DSO Displays: Almost as Good as Analog

With color and intensity gradients, the displays in new digital scopes nearly emulate those of analog models.

Martin Rowe, Senior Technical Editor -- Test & Measurement World, 2/1/2000

DSOs give you dozens of measurements that you can’t get with analog scopes. Yet, some signals have managed to defy digitizers. Analog scopes, with persistence and intensity in their displays, are still the favorite among engineers who measure modulated analog signals. Some digital circuit designers and communications engineers also like analog scopes because they provide information about a clock signal’s jitter.

Recently, new DSOs have nearly closed the last gap between digital scopes and analog scopes. DSOs now emulate the persistence and intensity found in analog scope displays. Indeed, these new DSOs go further; they let you measure characteristics such as pulse count and frequency of occurrence—measurements you can’t make accurately with analog scopes. They also let you perform mask testing on communications signals with the benefit of knowing what percentage of pulses are out of tolerance.

An analog scope can show different intensities, giving you a clue that more than one frequency is at work in a signal. As a result, engineers who need to view modulated signals have refrained from using DSOs because they can’t distinguish between a carrier and its modulation. That’s begun to change.

Figure 1. Digital scopes emulate the persistence of analog scopes, letting you distinguish a carrier from its modulation. (Courtesy of Tektronix.)

 Figure 1 shows how well some digital scopes can emulate analog displays. The low-frequency modulation is brighter than the high-frequency carrier. If you looked at this signal on a traditional DSO, you wouldn’t be able to distinguish the carrier from the modulation because they would have the same intensity.

DSOs can also display waveforms in color; analog scopes can’t. Therefore, DSOs can use color to differentiate between a signal’s characteristics. The technology in DSOs that adjusts color or intensity goes by trade names such as analog persistence, Digital Phosphor Oscilloscope (DPO), and TruTrace. All of these gradient displays can help you find signal problems. They give you some indication of what the problem looks like so you can set up a trigger to capture a particular signal characteristic. You may want to turn off the gradient display once you can set the scope to trigger on the event of interest.

While all gradient displays are based on similar concepts, the DSOs themselves are different. Some scopes use their gradient displays in single-shot mode (Gould Classic), while others work in repetitive mode only (Agilent Infiniium) and Cell USA (VDS2152 with ScopeManager PC software). Scopes from LeCroy (Waverunner and LC series), Tektronix (TDS 500, TDS 700 and TDS 3000), Yokogawa (DL7100) work in both modes. Regardless of trigger mode, a gradient DSO displays more information than a traditional DSO.

A DSO display comprises a matrix of pixels, typically 640 columns by 480 rows. The scope uses 500 columns and 250 rows for its waveform grid. Most DSOs have 8-bit (256-count) ADCs. So for all practical purposes, each row, representing amplitude, corresponds to one ADC count.

A DSO’s acquisition memory can store thousands or even millions of waveform samples, but a scope screen has just 500 columns in which to display all or part of that data. If you want to see more than 500 points at a time, you’ll have to look at a compressed representation of the data in the scope’s acquisition memory. Scopes with gradient displays can present some of the information through differing color or intensity.

Pixel Maps

A DSO sends data from its acquisition memory to a pixel map. The map helps the scope calculate which pixels to illuminate and with what color or intensity. Assume a DSO has a 250,000-point acquisition memory. When displaying the entire acquisition-memory contents, the scope has to represent 500 samples in each of the 500 columns.

A traditional DSO compresses the data by calculating the maximum and minimum values of those 500 points. Then it lights the pixels between those points with equal color and intensity, causing you to lose what’s between the maximum and minimum amplitudes and how often each point occurs.

Scopes with gradient displays use color or intensity to highlight activity between the maximum and minimum amplitudes in a column. The scope counts the number of occurrences of each ADC value in each column. A scope might have 8 or 16 possible intensities or colors. Each indicates a range of the number of times that the ADC’s counts occur—the number of times a pixel is “hit.” Figure 2 shows the extreme cases and an intermediate case.

Assume you can capture a square wave with perfectly flat levels that have no noise or overshoot. If the scope takes 500 samples for one column between the signal’s edges, then the scope’s ADC will produce the same count for every sample (Fig. 2a). The scope will display all 500 points on the same pixel using the brightest possible intensity (shown as darkest in the figure).

In the other extreme, assume that the scope takes 200 samples to capture the signal’s rising edge (Fig. 2b). Also assume that the rising edge results in ADC counts that increase by 1 with each succeeding sample. After compression, 200 of the 500 columns in a display get just one pixel hit. These 200 pixels will light, but at the lowest possible intensity.

Now look at Figure 2c. This time, assume the signal has ringing in the middle of its rise. Numerous ADC counts will occur multiple times. Those pixels will appear brighter than those that occur once but will be dimmer than if one pixel were hit for each of the 500 samples as in Figure 2a. In effect, a scope produces a histogram for each column, then represents that histogram with color or intensity (depending upon its capabilities; some scopes offer both options). Most often, engineers use intensity because that more closely represents the display of an analog scope.

Scope manufacturers group the number of ADC counts into ranges of color or intensity. Finding the best number of groupings proves tricky: Too few ranges, and you won’t get enough information about the waveform; too many shades, and you’ll get more than your eye can distinguish. In their DSOs with TruTrace, Gould divides the number of ADC counts in a vertical column into eight possible intensities—nine if you count “off” indicating zero pixel hits. Others may use fewer or more.

Gould’s TruTrace display uses gray scale only, so the display indicates activity in a column using intensity. TruTrace also works with the scope in single-shot mode only. Scopes from LeCroy, Tektronix, and Yokogawa let you use color to view the ranges of pixel hits in either single-shot or in repetitive mode, while Agilent’s color-gradient displays work in repetitive mode only.

Repetitive Hits

Many signals are repetitive; others are mostly repetitive but may contain anomalies. With analog scopes, you can see modulation easily, for it appears as a changing pattern or varying intensities. When using an analog scope to look at a digital signal, you may be able to see glitches that appear as faint lines. The faintness provides an indication as to a glitch’s frequency of occurrence. If you’re looking at clock jitter, you may see a line that’s relatively bright at its center, but fades as you move away. The change in intensity acts like a histogram of the clock’s jitter.

DSOs from Agilent, Tektronix, LeCroy, Yokogawa, and Cell USA can use a signal’s history to give you an indication of its frequency of occurrence and how the signal changes over time. In repetitive mode, the scopes first use data compression to create a pixel map. Then, they build a histogram of a signal, pixel-by-pixel, over many triggers. The scopes then translate the histogram into different colors or intensities. To do that, the scopes use an accumulator to count the occurrences of each pixel over time or, in other words, over many pixel maps.

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Figure 3. Accumulators build a histogram of pixel hits that scopes can translate into ranges of color or intensity. (Courtesy of LeCroy.)

Figure 3 shows the “third dimension” to a DSO’s display in terms of pixel hits over time. Each time a scope generates a pixel map, it stores that map in a buffer. The buffer contains one accumulator per pixel. Those pixels with more “hits” (taller bars in Figure 3) will display with the brightest intensity or with a color that indicates a higher frequency of occurrence. The accumulator may count to 14 bits (Yokogawa DL7100), 16 bits (LeCroy Waverunner), or 21 bits (Agilent Infiniium, Tektronix DPO).

All of these accumulators can generate more colors than the eye can distinguish. So, the scopes don’t display a unique color for each possible accumulator count. Instead, they divide the counts into ranges just as they did for counting ADC values during compression. Here, though, a different color represents a range of pixel hits over time rather than ADC counts in one acquisition. When I say “color” here I include “intensity” as well. After all, DSOs treat intensities as though each is a different color. LeCroy uses 16 color ranges, Tektronix uses 15, Yokogawa uses 8, and Agilent and Cell USA use 7. All add one more color, black, which indicates zero pixel hits. You can choose the colors for each range, except for black.

Weigh the Scales

The number of different color ranges that a scope has doesn’t tell the whole story of what you see on the screen. The scopes differ in the weights they assign to each range. Knowing how a scope sets the break points between count ranges helps you better understand what you see.

For example, LeCroy uses a linear scale where Agilent and Tektronix use binary-weighted scales. Yokogawa lets you set the break points in a menu.

LeCroy scopes take the 16-bit output of their accumulators and assigns a count of zero to black (as do all other scopes). The scope then has one of the 16 possible colors assigned to a count of one, which lets you find an anomaly that occurs just once. The scopes then divide the remaining counts evenly among the remaining 15 colors.

Agilent uses a binary-weighting scheme to determine the break points between seven different colors. The “hottest” color—or the one that is hit the most (normally white on the Infiniium scopes)—turns on when the accumulator’s most significant bit changes from 0 to 1. The next hottest color turns on when the accumulator changes the next most significant bit from 0 to 1. The range for the hottest color occurs from 111...1 down to 100...0, the next color has a range from 011...1 down to 010...0, and so on in power-of-two steps. Thus, you get finer color-change resolution at lower counts.

Some scopes let you “scale” the accumulator values. By doing so, you effectively increase the histogram’s resolution by “moving” more color changes into the range you want to see. Scopes that have this adjustment let you set the level at which the pixel accumulators saturate. The adjustment effectively cuts off the top of the vertical bars in Figure 3 when the saturation level is set to less than 100%.

Figure 4. At 100% saturation, you can’t see the details of which pulses occur how many times. (Courtesy of Catenary Scientific.)

Figure 5. Adjusting the saturation point lets you see details that otherwise get lost. (Courtesy of Catenary Scientific.)

Figure 6. Scopes use the information in accumulators for color-grade persistence (top trace) and for calculating histograms (bottom trace). (Courtesy of Catenary Scientific.)

Figure 7. Color gradients show distribution of pixel hits in a constellation diagram. (Courtesy of LeCroy.)

Figure 8. Color-grade persistence lets you use masks for testing eye-diagrams and other telecom signals. (Courtesy of Agilent Technologies.)

Figure 4 and Figure 5 demonstrate the use of saturation levels. Both figures contain data from the same data set. The figures demonstrate the saturation adjustment used to measure the pulse’s output from an x-ray detector. Chuck Parsons, president of Catenary Scientific (Groton, MA), uses the detector and scope to measure the manganese content in iron alloys. The amplitude of the pulse indicates the composition of the alloy. Each element produces a unique amplitude. The more a particular amplitude occurs, the higher the content of that element.

In Figure 4, the saturation point of the scope’s accumulator is set to 100%, which yields the best view of the most dominant signal. But not enough pixel hits occur for the other accumulators to produce any other colors. Figure 5 shows the effect of adjusting the saturation level to 2.1%. Here, you can see other colors because the lower saturation effectively boosts the gain by 50X so low amplitude counts appear as different colors.

The scaling of the saturation level is similar to the persistence you get when you deactivate the accumulators, which is the same as you get with traditional DSOs. See “Which Persistence?” for examples differentiating the two types of persistence.

Scopes that have the accumulators can also perform measurements on the accumulator data. For example, the falling edge of the waveform in Figure 6 has a considerable amount of jitter. The colors surrounding the brightest color (orange) indicate that the jitter has some distribution. The histogram of the jitter, shown below the waveform, gives you a better idea of the jitter’s distribution.

Other Measurements

You can also use the color-gradient displays for viewing frequency of occurrence in communication signals. Figure 7 shows a constellation diagram of a CDMA pilot channel. The red areas indicate the most often hit pixels.

Other communications signals such as eye diagrams and bit pulses have established limits for variances. Scopes from Agilent, LeCroy, and Tektronix have optional telecom masks that you can use in conjunction with the color-gradient displays. The mask in the center of Figure 8 shows the limits of the “eye” opening. Color-gradient displays, can indicate what percentage of signals fall within the limits of the eye diagram.

Despite their many advantages, gradient DSOs have difficulty emulating an analog scope in real time. According to Parsons, how well a DSO emulates a real-time analog scope depends partly on the screen update rate of the scope and on how much data the scope can get to the screen in each update. If you’d like to use a gradient display to view signals in near-real time, then ask about the screen update rate and number of triggers that the scope can process.

As I mentioned earlier, Gould’s TruTrace works on single-shot mode only. Agilent’s Infiniium scopes update the screen just once per second when you use the accumulators and gradient display. Scopes from LeCroy, Tektronix, and Yokogawa have screen update rates of 30 per second or faster. At 30 updates per second, the waveform appears to be “real time” because the screen update rate is fast enough for your eye to perceive that as so. Scopes from Cell USA connect to a host PC over a serial port, so the updates won’t appear as real time.

If you’re looking for a scope and plan to use the gradient displays, be sure to get a demo scope to try with your application. You’ll find that the scope manufacturers make various claims as to how well their scope emulates an analog scope. One company might claim that its scope can display more acquisitions in closer to real time than other scopes can. Another company might argue that its scopes can acquire more triggers or that its longer memory compensates for fewer acquisitions placed on the screen.

Keep in mind that no matter how well a digital scope emulates an analog scope, you’re still using a digitizer. All digitizers are subject to artifacts from aliasing. These scopes are fast, but not fast enough just yet. T&MW

 You can contact Martin Rowe at m.rowe@tmworld.com

Figure A. With standard persistence, the lit pixels decay over time. (Courtesy of Agilent Technologies.) Figure B. Color-grade persistence displays every written pixel until the scope’s accumulator clears. (Courtesy of Agilent Technologies.) Which Persistence? The displays you see from the “persistence” in scopes with gradient displays is different from the display persistence you get on analog scopes and on the persistence that traditional DSOs have had for years. (The scopes I discuss in this article also work as traditional DSOs.) Display persistence doesn’t need the accumulators that gradient displays require. You can set the display persistence through the scope’s controls. With no display persistence, the scope simply writes a new pixel map to the display each time it updates the screen. With infinite persistence, the scope doesn’t clear the screen at all; it just keeps writing new pixel maps on top of previous pixel maps to the display. With variable persistence, the scope digitally “ages” the pixels so that they change color over time as they “fade” out. Figure A and Figure B highlight the difference between display persistence and the persistence using the accumulators. Both show a sine wave of varying frequency. In Figure A, only the recent acquisitions appear with the most recent shown brightest. In Figure B, the pixels don’t age. The pixel hits from all acquisitions appear in the display. The colors indicate pixel hits since the last time the “persistence” mode was activated. The small bright area in the center of the screen shows those pixels with the most hits. —Martin Rowe