Abstract: A digital bandpass oscilloscope permits an operator to observe both time and frequency domain plots of analog signal components of frequency within a passband of selected center frequency during a time window of selected duration. The input signal is digitized, and the resulting digital data sequence is bandpass filtered and decimated by an adjustable decimation ratio to produce stored waveform data indicating the time dependent behavior of the passband during the time window. The oscilloscope, which utilizes the waveform data to produce both time and frequency domain displays of the passband, adjusts the decimation ratio to ensure that the number of data elements in the waveform data sequence is constant regardless of the selected duration of the time window and adjusts the bandwidth of the passband to minimize aliasing in the time domain display.

Abstract: A sequence of digital data values is processed by deriving the average W.sub.n of two of the data values X.sub.n-1 and X.sub.n+1. If W.sub.n lies within the quantization step q centered on the data value X.sub.n, W.sub.n is adopted as a modified value of X.sub.n, but if W.sub.n lies outside the quantization step the modified value of X.sub.n is taken to be X.sub.n +q/2 or X.sub.n -q/2, depending on whether W.sub.n is greater or less than X.sub.n.