Rise time

About: Rise time is a research topic. Over the lifetime, 4748 publications have been published within this topic receiving 47512 citations.

Multi-mode resonant gradient coil circuit for ultra high speed NMR imaging

Abstract: A multi-mode resonant gradient circuit is described for use in ultra high speed imaging techniques like echo-planar and echo-volumar imaging. The circuit behaves overall in a series resonant manner, but at a fixed number of discrete frequencies. By choosing the number of resonant modes, the circuit can be used to generate approximations to a square wave or a trapezoidal waveform. It it shown that trapezoidal waveforms of a given rise time may be described by a more convergent series expansion involving fewer harmonic components. Because the circuit exploits energy conserving principles, much faster current rise times can be achieved with a given driver amplifier and gradient coil than can be achieved by direct coil drive. Expressions are given for the choice of passive circuit components required to accommodate all odd harmonics up to seventh order.

Telephone line protection device for steep electromagnetic pulses - uses low pass LC filter with voltage dependent shunt impedances connected symmetrically in series and parallel with grounded chassis

Abstract: Two symmetrical conductors (8,9) link the input terminals (1,2) to respective inductors (14,15) and are connected by spark gaps (10,11) to the earthed screen between the two input stages (3,4). The inductors (14,15) are connected via feed-through insulators (18,19) into an output stage (6) contg. a low-pass filter (24) with 50 Hz cutoff frequency (for power line protection). The normal operating voltage (e.g. 220V) and current (100A) are passed without distortion, but perturbing pulses of higher frequency are greatly attenuated by Zener diode or voltage-dependent resistive shunts (20,21) at the filter input connections. A 10kV peak pulse with 10ns rise time is reduced to less than 100V at 1kV/microsecond at output terminals (33,34). Alternatively the two inductors are replaced by a screened 1:1 isolating transformer.

Experimental Study of Perpendicular Write Head Field Rise Time

Abstract: In this paper, a simple technique that can determine the write head field rise time based on electrical measurements is presented. A model that relates the head electrical properties to the frequency dependent head efficiency epsiv(omega) and head field rise time tau is described in detail. This method only requires knowledge of the complex impedances of a magnetically saturated head and an unsaturated head. The results obtained are verified with recording measurements of the rise time for the same heads. Very good agreement was found for all heads. It was also found that the presence of a soft underlayer does not seem to significantly affect the measured head field rise time. Therefore, it is concluded that the head field rise time is determined by the yoke structure and not by the pole tip area

Circuit for controlling the conduction of a switching device

Abstract: A circuit for controlling the conduction of a switching device, such as a silicon controlled rectifier, Thyratron tube, mercuryarc tube and the like, which turns on and off the switching device so as to produce a load voltage with a fast rise time, a fast fall time and a wide range of pulse widths at a high repetition rate. To obtain a fast rise time of the load voltage, the switching device is turned on in series with a direct current source with all elements interconnecting therebetween electrically decoupled. To obtain a fast fall time for the load voltage, a direct current voltage is applied in series with the switching device which is of a polarity opposite from the polarity of the direct current source used to furnish power to the load. This reverse voltage is of an appropriate magnitude and time duration for turning off the switching device, and in addition thereto, electrically decouples the load from any direct current source at the same time as the switching device is reversed biased. Toward this end, a reactive element is disposed in series with the direct current source and the switching device. It has been found that energy is stored in the reactive element during the turning off or the commutation period of the switching device. Such stored or trapped energy in the reactive element inhibits the succeeding application of the appropriate magnitude and time duration of reverse voltage to the switching device for turning off the switching device. Consequently, such stored trapped energy is a detriment to commutating the switching device in succeeding commutation intervals. Thus, one embodiment of the present invention is to dissipate the trapped energy stored in the reactive element before the next commutation period. Another embodiment of the present invention is to inhibit the accumulation of trapped energy in the reactive element. Both embodiments have a small commutation interval, and are therefore capable of high switching rates.

Note: A high-frequency signal generator based on direct digital synthesizer and field-programmable gate array

Abstract: A high-frequency signal generator based on direct digital synthesizer (DDS) and field-programmable gate array (FPGA) is presented. The FPGA provides the controlling time sequence for the DDS, which has a highest output frequency of 1.4 GHz and a frequency resolution of 190 pHz. At an output frequency of 1.2 GHz, the measured phase noise, including the contribution of the reference clock, is -65 dBc/Hz@1 Hz, while the intrinsic phase noise is -82 dBc/Hz@1 Hz. Time delay of the DDS is measured to be less than 150 ns. The signal generator is used to drive an acousto-optic modulator, and the rise time due to the whole link is 24 ns. The developed signal generator can be used in many precision measurement experiments in the fields of atomic, molecular, and optical physics.