Digital Measurements of Peak Signals

Uncertainties in Digital Measurements of Peak Signals

For most engineering parameters that require measurement, the test signal varies continuously over time. A plot of these signals, made on any analog data recorder as they vary from one instant of time to the next, produces a line consisting of an infinite number of data points. From a visual inspection of this line, we can immediately see the nature of the variable. Does it increase or decrease? Is it cyclical? What are the maximum and minimum values, and when did they occur? When measurements are made digitally, the time between each conversion of an analog measurement to a digital datum, and the finite data storage capacity of a computer, limits us to the measurement of only a few of the data points on the line of interest. The question then is how many digital data are needed to make a "good enough" reconstruction of the analog signal for us to obtain meaningful measurement results. That depends upon the nature of the analog signal, the digital sampling rate, when the samples are taken, and the accuracy required.

Suppose, as shown below, that a signal varies in a sinusoidal way with time. Further suppose that digital samples are taken at ten even intervals throughout each cycle, beginning at the point where the amplitude passes through zero. As we can further see when these measurement values are superimposed on the plot of the analog signal, it is possible to get a vague notion that the signal is sinusoidal. But, the largest values actually measured are only about 95% of the peak value.

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