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Engineers live in fear of the unknown. Any problem or trade-off, if adequately managed, can be mitigated by the engineer's skill. The problems that one doesn't see, such as intermittent signal anomalies (a glitch), become increasingly expensive as they propagate to quality testing, manufacturing or to customers. A fast update rate—a frequently misunderstood characteristic— is critical to an oscilloscope's ability to display these problems.
The case for update rate as a "banner specification"
The current pantheon of banner specifications (the ones that oscilloscope vendors typically use at the top of their advertising) includes bandwidth, sample rate, memory depth, and price. However, update rate is equally important because it characterizes the oscilloscope's ability to capture both intermittent and repetitive events. It doesn't matter how much data the scope acquires—or how fast—if it spends disproportionably more time displaying data than acquiring it. And the speed of the display system doesn't matter if the oscilloscope's triggering circuits are slow to "re-arm" for the next acquisition.
Update rate is significant because it can impact debugging methodology. Consider the glitch mentioned above. If an engineer knows the glitch exists, it's easy to isolate it with a pulse-width trigger. However, it's the glitch that isn't suspected that causes the biggest problems. When a user gets a new circuit board, most "browse" from pin to pin. An oscilloscope with a fast update rate increases chances of finding the glitch during casual inspection—increasing the user's confidence in the board. If one isn't confident in the scope's display capability, the user will have to rely on its triggering system to search for each potential problem on each pin.
Another useful application of a fast update rate is manufacturing test. Many tests require multiple acquisitions on the same test point to increase measurement confidence. Large samples lead to better characterization of margins. Mask tests are an excellent example. A slow update rate forces a trade-off between lower test throughput and lower measurement confidence.
Even though a typical LCD or CRT updates at just 60Hz, one can still benefit from update rates on the order of hundreds of thousands of waveforms per second. All data is displayed, but each acquisition is overlaid using "persistence" algorithms which use color or intensity gradation to show frequency of occurrence. It's like looking at a histogram of the traces from above.
Characterizing update rate
Update rate is a dynamic characteristic. It is relevant to repetitive acquisitions, not "single-shot" measurements. And, it can vary with the scope's timebase settings, modes of operation, and its architecture.
Update rate is constrained by the "dead time" between acquisitions. The oscilloscope is blind to any events that occur during the dead time (Figure 1). There are several sources of dead time. The most important is the time it takes to display data in the acquisition memory.
1. Events that occur during the "dead time between acquisitions" can be missed.
At a fundamental level, some scopes simply have a faster data connection between acquisition and display than others. Architecture also matters. Some designs empty all data to the display before refilling it. Another approach is to "ping-pong" data from different acquisition memory banks to the display. A third technique is to queue data to the display. Other factors include the amount of memory processed and delay inherent in the triggering system.
Many oscilloscopes have special modes that can accelerate the update rate. They accomplish this by reducing the memory depth or bypassing most triggering circuitry. As a rule, these special modes require performance trade-offs (such as sample rate reductions or inability to execute even simple triggers) and should be used carefully.
If a scope has an external trigger and offers a frequency counter measurement, such as the Agilent DSO5054A, this is a simple experiment to perform. Use a 50Ω BNC cable to connect the external trigger to Channel 1 of the oscilloscope. Set the scope to auto trigger. The frequency counter measurement will count the number of triggers per second, which is a close approximation of the maximum update rate. If the scope does not offer an external trigger, the user can substitute a high-frequency source as the input.
Duty cycle
The concepts of update rate and dead time can be easier to visualize when placed in the context of duty cycle. For an oscilloscope, duty cycle is the percentage of time the scope is acquiring data. The more time spent acquiring data, the better chances are of seeing intermittent events.
If the oscilloscope's update rate is known, it is easy to calculate the update rate. We start with a more traditional definition of duty cycle:
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