Capturing Intermittent or Infrequent Events?

Blog Post created by rlashlee Employee on Sep 1, 2016

Why You Should Care About the Update Rate of Your Oscilloscope

Oscilloscopes have a lot of specifications – some more readily understood than others. One specification that has recently become more frequently discussed is update rate. Despite its importance, there may still be many oscilloscope users who do not understand update rate nor why it impacts their measurements.

When you debug new designs, waveform and decode update 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 a scope’s probability of capturing elusive events. To understand why this is true, you must first understand what is known as oscilloscope “dead time”.

Every oscilloscope has an inherent characteristic called “dead time” or “blind time”. This is defined as 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. And during this dead time, any signal activity that may be occurring will be missed by the oscilloscope, as shown in the figure below. The waveform update rate specification tells you the number of acquisitions by the oscilloscope per second. The larger the waveform update rate, the more acquisitions per second, and the shorter the dead time.

This issue of large dead times becomes particularly problematic when capturing random or infrequent events as you are essentially rolling the dice on whether you will capture these or not. The shorter this dead time, the more likely you are to successfully capture an elusive event. As an example, many of the Keysight InfiniiVision oscilloscopeshave waveform update rates of up to 1,000,000 wfms/sec. When capturing an infrequent metastable state (glitch) that occurs approximately 5 times per second using a oscilloscope with an update rate of 1,000,000 waveforms per second, this scope has a 92% probability of capturing this glitch within 5 seconds. In comparison, other oscilloscopes in this class may update waveforms only 2000 to 3000 times per second. These scopes would have less than a 1% probability of capturing and displaying an infrequent glitch such as this within 5 seconds.

It is also useful to understand how an oscilloscope’s update rate is impacted by other features or functionality. For example, some scopes spec a best-case scenario, but when features such as MSO, protocol decode, or math functions are turned on, the update rate can drop significantly. Therefore, it is important to know how the features you will be using for your applications impact the update rate of your oscilloscope.

Want to learn more?

Here is an application note that goes into greater depths on this topic, including how to measure your oscilloscope’s update rate: Oscilloscope Waveform Update Rate Determines Ability to Capture Elusive Events.

See update rate in action with Keysight’s video on debugging infrequent events.