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FiGurE 1 The scope’s fast waveform update rate reveals an infrequent non-monotonic edge. FiGurE 2 InfiniiScan Zone trigger isolates the non-monotonic edge waveform. Get a quick quote today at www.agilent.com/find/quick-quote Capturing ghosts: the oscilloscope specifications that matter the most A flickering “waveform ghost” is a classic intermittent signal—sometimes it’s there; sometimes it isn’t. A ghost can be an infrequent narrow glitch, an infrequent shift in timing, an infrequent runt pulse, or any inconsistent and unexpected waveform. These anomalies are among the toughest troubleshooting challenges, so it’s vital to understand how scope performance affects your ability to capture, identify, and fix these difficult creatures. Bandwidth and sample rate The most important oscilloscope specifications to consider are bandwidth and sample rate. A scope’s real-time bandwidth and its associated sample rate determine the level of detail in which signals can be captured. If an infrequent glitch has a very fast transition time or is very narrow, a low bandwidth scope may filter out the glitch entirely, and you’ll never know it’s there. memory depth and display update rate Deep memory is a powerful tool for catching ghosts because it allows you to sample at higher rates over a longer period of time. However, even if an infrequent event is randomly captured in your scope’s deep acquisition memory buffer, will you know it’s there? And if you can’t readily see it on the scope’s display, how do you even know that you need to search for it, or what to search for? A fast display system greatly enhances a scope’s ability to make those occasional waveform ghosts more visible. The higher the waveform update rate, the more likely it will catch and display infrequent anomalies, even when you are not specifically looking for them. Figure 1 shows an example of a waveform ghost captured on Agilent’s new InfiniiVision 4000 X-Series oscilloscope while updating at 1,000,000 waveforms per second. Scopes with slower update rates may never reveal this ghost of a waveform (an infrequent non-monotonic edge). Assuming that your oscilloscope’s update rate is fast enough, the next step is typically to set up the scope to uniquely trigger on it in order to isolate it so that you can determine its root cause. If the infrequent anomaly is a narrow glitch, try using your scope’s pulse-width trigger mode. If the infrequent anomaly is a pulse with insufficient amplitude, try using your scope’s runt trigger mode. Or if the anomaly is an infrequent non-monotonic edge as in this particular example, try the rise-time trigger mode. Zone triggering Chances are you’ll encounter situations in which using your scope’s advanced parametric/violation triggering modes is easier said than done. A simpler alternative in many cases is zone triggering, which is available on Agilent’s InfiniiVision 4000 X-Series oscilloscopes. Simply draw a box (zone) around the area of the waveform ghost using the scope’s capacitive touch screen as shown in Figure 2, and the scope will display only the anomalous waveforms that intersect that zone. To learn more about capturing elusive signals with today’s advanced scopes, visit www. agilent.com/find/zonetrigger sPotliGHt


EETE APRIL 2013
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