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What is waveform update rate and why does it matter?

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Although often overlooked when evaluating oscilloscope performance, waveform update rates can be extremely important — sometimes just as important as the traditional banner specifications including bandwidth, sample rate, and memory depth.

All oscilloscopes have an inherent characteristic called “dead-time” or “blind time”. This is the time between each repetitive acquisition of the scope when it is processing the previously acquired waveform. Unfortunately, oscilloscope dead-times can sometimes be orders of magnitude longer than acquisition times. During the oscilloscope’s dead-time, any signal activity that may be occurring will be missed as shown here.

Because of oscilloscope dead-time, capturing random and infrequent events with a scope becomes a gamble — much like rolling dice. The more times you roll the dice, the higher the probability of obtaining a specific combination of numbers. Likewise, the more often a scope updates waveforms for a given amount of observation time, the higher the probability of capturing and viewing an elusive event — one that you may not even know exists.

When you debug new designs, waveform and serial bus decode rates can be critical — especially when you are attempting to find and debug infrequent or intermittent problems. These are the toughest kinds of problems to solve. Faster waveform and decode update rates improve the scope’s probability of capturing illusive events and serial bus communication errors.

An example of these faster waveform update rates can be found in the Agilent InfiniiVision X-Series oscilloscopes with waveform update rates of up to 1,000,000 wfms/sec. For example, when capturing an infrequent metastable state (glitch) that occurs approximately 5 times per second. With a maximum waveform update rate of more than 1,000,000 waveforms per second, this scope has a 92% probability of capturing this glitch within 5 seconds. In comparison other scopes in this class may update waveforms only 2000 to 3000 times per second. This means that these scopes would have less than a 1% probability of capturing and displaying an infrequent glitch such as this within 5 seconds.