The other day, I received a request for information on digital oscilloscope fundamentals. A very nice gentleman, Ed, had just purchased his first digital scope and was looking for help in learning its basic operation. Ed is an old-time Tektronix user. He lamented that he had two Tektronix scopes “that I like, but both are ancient. But, as you know, they go on forever” He had an application that required him to capture his waveforms for analysis. That simple need had forced him to finally upgrade to a basic digital oscilloscope.
In chatting with Ed, I was struck by the fact that this seemingly simple requirement—capturing data—had led him to buy a new scope. This made me curious; when did digital oscilloscopes first appear in the market? While they've been around since the early 90s, analog scopes continued in the mainstream for some time. Not so long ago, an engineer had to print off a picture of the oscilloscope screen to capture what was happening. You couldn't save the waveform to load up later for more analysis or comparison, nor could you take the waveform data back to your desk for analysis on a computer. You could only print a picture. Today, even the cheapest general-purpose digital oscilloscope has a front-panel USB port for easily saving screen shots and waveforms.
When the first digital scopes came out, they did a great job of capturing data. However, those of us in the test and measurement industry got an earful about the lack of phosphor displays. Many digital design engineers felt hindered by their scope's inability to monitor what was happening in their signal. They missed the unique persistence characteristics of their analog scope's phosphor display, which showed them how their signal was changing over time (see Figure 1). And, they missed the “liveliness” or fast update rate of their analog scope which allowed them to see even fast changing signal details.
The industry has since responded with innovations like digital phosphor displays that effectively eliminate the last remaining advantage of analog scopes. With digital phosphor displays, waveform after waveform is overlaid on the display, meaning frequently recurring waveforms are more “intense,” just like the phosphor on a cathode ray tube (CRT). This means engineers can see how their signal is changing.
But where analog scopes had reached maturity, digital scopes were just getting started. For instance, by adding a fast acquisition system to the front-end, digital scopes can capture over 300,000 waveforms/s, offering significantly faster update rates than the fastest analog scope. And, the addition of color-coded “heat map” displays (see Figure 2) lets engineers discover problems faster than their old analog scope, and capture the problem for analysis.
With the transition to digital architectures, scopes can now offer more advanced triggering. Not so long ago, the only trigger option on a basic scope was an edge trigger. Today, basic oscilloscopes offer advanced triggers like pulse width, run, rise time, and logic. Scopes below $3000 provide triggers for specific serial packets and parallel bus data, and setup/hold violations. In a higher performance oscilloscope, you'll find over 1400 trigger combinations available.
Another benefit of a digital architecture is automated measurements. With all waveform data represented in bits, the oscilloscope can repeatedly and accurately make complex measurements at the press of a button. Gone are the days of grease pencils and “eyeballing” divisions on the scope display to determine pulse width or slew rate.
In the last few years, we've also seen the birth of moderately priced scopes that offer signal visualization and other more advanced capabilities, but at prices substantially less than traditional performance oscilloscopes. With over 60% of scope users working with serial buses, engineers today often need to look at long data streams to verify data integrity in their communication buses and to see interactions between different components in their system. Given the difficulty of manually interpreting serial data, many bench scopes offer decode for common serial buses along with the ability to trigger on a specific packet or byte content.
When I look back at the last 15 years of general-purpose scopes, it amazes me at how far we've come. In the scope, we can see evidence of all the trends that have been propelling electronics to the next generation: convergence of many features in one instrument; IC integration to enable all those features in the same size package or even smaller; higher levels of performance by going digital; and generally more value.
When you look at your scope, what do you see? Is it time for an upgrade?
About the author
Gina Bonini is the Worldwide Embedded Systems Technical Marketing Manager for Tektronix. She has worked extensively in various Test and Measurement positions for over 15 years, including Product Planning, Product Marketing, Business and Market Development. She holds a BSChE from the University of California, Berkeley and a MSEE from Stanford University. She can be reached at .