Electronic Model Studies of TC

Figure 8.2-4 illustrates the system configuration Wu (1970) used to compute the TC function of a known bandpass filter. A broadband Gaussian noise generator (Quan-Tech model 420) was used as the source. Its output was conditioned by a bandpass filter with adjustable -3 dB frequency and a -36 dB/octave rolloff (SKL model 302). The filter output was amplified by a broadband amplifier whose output was x(t). x(t) was the input to a linear bandpass filter set up to emulate H(s) in the TC model. The BPF output voltage was y(t). y(t) was the input to a comparator-pulse generator. When y(t) exceeded the comparator threshold voltage, b, the comparator output went high. This event was sensed by a one-shot multivibrator (74123) that produced a 50-|im TTL output pulse only for positive-going crossings of b. Both x(t) and the output pulses were digitized and recorded on magnetic tape using a PI model 6200 analog instrumentation tape recorder. This recorder is unique in that it can play back in reverse at the same speed that the data were recorded in the forward direction. To compute individual x+(x), whenever a recorded pulse occurred, the reverse x(t) was averaged by a signal averager. This process is given physical significance in Figure 8.2-5 where the kth pulse has just occurred. By reverse playback, one can obtain x(tk - t) for 0 3 t 3 TA . x(tk - t) is then plotted in positive time as x+k(x). TA is the maximum analysis time over which each x+k(T) is averaged. The average of

Broadband, Gaussian Lowpass filter

Broadband, Gaussian Lowpass filter

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Pulse

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