Real-Time & Sampling Oscilloscopes – Part 1

If you are wondering what a real-time oscilloscope is, what “real-time” means in this context, and how a real-time oscilloscope differs from an equivalent-time or sampling oscilloscope, you are not alone.

Keep in mind that pretty much every oscilloscope available today is a digital storage oscilloscope. A real-time digital oscilloscope replaces an analog oscilloscope’s sweep across the display (a trigger event starts the sweep) with a sequence of samples from a high-speed digitizer; these samples go into the instrument’s memory and, when the memory is full, the oscilloscope’s processor reads the memory and renders a trace on the display. The sampling is consecutive, which is to say that if you’re running at 1 GS/s, you will get a new sample point every 1 ns. A 1000 point memory would then translate into 1us of continuous time on the oscilloscope’s display. If you need to store more time, you could either opt for an oscilloscope with more memory or drop the sample rate; for example, at one thousand points, you can store 2 us at 500 MS/s — if you want to stay at 1 GS/s, you would need an oscilloscope with two thousand points of memory.

The main frustration with digital storage oscilloscope architecture is the significant amount of time it takes to move data from memory to the processing system in order to create a trace raster on the display, meaning that the trace update on the oscilloscope’s display is much slower than an analog display, which is why digital oscilloscopes were initially unpopular. This issue was eventually addressed with a dedicated application-specific integrated circuit (ASIC) for rendering traces and displaying them on-screen, resulting in trace updates comparable to analog oscilloscopes (these instruments are sometimes called digital phosphor oscilloscopes, or DPO).

A real-time oscilloscope, however, does not need to trigger: it can capture the next asynchronous sample of data and display it immediately — this is known as an auto trigger and it functions much like an analog oscilloscope. Real-time oscilloscopes can trigger on data as well. On an analog oscilloscope, the trigger is the start; on a digital oscilloscope, the trigger is the stop.

Real-time oscilloscopes are referred to as “real time” because they have real-time triggers which continually monitor data and digitize a signal with sequential real-time samples.



George Leger has a Masters in Electrical Engineering from Stanford University, worked in private industry pioneering surface-mount technology and in government research labs for twenty years, published several papers on surface-mount technology, co-authored papers published in national symposiums on accelerator technology, was past president of SMTA and an adjunct professor at the community college level, holds a patent, and is a certified microchip design partner, serving as a consultant to many companies developing electronic circuits.

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