Rise time

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

A Fixed-Angle Heat Spreading Model for Dynamic Thermal Characterization of Rear-Cooled Substrates

Abstract: During a period of almost 40 years already, various fixed-angle heat spreading models have been developed in the literature. These models can be used by thermal engineers as approximations for the thermal steady state resistance of a heat source on a rear-cooled substrate. In this paper an extension of these models to dynamic (time-dependent) phenomena is proposed. The heat dissipated by a square source (side a) is assumed to spread out into the substrate (thickness ts) under a fixed angle Phi. An analytical solution for the complex thermal impedance Zth(jomega) in phasor notation is derived. The obtained expression, in which Phi is used as a fitting parameter, is compared with exact results. A very good agreement is observed (average relative error less than 6%) for a wide range of the normalized thickness lambda = ts/a. A compact expression for the optimal heat spreading angle as a function of lambda is given. Finally the temperature response to a heat power step is investigated. A simple formula for the thermal rise time is provided, allowing a thermal designer to make quick yet accurate estimations about the dynamic behavior of the device.

Picosecond response of gallium-nitride metal-semiconductor-metal photodetectors

Abstract: Metal–semiconductor–metal ultraviolet photodiodes fabricated on GaN were tested in the picosecond regime with an electro-optic sampling system. A device with a feature size of 1 μm showed a response with 1.4 ps rise time and 3.5 ps full width at half maximum. The derived electron velocity, 1.43×107 cm/s, is in good agreement with independent photoexcitation measurements. A slower impulse response was observed in a device with smaller feature size of 0.5 μm.

A compact, high repetition-rate, nanosecond pulse generator based on magnetic pulse compression system

Abstract: Magnetic pulse compression (MPC) system has been widely-used over a few decades as a technique for producing short duration, high peak power pulses reliably. A compact pulse generator with a 3-stage MPC system (assembled with amorphous cores and ferrite cores) is constructed to generate repetitive pulses of maximum 7.5 MW peak power with pulse width (FWHM) of 70 ns and rise time of 30 ns at the maximum pulse repetition rate of 2 kHz into a 307 resistive load. In this paper, the detailed system design and operational characteristics of the MPC modulator are presented. The MPC has 3-stage pulse compression circuit using two saturable pulse transformers and a magnetic switch. The saturable pulse transformer has two functions, a step-up transformer and a magnetic switch. The generator consists of 2 units with distinct functions. The resonant charge unit is a diode based auto L-C resonator recharger. The 3-stage MPC unit comprises a saturable pulse transformer which compress the pulse width to 1.5 μs and step up the amplitude to 7 kV, and a 2-stage magnetic pulse compressor using another saturable pulse transformer and a magnetic switch, which compress the pulse rise time to 30 ns and step up the amplitude to 62 kV at most (1 kΩ resistive load).

Increasing the speed of microstrip-line-type polymer-dispersed liquid-crystal loaded variable phase shifter

Abstract: We discuss a method for the production of microwave polymer-dispersed liquid-crystal devices. We also show how to obtain a fine grained uniform fibrous polymer network. We then measure how the microwave-band dielectric properties and response-time characteristics of microstrip-line-type polymer-dispersed liquid-crystal devices vary with polymer concentration, and we measure how their response-time characteristics vary with the thickness of the polymer-dispersed liquid-crystal layer. With a layer thickness of 50 /spl mu/m, an applied voltage of 100 V and a polymer concentration of 7 wt%, we show that the decay time can be reduced to approximately 1/30th that of a plain liquid crystal at the expense of an increase in rise time of approximately 2.5 times and a deterioration of dielectric birefringence of approximately 50%. Finally, we discuss the results obtained with a prototype polymer-dispersed liquid-crystal loaded variable phase shifter in the 15-GHz band. By comparing the variable phase and response time of this phase shifter with basic experimental data obtained by the microwave resonance method, we verified the validity of these values.

Time domain reflectometer

Abstract: A typical sampling time domain reflectometer (T.D.R.) apparatus is modified by the addition of an integrator which can be selectively switched into and out of the vertical signal path. The integrator is utilized to integrate signals reflected from any reactive discontinuity in a transmission line under test. The amplitude of the integrated signal is utilized to enable the accurate determination of the reflection coefficient. The derived results are substantially unaffected by variations in the rise time of the step function signal. In addition, the apparatus'' independence from the step function signal rise time enables the use of a longer rise time and thus the system''s bandwidth requirement is lessened to the point where a real time T.D.R. apparatus may be utilized.