Digital Electronics: Principles, Devices and Applications

676 Digital ElectronicsFigure 16.18Effective sampling rate – time-base setting graph.Figure 16.18 shows the changes in sample rate as a function of time-base setting for digitaloscilloscopes of different memory lengths. Given two oscilloscopes with identical sample rate andsingle-shot bandwidth specifications, the one with the longer acquisition memory has a decisive edge.Hence, it must be accorded due importance when choosing one to meet your requirements. For a giventime resolution, a longer memory enables events of longer duration to be recorded. For instance, aDSO with a 1K memory can record a1stransient with a time resolution of 1 ms, whereas a DSOwith a 10K memory can record a 10 s long event with the same time resolution. In other words, forthe same transient duration, longer memories give enhanced time resolution. Long memories also helpin acquiring hard-to-catch signals and also minimize signal reconstruction distortion.16.12.3.3 Vertical Accuracy and ResolutionThe accuracy specification tells us how closely the measurement matches the actual value. Theaccuracy of a DSO is affected by various sources of error, including gain and offset errors, differentialnonlinearity, quantization error and so on. The quantization error indirectly indicates vertical resolution,i.e. uncertainty associated with any reading or the ability of the oscilloscope to see small changes inamplitude measurements. Choosing a scope with fewer than eight bits of resolution is not recommended.Resolution specification must not be considered in isolation from accuracy specification. For instance,more than eight bits of resolution is meaningless when the overall accuracy itself is ±1%. An eight-bitresolution gives a ±04% uncertainty, which is fairly acceptable if the overall accuracy is ±1%, ascan be seen from Table 16.3. Also, digital oscilloscopes with more than seven bits of resolution canresolve signal details better than visual measurements made with analogue oscilloscopes.To sum up our discussion on the available oscilloscope types and the selection criteria for choosingthe right one, it can be said that both analogue and digital oscilloscopes have their advantagesand shortcomings. The suitability of a particular type must always be viewed in terms of intendedapplication. Although digital oscilloscopes can perform many functions that analogue versions cannot,analogue oscilloscope technology, too, has reached high performance standards. It is important to