Pulser

In a few seconds, the capacitor fills up, and the light suddenly goes out again, similar to the sudden turn-offs in the astable circuit. Again, an optional extension of this experiment would be to attach a voltmeter. 

This circuit is referred to as being monostable, because it is only stable in the off condition for long times, but not in the on condition. With a small capacitor, it is used to make short pulses of electricity, and this variation is called a pulse generator. It is a useful item to have in one s intellectual toolbox, whenever a short burst of voltage or current is needed. 

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In order to obtain a high signal-to-noise ratio sufficient acoustical power is necessary. For special applications a programmable pulser (transmitter) is required in order to optimize the frequency spectrum. 

The VMOS-pulser with a rise time lower than 6 ns provides high axial resolution and high-frequency inspections above 10 MHz with an excellent signal-to-noise ratio. The output voltage amounts to about 228 V without load, and 194 V with a load of 75 H, A damping control from 75 Q to 360 Q matches the impedance to the transducer. 

The pulser/receiver is completely shielded and therefore receives no noise coming from the PC components. The input connector is a Lemo 00 coupler. The extremely low noise amplifier with a maximum amplification of about 90 dB accommodates very small signals. A precise gain setting is possible up to 106 dB. 

The external trigger input allows automatic inspections and ultrasonic imaging. The clock signal of an encoder or of stepper motors can be used as a trigger for the pulser. An internal software controlled divider allows different scan resolutions. 

In order to test a wide range of materials a series of ultrasonic pulser/receiver boards has been developed. In combination with an ADC-board (analogue-to-digital converter) a PC can be converted to a high performance ultrasonic system not only for manual testing but also for automatic testing and imaging systems. 

The internal pulser generates an output voltage of 228 V with a rise time of lower than 6 ns, which provides high resolution for frequencies above 10 MHz. The external trigger input enables synehronization with manipulators. 

The pulser/receiver HILL-SCAN 30XX boards satisfy DIN 25450. Typical applications are ultrasonie imaging systems for nuclear power stations and for aircraft, material characterization, transducer qualification, replacement of portable flaw detectors (inspections of welded joints), inspection of new materials, measurement systems with air eoupling. ... 

Fig. 4. A schematic of the plasma source ion implantation system, a plasma source chamber linked to a high voltage pulser. The plasma can be created from...

FIG. IS-S Pulsed columns a) Perforated-plate column with pump pulse generator, (h) Packed column with air pulser. 

The following terms are generally used to describe the pulse action Frequency is the rate of application of the pulse action, cycles/time. Amplitude is the linear distance between extreme positions of the liquid in the column (not of the pulser) produced by pulsing. Pulsed volume = amplitude X frequency X column crosssectional area = volumetric rate of movement of hquid, expressed as volume/time or vol-ume/(time-area). 

Equation 4-180 can be used to calculate the pressure change inside the drill collars by changing the cross-sectional area from bit nozzles only to bit nozzles plus the pulser nozzle. 

Fluidic Pulser System. A new type of pulser is being developed at Louisiana State University. It is based on a patent by A. B. Holmes [107]. The throttling of the mud is obtained by creating a turbulent flow in a chamber as shown in Figure 4-249. 

Figure 4-249. Fluidic mud pulser principlet. (Courtesy Louisiana State University [107]. ...

The mud flow rate is 500 gal/min and the fluidic pulser has 4 x 24/32 in. diameter nozzles in parallel. Compute the pressure loss in the fluidic pulser in the minimum loss mode (C = 1). 

Holmes, A. B., Fluidic mud pulser concepts in MWD, Proceedings MWD Symposium, Louisiana State University, Baton Rouge, February 26-27, 1990. 

Timing devices, including a pulser/sequencer unit... 

One thus arrives at the concept of pulser sequences (rather than RF pulse sequence) in which the programmable pulser generates a sequence of synchronized events and controls a number of distinct devices. Analogously, the term RF phase-cycling is no longer appropriate (in Stelar terminology, it is replaced by X-device cycling). 

In FFC relaxometry, the most conspicuous pulser-controlled device (apart from the RF excitation channel) is the magnet system. In other words, we generate Bq field pulses of considerable amplitude, often switching the magnet field between zero and a maximum value of over 1 T, and we rigorously synchronize such Bq pulses with the RF signal-excitation and/or preparation pulses. This, moreover, does not exclude the possibility to control other devices as well. 

Fig. 20. Schematic illustration of a main-field switching interval. The single pulser interval Swt (switching time), nested between a previous interval Pand a next interval N, is actually composed of three distinct phases (1,2,3). During phase (1) the magnetic field B is actively driven along a linear ramp from its previous value Bp to a close vicinity of its next value During the subsequent phase (2) it settles to its final...

Suppression of instrumental imperfections and/or selection of particular signal components are both based on the technique of phase cycling which exploits the dependence of NMR signals on the variations of the RF phases of the transmitter pulse(s) S (since phase-cycling is used in every branch of NMR, we assume that the reader is acquainted with the technique) (we will provide more information later, while discussing signal detection methods). At this point we just wish to point out that phase-cycling is extensively used in FFC and has to be supported by the console hardware - a requirement which implies pulser control of RF phases. 

The relative magnetization-variation factor v = (M ax - min)/A/inax is related to the fact that if the magnetization did not vary with respect to x, we could never determine R, no matter how large p and p might be. The reason why we consider this factor independently of p is that, theoretically, it depends only on the pulser sequence type and, in pre-polarized sequences, on the ratio % = M IMp = B jBp. For example, one can easily verify that for the basic non-polarized and pre-polarized sequences v p = 1 and... 

Figure 20.6 Centerline temperature vs. axial distance high forcing - pulser off for configuration 1 (Tw represents the corresponding centerline temperature for the unforced case). 1 — No swirl 2 — low swirl and 3 — high swirl...

Instrumentation. The steady-state fluorescence spectra were measured with Perkin-Elmer MPF-44B fluorescence spectrophotometer. The single-photon counting instrument for fluorescence lifetime measurements was assembled in-house from components obtained from EG G ORTEC. A PRA-510B light pulser filled with gas was used as the excitation source. Instrument response function was obtained with DuPont Ludox scatter solution at the excitation wavelength. 

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