Pulse frequency 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. 

The muscle model identification problem can be categorized by the following factors (1) time domain continuous-time or discrete-time models (2) input types stimulus period (SP), that is, pulse frequency modulation, pulse width (PW) modulation, or a combination of the two (3) model outputs for example, muscle torque or force and muscle length or position (4) loading conditions isometric or nonisometric loads and load transitions and (5) model type linear models, nonlinear Hill-type models, and other nonlinear models. 

The most popiilar form of motor speed control for adjustable-speed pumping is the voltage-controlled pulse-width-modulated (PWM) frequency synthesizer and AC squirrel-cage induction motor combination. The flexibility of apphcation of the PWM motor drive and its 90 percent- - electric efficiency along with the proven ruggedness of the traditional AC induction motor makes this combination popular. 

This is Ihe most commonly used inverter for Ihe control of a.c. motors and is shown in Figure 6.28(a). The fixed d.c. voltage from the uncontrolled rectifier converter acts as a voltage source to the inverter. The voltage in Ihe inverter unit is varied to Ihe required level by using a pulse width modulation, as noted earlier. Through Ihe switching circuit of Ihe inverter Ihe frequency of the... 

Selective experiments can also be performed by the tailored excitation method of Tomlinson and Hill. The selective pulse is frequency-modulated with a function designed to yield zero effective field at the resonance offset of the neighboring nuclei. Although this technique is especially promising for studies of more-complex spin systems, its use is as yet very limited, in part because the instrumentation needed is not yet commercially available. 

Since TIRF produces an evanescent wave of typically 80 nm depth and several tens of microns width, detection of TIRF-induced fluorescence requires a camera-based (imaging) detector. Hence, implementing TIRF on scanning FLIM systems or multiphoton FLIM systems is generally not possible. To combine it with FLIM, a nanosecond-gated or high-frequency-modulated imaging detector is required in addition to a pulsed or modulated laser source. In this chapter, the implementation with of TIRF into a frequency-domain wide-field FLIM system is described. 

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])...

Having defined the utility of a waveform library we go on to investigate the utilities of a few libraries. Specifically, we consider libraries generated from a fixed waveform 4>o, usually an unmodulated pulse of some fixed duration, by symplectic transformations. Such transformations form a group of unitary transformations on L2(R) and include linear frequency modulation as well as the Fractional Fourier transform (FrFT) in a sense that we shall make clear. 

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... 

As a method to control wavepackets, alternative to the use of ultra-short pulses, I would like to propose use of frequency-modulated light. Since it is very difficult to obtain a well-controlled pulse shape without any chirp, it is even easier to control the frequency by the electro-optic effect and also by appropriate superposition of several continuous-wave tunable laser light beams. 

It would be impossible to handle ternary solvent systems such as acetonitrile/ water/methanol or mixtures which contain further additives such as triethyla-mine. Therefore most techniques nowadays use modulated shaped pulses for multiple presaturation or pulse frequency generation (PFG) techniques. With basic hardware, it is then possible to suppress an arbitrary number of solvents. 

Note that the stationarity assumption results in a spectrum consisting of identically shaped (window) pulses placed at the sine-wave frequencies. Most signals of interest however are generally nonstationary (e.g., the frequencies may change over the window extent due to amplitude and frequency modulation), and so the window transform may deviate from this fixed shape. Naylor and Porter [Naylor and Boll, 1986] have developed an extension of the approach of this section that accounts for the deviation from the ideal case. 

Key et al., 1959] Key, E., Fowle, E., and Haggarty, R. (1959). A method of pulse compression employing nonlinear frequency modulation, Technical report 207, Lincoln Laboratory, M.I.T. 

At first sight, it may seem difficult to get high-frequency modulated fundamental light, especially when still thinking about 10 Hz nanosecond lasers, as used in nanosecond HRS experiments. However, the repetitive pulse laser, as used in femtosecond HRS, naturally provides high frequencies The harmonic content in the frequency domain of the very short pulse in the time domain extends to well in the GHz range. 

Microwave switches are beam-breaker-type point sensors with an accuracy of 13 mm (0.5 in.) and with pressure and temperature ratings up to 28 bar (400 psig) and 300°C (600°F). Pulse-type radar gauges have ranges up to 200 m (650 ft) and are accurate to 0.5% FS, whereas frequency-modulated carrier wave (FMCW) units have errors from 1 to 3 mm (0.04-0.125 in.). Their pressure and temperature ratings are up to 80 bar (1,200 psig) and up to 400°C (750°F). 

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