在回波系统中,返回信号幅度是随 目标的距离而变化的,在数学上 可作为时间函数来表示。采用增益固定的前置放大器的回波系统呈现很差的动态范围,因为短距离目标(短返回时间)具有很大的信号幅度,而长距离目标(长返回时间)产生低得多的幅度。在增益固定的系统中,最大的信号确立前置放大器增益上限。但是,这一增益可能还没有高到足以恰如其分地处理低电平返回信号。 一种解决方案是采用一种具有与时间成正比的增益的前置放大器,这样,对短距离目标来说,增益将很小,而对长距离目标来说,增益将很大。前置放大器仍然要遵循所有其他常规的前置放大器准则,如带宽和噪声性能。而且,与时间有关的附加增益函数一定不 In an echo system, the amplitude of the return signal varies with the distance of the target and is mathematically expressed as a function of time. Echo systems with fixed-gain preamplifiers exhibit poor dynamic range because of the large signal amplitude of short-range targets (short return times) and much lower amplitudes of long-range targets (long return times). In systems with fixed gain, the maximum signal establishes the upper limit of the preamplifier gain. However, this gain may not be high enough to properly handle low-level return signals. One solution is to use a preamplifier with a gain proportional to the time, so that for short-range targets the gain will be small and for long-distance targets the gain will be large. The preamplifier still has to follow all other conventional preamplifier criteria such as bandwidth and noise performance. Moreover, the time-dependent additional gain function must not