure 11 for an example). Data are summarized in Table 4. To the best of our knowledge, the first chlorine SSNMR report for a powdered hydrochloride salt appears to be that of Pines and co-workers, who studied cocaine hydrochloride in 1995. The study utilized multiple techniques to study the hydrochloride salt, including N NQR. The chlorine-35 SSNMR experiment was carried out at 7.0 T using a Hahn-echo pulse sequence, and a chlorine-35 Cq of 5.027 MHz was reported. To avoid the intensity distortions that result from a finite pulse applied to a broad line shape, a variable frequency offset approach, in which the frequency was stepped in 2 or 4 kHz increments over the entire spectral width, was used to acquire the spectrum. 

The majority of detectors with radioisotope sources in current use are based on either the coaxial cylinder or asymmetric configurations, as shown in Figure 3. The low specific activity of the Ni source requires a relatively larger source area to provide a suitable background current that is easier to accommodate in these designs. Virtually all contemporary detectors employ pulse-sampling techniques to collect the thermal electrons based on the variable frequency constant... 

Power control in a full bridge can be obtained by either variable-frequency control or fixed-frequency pulse width modulated (PWM) control. The variable frequency control method used in resonant inverters is same as that explained in Sec. 10.5.2. The fixed-frequency control method is explained in Sec. 10.5.4. 

To overcome some of the problems associated with the variable frequency control of resonant converters, they are operated at a fixed frequency (Kit Sum, 1988 Bhat, 1988). A number of configurations and control methods for fixed-frequency operation are available in the literature (Bhat (1988) gives a list of papers). One of the most popular methods of control is the phase-shift control (also called clamped mode or PWM operation) method. Figure 10.89(b) illustrates the clamped mode fixed-frequency operation of the modified SRC shown in Fig. 10.89(a). The load power control is achieved by changing the phase-shift angle

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One method of controlling the speed of ac motors is with a pulse-width-modulated (PWM) variable-frequency drive (VFD). In the simplest terms, the VFD converts ac line voltage into variable voltage and frequency, which is then used to control the speed of ac squirrel-cage motors. As a controller, start/stop, hand-off-auto, and various other control schemes may be applied. 

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