Digital Electronics: Principles, Devices and Applications

Troubleshooting Digital Circuits and Test Equipment 6711 1f s f s1TriggersMf s11Mf s1f s f sFigure 16.13Sequential sweep equivalent-time sampling.trigger points, which gives this technique a ‘pretrigger view’ capability not available in the first twoequivalent-time sampling techniques, as both methods gather signals only following the receipt of atrigger.If we wanted to view a 1 GHz signal, the sweep speed requirement would be enormous. Even ifwe were successful in achieving this high speed, the beam would be almost invisible. We have oftennoticed that, as the time-base setting is made faster, we are forced to adjust the intensity controlto maintain an acceptable intensity level setting. Another major problem in designing a real-timeoscilloscope for viewing very high-frequency signals (in the GHz range) is the difficulty in buildingsuch a high bandwidth in the vertical amplifier. A sampling oscilloscope using any of the equivalenttimesampling techniques outlined above is an answer to all these problems. In such scopes it is notimperative to take a sample or a group of samples from each cycle of the signal to be viewed. Thenext adjacent sample or group of samples may be 10 000 cycles away. As a result, the bandwidth ofthe vertical amplifier can afford to be much lower than the frequency of the signal.Another type of sampling oscilloscope, although not very common in use, is the analogue samplingoscilloscope, where a conventional sample/hold circuit consisting of an electronic switch and a capacitoris used for signal acquisition (Fig. 16.14). It can be used to view high-frequency repetitive signals innonstorage mode, unlike the digital sampling scopes where the signal is sampled digitally and thenstored in semiconductor memory for subsequent retrieval. It can also be used for viewing high-frequencyrepetitive signals in storage mode, although not in real time (Fig. 16.15).Digital storage oscilloscopes are also available in a large variety of sizes, shapes, performancefeatures and specifications. Battery-operated, handheld digital storage oscilloscopes with a bandwidthas high as 200 MHz are common (Fig. 16.16). The digital phosphor oscilloscope (DPO) is a big stepforward in DSO technology. It captures, stores, displays and analyses, in real time, three dimensions ofsignal information, i.e. amplitude, time and distribution of amplitude over time. This third dimensionSignalSHTriggerGeneratorBufferVert I/PofOscilloscopeFigure 16.14Corporation.Analogue sampling oscilloscope (nonstorage mode). Reproduced with permission of Fluke