外文翻译--底土的土壤结构和饱和导水率(编辑修改稿)

外文翻译--底土的土壤结构和饱和导水率(编辑修改稿)内容摘要：

ltaneous pulses with different amplitudes 14AA have to be generated for each simulated event. These amplitudes are sent by the digital module to the analog module, together with a start pulse (see Fig 2). The munication is performed through a coupler circuit for isolation purposes. The start signal is sent to a reference pulse generator, which generates a pulse of constant amplitude, rise and fall times. One of the inputs of each multiplier is this reference pulse, and the other is one of the DAC amplitude signals. Thus, every multiplier acts as a modulator: when the reference pulse arrives, the multiplier generates a similar pulse whose amplitude is the respective voltage given by the digital module. The reference pulse generator is the most critical element in the system, because any noise in the reference pulse will be present (and not independently) in each of the output signals. The core of this element is the circuit shown in Fig 3. Before a start pulse arrives to the reference pulse generator, the capacitor 1C is charged at voltage 1 5refCV A V , ultraprecision, guaranteed longterm drift voltage reference has been used for this purpose (MAX677BCPP). Once the capacitor 1C present stable voltage and a start pulse is generated, this capacitor is connected to 1R switching the relay 1K . In order to avoid the characteristic glitches of the mechanic relays, a mercury relay has been used. Mikhailov[2]has pointed out recently the limited pulse rate (~100 pps) achievable with mercury relays, but we focuses on modulating the pulse amplitude rather than reaching a high pulse rate. When 1C is connected to 1R , the equations describing the evolution of the circuit of Fig3 are the following:  1 1 2 1 2 111 0R i R i i i d tC     2 1 2 221 i i i d tC   (2) The solution of this linear system with the conditions  1 0 refvV and  2 00v  gives an output pulse 2vt with the functional form (1) and amplitude refV . Though the rise and fall times depend on resistor and capacitor values through a plicated algebraic expression, for 12CC (condition fulfilled here) the following approximate expressions hold: 12 212RRR CRR    1 2 1F R R C  (3) The values and characteristics of capacitors, resistors and reference voltage are given in Table 1。 for these values 5R  ns and 10F  s . The shape, rise and fall time of the reference pulse are shown in Fig 4 . Fig 4. Oscilloscope images of the reference pulse rise (left) and fall (right) flanges. The quoted values of rise and fall time refer to 1090%of the amplitude. The values of R。