Pulse position modulation

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... 

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. 2.13 Pulse position modulation scheme for radiofrequency communication between pacemaker and programmer.

FIGURE 12.49 llustration of practical sampling of a signal and pulse modulation techniques (a) pulse sampling function, (b) signal to be sampled, (c) natural sampling, (d) flat top sampling or pulse-amplitude modulation, (e) pulse-width modulation, (f) pulse-position modulation. 

Pulse-position modulation (PPM) A recording technique in which a mark on the disk signifies a binary 1 and its absence signifies a binary 0. A 1001 bit sequence is mark-space-space-mark. 

The PGSE NMR method relies on the use of two sharp gradient pulses separated by a well-defined time interval and is therefore naturally suited to time-domain analysis of motion. However, it is important to realize that this particular form of two-pulse gradient modulation is not unique. In particular, a number of other time-modulation schemes are possible in which the molecular motion is detected in a different manner. However, as we shall see, whenever modulated gradients are used to encode the spin magnetization for motion rather than position, it is appropriate to refocus any phase shift due to absolute spin position by means of a spin echo. Consequently, we refer to this more general type of experiment as modu-... 

In implantable stimulators and electrodes, the stimulation parameters greatly depend on the implantation site. When the electrodes are positioned on or around the target nerve, the stimulation amplitudes are on the order of a few milliamperes or less. Electrodes positioned on the muscle surface (epimysial electrodes) or in the muscle itself (intramuscular electrodes), employ up to ten times higher amplitudes. For muscle force control, implantable stimulators rely either on pulse-width modulation or amplitude modulation. For example, in upper extremity applications, the current amplitude is usually a fixed parameter set to 16 or 20 mA, while the muscle force is modulated with pulse widths within 0-200 p,s. 

The principle of pulse-width modulation is shown in Figure 10.11. The same circuit as shown in Figure 10.9 is used. In the positive cycle, only switch D is on all the time, and switch A is on intermittently. When A is on, current builds up in the load. When A is off, the current continues to flow, because of the load inductance, through switch D and the free-wheeling diode in parallel with switch C, around the bottom right loop of the circuit. 

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. 

Figure 5.7 (A) Pulse sequence for gated decoupled /-resolved spectroscopy. It involves decoupling only during the first half of the evolution period

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