Memory Specification Is an Important Oscilloscope Performance Indicator

An oscilloscope is nothing less than a nearly magical window that allows a designer or troubleshooter to visualize the details of electronic signals. Visualization is extremely important to understanding as we are, by nature, visually oriented creatures.

The earliest oscilloscopes only gave qualitative insight about how a voltage varied with respect to time or its wave shape. Improved oscilloscopes showed quantitatively, in graphical form on calibrated axes, how much a voltage varied in amplitude and how fast this change occurred.

Multichannel oscilloscopes then allowed the time comparison of signals to be visualized. Oscilloscope cameras and storage capability compared and analyzed a single occurrence or infrequent events.

More recently, color became another method of displaying greater quantities of information on the oscilloscope screen. Concurrent with these advances were increases in bandwidth and stability which expanded the window of the visual insight into electrical signals provided by oscilloscopes.

DSOs vs ARTs

As happens to most devices that become increasingly sophisticated, oscilloscopes are beginning to merge with computers to provide even more impressive features. These capabilities include advances in triggering, control functions, signal processing, higher resolution, hard-copy output, and examination of a small portion of a captured waveform in detail without reacquiring it.

Another consequence of increasing sophistication is specialization. Where an oscilloscope used to be a simple cathode ray tube and an amplifier, the much wider range of capabilities demanded of modern scopes has led to designs along separate paths. Each configuration has characteristics that are more useful in some applications than others.

The original oscilloscope configuration is the analog real-time (ART) oscilloscope. A later development is the digital storage oscilloscope (DSO).

The DSO, by virtue of producing the signal in sampled or digital format, is more amicable to interfacing with computer functions. This versatility is achieved at the loss of lower frequency response and the intensity or Z-axis information that is present on an ART.

Also, the associated visualization of waveform variations in successive sweeps that show up as blurring on an ART scope generally is lost on a DSO. On the other hand, characterizing the overshoot on the trailing edge of a modulated pulse is almost impossible on most ART scopes, but quite simple on most DSOs.

Just as ART scopes can be conceptualized as a cathode ray tube attached to an amplifier, DSOs can be described as a sampled signal fed to an analog-to-digital converter operated by a computer. This description is the basis for another difference in DSO configurations.

The most prevalent configuration houses the oscilloscope in a box with all the necessary components pre-assembled in one functioning unit. The other configuration installs an add-on card in an existing computer, where the plug-in card achieves the input signal sampling function.

The other functions necessary to produce an operable oscilloscope are provided through computer resources by means of appropriate software. This approach is especially attractive in ATE applications where a special-purpose scope normally would be required, but the function can be programmed into a virtual instrument using general-purpose computers and data acquisition cards.

The subject of applying PC card oscilloscopes to ATE was addressed by Ed McConnell, computer-based instruments marketing manager at National Instruments: “There are two ways to leverage computer technologies. Either the computer is designed into the oscilloscope or the oscilloscope is made to use with a standard computer.

“For ATE systems, substantial cost savings result from putting oscilloscopes inside the computer. Not only is the price of the raw instrument less, but the test performance and automation speed also are dramatically improved because the instrument transfers readings directly to the computer memory,” he said.

An even newer configuration, the digital phosphor oscilloscope, is discussed in a product focus in this issue. It combines features of ART scopes and DSOs.

In the case of the ART scope, parameters of bandwidth, sweep rate, and sensitivity pretty well define the operational limitations that can be expected in a given oscilloscope. Nailing down the operational parameters of DSOs is a little more complicated, especially since more options are becoming available almost daily.

Memory as a Performance Indicator

One parameter common to all DSOs and indicative of the capabilities of the instrument is the size of the memory available for storage operations. While RAM is not expensive itself, the features in oscilloscopes that mandate large amounts of memory—including pre- and post-triggering, high sample rates, many channels, and waveform comparison—are high-performance characteristics. While no guarantee of performance, the available memory is indicative of the limits to the capability of a DSO.

A simple DSO might digitize 256 samples of 8 bits to produce one trace. This would require only 256 bytes of storage memory. Two traces would double this to 512 bytes. Add two screen widths of pre- and post-trigger capability, and we need 2 kB of memory.

Add to this a high-resolution option that allows you to zoom a captured waveform by a factor of 10 in the horizontal axis, and we need 20 kB. Incorporating enough vertical resolution to allow a similar zoom in amplitude now requires 40 kB in this example. Some oscilloscopes have significantly greater capabilities and contain as much as 8 MB of memory to support these and other capabilities.

Insight into the memory requirements of current applications and a rule to estimate requirements for individual applications were given by Dr. Michael Lauterbach, director of product management at LeCroy. “Typically, the usable bandwidth or digital bandwidth of a digital scope is one-quarter of its sampling rate,” he explained. “A scope with only 50,000 points of memory would have to spread those samples 2 ns apart to capture 100 µs of data. This gives a rate of 500 MS/s and a usable bandwidth of 125 MHz.”

Memory requirements are not limited to the storage of acquired waveforms. DSOs can do much more than acquire and display waveforms. However, added capabilities require additional memory.

“The real power in the implementation of long memory comes from incorporating technology that allows the scope to display key signal characteristics that are captured in the long memory and to make computations of signal performance,” Dr. Lauterbach explained. Examples of computational capabilities that consume memory are FFT analysis to determine the frequency content of a waveform, comparison to a reference or stored waveform, or comparison to a specified envelope to determine out-of-tolerance conditions.

Pete Cipriani, national sales manager for oscilloscopes at Gould Instrument Systems, put the memory specification in perspective. “Just like any of the other banner specifications, bandwidth, sample rate, and channel count, determining the right memory depth is completely application-specific. In today’s market, 50 kS per channel seem to be the standard. Since DSOs lend themselves very well to general-purpose testing, it would be wise to consider an instrument that can be upgraded in memory length should the application grow or a new one arise.”

Oscilloscope Selection

From a practical view, we rarely can pin down the exact requirements of a test instrument before a project is underway or a troubleshooting problem is the current crisis. In either of these situations, the oscilloscope to be used was ordered months before. The only practical approach to this dilemma is to specify the best oscilloscope possible so that the resources are available when needed.

There is a caution that goes along with this approach. More features and capabilities included in any test equipment generally require more training and setup time to use properly. A difficult-to-operate piece of test equipment can be a liability, especially in a high-pressure situation.

This issue was addressed by Tim Coll, product manager for Infinium Oscilloscopes at Hewlett-Packard. “Some oscilloscope vendors have become so absorbed with increasing performance and adding features that they have forgotten what an engineer’s daily work is really like. Today’s engineers may wonder if vendors have forgotten that the main reason for using an oscilloscope, even a high-performance model, is to view waveforms,” he said.

The lesson here is not to shun high-performance features, but to ascertain that they are relatively easy to use and easy to disable when they are not required. Embedded microcontrollers and programmable or soft controls are effective approaches to easing this situation.

Several vendors provide push-button controls with functions described by legends that appear on the edges of the screen. The functions of these controls change according to how the scope is set up, and only functions germane to an operational mode are present.

This approach provides a much less intimidating array of controls than the classical wall of knobs on high-performance, hardwired oscilloscopes. Some models even guide the operator by highlighting legends to which control needs to be adjusted.

To provide you with the latest specifications on oscilloscopes, we have compiled comparison charts listing models currently on the market. For more information on any scope, please circle the appropriate number on the Reader Service Card accompanying this issue, or visit our web site at www.evaluationengineering.com and click on Product Link.

Copyright 1998 Nelson Publishing Inc.

July 1998

 

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