Digital Electronics: Principles, Devices and Applications

670 Digital ElectronicsTriggerFigure 16.11Real-time sampling.single-shot signals. In digital storage oscilloscopes, the digitizer samples the entire input waveformwith a single trigger. Sampling oscilloscopes use equivalent-time sampling and are limited to capturingrepetitive signals. Some digital storage oscilloscopes also use equivalent-time sampling to extend theiruseful frequency range for capturing repetitive signals. The equivalent-time sampling technique is thusapplicable to only stable repetitive signals and can be implemented in at least three different ways,namely sequential single-sample, sequential sweep and random interleaved sampling (RIS).In the sequential single-sample technique (Fig. 16.12), the digitizer acquires a single sample withthe first trigger pulse. It then waits for the second trigger, and, on receipt of the second trigger, a timedelay equal to the reciprocal of the desired sampling rate is executed and then the second sample isacquired. The trigger-to-acquisition delay is incremented by the desired intersample period t for eachsubsequent acquisition. The resulting capture has thus an equivalent sample rate of 1/t. Clearly, thismethod is slow, as N trigger cycles would be needed to gather N samples, and the scopes using thistype of digitizing technique cannot provide real-time operation.In sequential sweep equivalent-time sampling (Fig. 16.13), a sweep of samples spanning the desireddisplay time range is acquired for each trigger. Here, N samples are acquired in M trigger cycles,where N = kM. On receipt of each trigger, k sequential samples are acquired at sample rate f s . Theseare stored in every Mth location of the acquisition memory allocated for N samples. k samples ofthe first sweep are acquired directly on receipt of the trigger. Subsequent sweeps have an increasingdelay between trigger receipt and sweep initiation, with the delay increment being equal to 1/Mf s withreference to trigger detection in order to give an apparent sample rate of Mf s .The random interleaved sampling (RIS) technique uses a memory distribution scheme that isphilosophically similar to that of sequential sweep equivalent-time sampling, with the difference thatthe samples are random with respect to the trigger. Sampling in this case occurs on both sides ofFirstSampleSecondSampleThirdSampleReal TimeBetween SamplesTriggersEquivalent TimeBetween SampleFigure 16.12Sequential single-sample technique.