Pulse period modulation

A variation on PWM is pulse position modulation (PPM), also known as pulse period modulation or pulse frequency modulation (PFM). In this case, the duty-cycle pulse remains on for a fixed time while the base period is varied. The frequency of the pulses (how close together the pulses are) determines the voltage level. The neuromuscular system is an example of a pulse position modulation system. A muscle is made up of many discrete motor units. A motor unit has an all or nothing response to a nerve impulse in much the same way that a nerve impulse is a nonlinear (thresholded) all-or-nothing event. The level of sustained force output of a motor unit is dictated by the frequency of incidence of the nerve impulses, with the motor units dynamics [mechanical properties—inertial and damping properties (acts as a mechanical filter)] holding the force output smooth between incoming impulses. The motor unit is pulse frequency modulated by the nervous system. 

Fig. 5 Radio frequency pulse sequences for measurements of Sj and Si in DSQ-REDOR experiments. The MAS period rR is 100 ps. XY represents a train of 15N n pulses with XY-16 phase patterns [98]. TPPM represents two-pulse phase modulation [99]. In these experiments, M = Nt 4, N2+ N3 = 48, and N2 is incremented from 0 to 48 to produce effective dephasing times from 0 to 9.6 ms. Signals arising from intraresidue 15N-13C DSQ coherence (Si) are selected by standard phase cycling. Signal decay due to the pulse imperfection of 15N pulses is estimated by S2. Decay due to the intermolecular 15N-I3C dipole-dipole couplings is calculated as Si(N2)/S2(N2). The phase cycling scheme can be found in the original figure and caption. (Figure and caption adapted from [45])...

Summary. Coherent optical phonons are the lattice atoms vibrating in phase with each other over a macroscopic spatial region. With sub-10 fs laser pulses, one can impulsively excite the coherent phonons of a frequency up to 50THz, and detect them optically as a periodic modulation of electric susceptibility. The generation and relaxation processes depend critically on the coupling of the phonon mode to photoexcited electrons. Real-time observation of coherent phonons can thus offer crucial insight into the dynamic nature of the coupling, especially in extremely nonequilibrium conditions under intense photoexcitation. 

With development of ultrashort pulsed lasers, coherently generated lattice dynamics was found, first as the periodic modulation in the transient grating signal from perylene in 1985 by De Silvestri and coworkers [1], Shortly later, similar modulation was observed in the reflectivity of Bi and Sb [2] and of GaAs [3], as well as in the transmissivity of YBCO [4] by different groups. Since then, the coherent optical phonon spectroscopy has been a simple and powerful tool to probe femtosecond lattice dynamics in a wide range of solid... 

Proton-proton homonuclear decoupling has been performed by the ESLG decoupling sequence [46]. Quadrature detection in coj was achieved by using the time proportional phase increment method (TPPI) [47]. During the acquisition period, two pulse phase modulation (TPPM) heteronuclear decouphng ]48] was applied (Figure 7.6). 

Fig. 2.51.. /-Modulated spin-echo sequence with gated proton decoupling for acquisition of -/-resolved two-dimensional 13C NMR spectra, and the CH magnetization vectors in the x y plane controlled by pulses and. /-modulation. During the preparation period between successive experiments, nuclear Overhauser enhancement of 13C magnetization is retained by minimum proton decoupling.

Sensor measurements are recorded in situ by a battery-powered, eight-channel, miniature digital tape recorder (MDTR) (Oxford Medical Systems Ltd.) with range, 0-1.023 V resolution, 0.1% scan period, 15 s (alternatively 5 s) duration (C120 cassette), 25 h (alternatively 8 h) and data capacity, 6000 scans. Direct (bandwidth 100 Hz) or pulse-width modulated analog signals (range, 120 mV bandwidth, 0-8 Hz and resolution, 1-2 %) can be recorded on the additional three tracks of the same cassette. 

The results of our numerical calculations based on Eqs. (7a) and (7b), which use the exact expressions for e(q), uq and vq instead of the HF approximation, clearly reveal a deviation from exponential decay for small times. Under such conditions, frequent measurements would accelerate the decay, causing the anti-Zeno effect (AZE). Alternatively, one may accelerate the decay by periodically modulating the coupling of the initial state to the continuum, [Kofman 2000 Kofman 2001 (a)], instead of repeated projective measurements. This can be done by changing the impurity velocity using a sequence of Bragg or Raman laser pulses [Stenger 1999 Steinhauer 2002],... 

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. 

Several deteetors are eonneeted via a router to the same TCSPC module. The photon pulses from the seeond deteetor are delayed by more than the dead time of the TCSPC module. More than two detectors ean be used if their delay lines are different by more than the module dead time. The stop pulses for the TCSPC module come from the pulsed laser, or, if a CW laser is used, from an external eloek generator. Due to the different delay of the deteetor signals, photons deteeted simultaneously do not arrive simultaneously at the router inputs. Therefore, photons detected in the same laser pulse period are reeorded at different times and stored in the FIFO data file with a macro time offset. The differenees in the maero times caused by the delay lines in front of the router are known and ean easily be corrected when the photons are correlated. 

Fig. 7.88 Calibration of a TCSPC module by a pulse sequence of known pulse period. Detector H5783P, time scale 4 ns/div. Left Pulses from a blue and a yellow LED driven from a HPl 1 lOA pulse generator. Period 10 ns, electrical pulse width 3.6 ns. Right 5 mW, 650 nm laser diode driven from a HP8131A pulse generator. Period 10 ns, electrical pulse...

The electrodes are placed in solutions containing the electroactive probe 2,5-dihydroxyphenylacetic acid (10 pM) and the analyte 2,4-D in 20 mM phosphate bufier, pH 7,10% methanol, for 1 h. Following incubation, the electrodes are rinsed briefly with ultrapure water to avoid carryover of the probe. The electrodes are then transferred into a solution ofO.l M KCl in 20 mM phosphate buffer, pH 7,10% methanol, and the amount of bound probe is quantified by differential-pulse voltammetry. The measurement parameters are as follows potential window 100-550 mV, scan rate 30 mV/s, sample width 17 ms, pulse amplitude 50 mV, pulse width (modulation time) 50 ms, pulse period (interval) 200 ms, quiet time 2 s, and sensitivity 10-5 A/V. Peak currents are determined either after subtraction of a manually added baseline or as absolute peak heights above zero. 

Often an acousto-optic switch is used, for example, for argon lasers and cw dye lasers [648]. Its basic principle is explained in Fig. 6.6. A short ultrasonic pulse with acoustic frequency / and pulse duration T 1 //s is sent nit = to through a fused quartz plate inside the laser resonator. The acoustic wave produces a time-dependent spatially periodic modulation of the refractive index n(t,z), which acts as a Bragg grating with the grating constant A = Cs//, equal to the acoustic wavelength A where Cg is the sound velocity. When an optical wave Eocos((ot — k r) with the... 

The final orthogonal format which we consider is binary pulse-position modulation (PPM/IM). In this scheme, each bit period is divided into two equal subintervals. If a 1(0) is transmitted, the pulse is caused to occur in the first (second) subinterval. A block diagram for one implementation of such a system... 

Pulse width modulation (PWM) Amodulation technique by which the width of arectangular waveshape is varied in accordance with a particular error signal. Typically, the overall period is held constant, and the duty cycle is therefore varied. 

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