The history of electronic measurement is a succession of steps toward viewing ever faster, more complex waveforms and signal details. Consistently, the oscilloscope's display has been the final judge of electronic circuit performance. For decades, a key tool in acquiring the most challenging waveform events has been digital sampling, and more recently the digital storage oscilloscope (DSO).
DSOs now dominate most scope applications because they offer crucial advantages over analog oscilloscopes. Digital scopes document, analyze, and compare waveforms far more efficiently. They present stable, clear images of complex waveforms. Even so, several limitations of conventional DSOs have hampered their acceptance in some applications.
The Last Strongholds of Analog Scopes
Even though the DSO has supplanted analog scopes in most applications, customers in the manufacturing test, education, and service markets remain loyal to their analog scopes. Why? Because they believe a combination of factors-cost, performance, ease of use, and more-limits the utility of digital instruments in their applications. These users have seen generations of DSOs come and go, with too little attention paid to the need for a low-cost tool that feels like an analog scope, yet offers the many advantages of digital measurements.
Perhaps the most important traditional objection to DSOs among these markets is the instruments' relatively high cost. Early DSOs cost far more than their analog ancestors and even after years of price adjustments, in 1996 an adequate digital scope costs about twice as much as an analog model with the same bandwidth and features. Manufacturers of consumer products like digital telephones, trade schools teaching electronics, and service facilities with large staffs of technicians simply can't afford to put a "premium" instrument on every bench. They must equip their personnel at the lowest possible cost. Historically the analog scope has fulfilled that need. At best, such customers will buy one lab-quality instrument to be used as a high-performance reference for other, lesser scopes in their facility. And even this might be just a faster, more capable analog scope.
A second concern is ease of use. The beauty of analog scopes is that they are simple to use, understand, and interpret. Scope users in the manufacturing test and service markets are usually not engineers or scientists; more commonly, they are technicians charged with routine tasks like calibration and troubleshooting. Theirs is not a job of probing deeply into microprocessor code or analyzing fast signal edge details. For their measurement procedures, a basic analog scope is an attractive tool because it acquires and continuously displays repeating waveforms and has minimal controls. Its display provides an inherently varying brightness or "greyscaling" effect that helps them distinguish, at a glance, signals that fall outside the predicted parameters. In addition, the analog scope is free from the misleading digital artifacts of conventional real-time and ET sampling. And importantly, most technical personnel were trained on analog scopes.
Finally, many analog users are skeptical of some digital scopes' capabilities. They see many of the automated measurement and analysis features as unnecessary bells and whistles, difficult to activate and not very useful for everyday measurement tasks. They are unfamiliar with the controls and displays of most digital storage oscilloscopes. These users, always watching for ways to cut cost, certainly are not willing to pay more for a DSO that includes features they don't value.
Though familiar and easy to use, analog scopes have their own limitations. Users would be happy to see these resolved. While the analog scope does indeed capture a full screen of waveform information for every trigger event, that information may be far from legible. Consider, for example, a signal consisting of 100 ns pulses occurring every 50 ms. The scope readily displays the stream of pulses when set to a medium sweep range. To examine individual pulses, though, it's necessary to increase the sweep speed to the point where the signal becomes dim and difficult to read. Details are lost. Even though the signal triggers the scope, it may not be adequately visible. Other well-known analog limitations include display flicker, writing speed problems at high bandwidths, and the inability to draw a continuous trace when running at very slow speeds.
When viewing fast, infrequent events, the analog scope obscures detail at slow sweep speeds (left) and presents a dim, difficult-to-see waveform at high sweep speeds.
Against a backdrop of entrenched analog loyalty, digital scopes have had relative difficulty in earning a place in many service depots and on the technician's benches. Even though DSOs, with their automated measurement and setup features, offer potentially greater productivity, they have failed to convince production managers and service shop owners that the added benefits are worth a higher price.
Given all these concerns, what would be required to make the digital scope attractive to manufacturing, service, and education customers? Would it be enough to simply cut the cost of existing conventional DSOs in half? Which aspects of analog performance would need to be preserved intact, which could be enhanced, and which discarded?
The answer to these questions lies in one simple premise: make the transition from analog to digital a comfortable one. The digital scope that really has the potential to win over analog loyalists is the scope that looks, feels, and costs "analog" yet delivers productivity-enhancing digital scope benefits. To model such a scope, it's necessary to examine the analog tool very closely.
This analog scope has had more than 50 years to mature. The front panel layout, control interactivity, adjustment ranges and even the actual knob sizes of the analog scope are accepted as the "standard." While high-end digital scopes can pack a hundred functions into a multi-level menu and a single rotary control, a manufacturing technician, for example, would find such an interface slow and cumbersome. The technician's hands "know" the analog scope's front panel and go instinctively to the right knob for each step in a test procedure. Direct hands-on access to critical controls is essential.
The display of the analog scope has created a set of expectations about how waveforms look. Moreover, the analog scope's wide viewing angle is valued in the educational environment, where groups of students must gather around a single scope for classroom demonstrations. The analog scope's display is legible near and far, from almost any perspective. The would-be replacement for analog scopes must offer equivalent viewing performance.
The final step in modeling the analog scope is, of course, cost. Analog tools base their price points on economies of scale, inexpensive off-the-shelf components, and advanced manufacturing technologies. Any digital scope that hopes to challenge analog must somehow achieve these same economies, even though conventional DSO architectures are inherently more expensive. This is purely a cost issue.
But simply emulating analog scope capability and cost is not enough to motivate entrenched analog users to move to digital scopes. While meeting the necessary cost goals, digital scopes must offer a better price/performance ratio than analog scopes. Users need to see the digital scope as the choice that saves time on the production floor or service bench, with no increase in purchase cost.
While features like one-touch measurements, math capability, and stored waveform templates are available in many DSOs, bringing the digital platform to price parity with analog has never before been possible. Now Tektronix has found a low-cost technology-Digital Real-Time-that supports digital scopes at all price points. Before explaining the characteristics that make DRT the most powerful sampling architecture in the world, it's worthwhile to look at the technologies that preceded it to better understand why DRT is such a breakthrough.
Sampling Solutions: An Overview
Twenty years of DSO competing claims have led to a confusing array of sampling and bandwidth ratings: "Real-Time," "Interleaved Real-Time," "Equivalent-Time," and other proprietary terms. What is meant by these terms, and how do they reflect a DSO's actual measurement capability? In fact, each of them represents a legitimate, usable acquisition approach, but each compromises measurement capability in some way.
Before looking at sampling methodologies, a discussion of "analog bandwidth" as it applies to DSOs is in order. Analog bandwidth specifications can be misleading because they express only the frequency response of the DSO's input channels. A DSO's sampling architecture has a profound impact on the ultimate accuracy of the signal representation. A 100 MHz analog bandwidth simply ensures that the 100 MHz signal will reach the scope's digitizing circuits, limited only by the vertical amplifier. The specification says nothing about what effect the digitizer itself will have.
Equivalent-Time sampling (ET) was for years the only available sampling architecture. ET uses relatively slow sample rates-a fraction of the input frequency-to gather samples across many cycles of the input signal. Inherently, the update rate is slow because it takes so many cycles to build a record to display onscreen. If the input signal is perfectly repetitive, the ET scope will produce an accurate waveform image. If the waveform changes during the sampling cycle, the scope will create false data points ("aliasing"), rendering the image meaningless. This characteristic prohibits equivalent- time sampling from capturing one-time momentary events.
Equivalent-Time Sampling: Data points are acquired from many cycles to build one screen image
Real-Time sampling is a more straightforward sampling approach. It's also the only way to capture transient (non-repetitive) events. Unfortunately, the low sample rates in most of today's oscilloscopes are not adequate to support the instruments' full analog bandwidth. A common solution to this shortcoming is Interleaved Real-Time sampling, which draws on the conversion circuits behind unused input channels to extend the sampling performance of multichannel instruments. Interleaving two channels will double the real-time sample rate; using four channels will quadruple it. Unfortunately, interleaving disables the supporting channels, making a four-channel scope into a basic one-channel tool. Most scope users need two or more channels for almost every measurement they make.
Most DSOs use a combination of real-time and ET sampling techniques. At lower frequencies real-time acquisition provides enough sample points to define the waveform. As a scope approaches its analog bandwidth, it will automatically switch to ET. In other words, a conventional DSO trades off critical transient capture capabilities to achieve its specified bandwidth.
The Real World Wants Real-Time
Applications ranging from digital design to production troubleshooting are best served by real-time measurement techniques. In today's telecommunications, computer, video, and network technologies, device speeds commonly exceed the real-time bandwidth of conventional DSOs. Frequently these same high-speed signal environments also require reliable transient capture and/or simultaneous acquisition from multiple test points. Clearly, these needs cannot all be met with interleaved real-time or ET measurement methods. Real-world applications need real-time measurement performance. The best of all worlds would be a DSO architecture that provides uncompromised real-time acquisition at full analog bandwidth, on multiple channels concurrently.
Just such a solution is now available: Tektronix' Digital Real-Time acquisition technology.
Digital Real-Time Technology
Digital Real-Time (DRT) is an oversampling technique that capitalizes on a series of Tektronix innovations in CMOS circuit integration, clock architecture, and analog-to-digital conversion technologies. It is aptly named because it acquires samples in true real-time fashion, at extremely high clock rates. DRT instruments do not resort to interleaving or ET schemes to deliver their specified performance; in fact, these capabilities would be redundant on a DRT scope.
Tektronix Digital Real-Time oscilloscopes achieve real-time acquisition up to their full analog bandwidth, both for repetitive and single-shot events. A DRT scope oversamples 5, 7, 10, or even 16 times its bandwidth. A trigger event prompts the acquisition of a full record-enough data points to display the waveform onscreen in exacting detail.
Digital Real-Time Sampling: Each waveform cycle produces more than enough data points to build a full record (screen display)
Each channel in a Tektronix DRT scope is fully supported by an independent DRT acquisition system. Consequently, the Digital Real-Time scope provides uncompromised acquisition on all channels simultaneously. This delivers clear, easily understood displays even on fast single-shot events, minimizing the distortion and aliasing common to conventional undersampled DSOs.
Tektronix' Digital Real-Time is a transparent technology. Since it works equally well on repetitive or single-shot events, there is no need to turn it on or off. DRT users enjoy analog-like operation while gaining the precision and analysis capabilities of a digital instrument. Screen updates are very fast because all samples are captured in a single acquisition.
Because Tektronix' patented Digital Real-Time architecture boasts a minimal chipset, low power consumption, and fast signal acquisition, Tektronix is able to adapt it for different form factors and price/performance points. Originally developed for Tektronix' top-of-the-line lab instruments, DRT is a very cost-effective platform whose development cost has been amortized over many products during the past few years. DRT has enabled Tektronix to develop high-performance digital scopes to compete at every price point.
The ultimate promise of Digital Real-Time is to bring the productivity and power of the digital oscilloscope to all scope users. Toward that end, Tektronix has introduced a wide range of DRT models, including handheld battery-operated field instruments, mid- and high-range lab scopes and now, compact low-cost benchtop tools.
The Tektronix TDS 200 Series: The Best of Both Worlds
Tektronix has capitalized on its Digital Real-Time technology with a new series of digital oscilloscopes, the TDS 200 Series, that can vault the analog scope barriers to entry in the manufacturing, service, and education markets.
The TDS 200 Series consists of two models, the 60 MHZ TDS 210 and the 100 MHz TDS 220. Both models are DRT-based and both are contenders for applications currently addressed by analog scopes, from equipment maintenance to TV repair to pre-shipping calibration. The new scopes are the result of exhaustive research involving customers in the manufacturing, service, and education markets. This series is designed to address the historical concerns about DSOs in these applications and to offer much-enhanced performance (relative to the existing analog tools) at no cost penalty.
The TDS 200 Series' front-panel controls are scaled and laid out in the manner of a classic analog scope. Familiar front-panel controls like vertical scaling, trigger level, cursors, and trace positioning are all accessed with conventional knobs. The analog user will find these controls to be responsive and linear, just like those of an analog instrument.
Supplementary digital features like automatic waveform measurements and waveform storage reside within a flat menu structure that is easy to reach and easy to understand. Many such features are activated with just a single button-push.
The TDS 200 Series' display is a bright, backlit 11.5 cm x 8.6 cm LCD screen with a wide viewing angle. Its clarity matches any analog scope, and thanks to DRT acquisition, it does not rely on vague waveform "shadows" to convey the details about the signal. DRT oversampling (10X for the TDS 220, 16X for the TDS 210) captures fast edges, transients, and ordinary repetitive waveforms equally well. Any signal that triggers the scope will produce a clear, crisp, highly visible waveform display.
Digital Real-Time scopes display fast, infrequent events with full contrast and clarity at both slow and fast sweep speeds.
Unlike earlier LCDs, the TDS 200 Series' display has a wide viewing angle that enhances off-axis legibility. Perhaps more importantly, its life expectancy and reliability are actually better than traditional CRT displays. The LCD employs newly-developed technologies that have been proven to maintain usable contrast longer than CRTs. And because the LCDs do not rely on critically-aligned beam-positioning elements, they are more impact-resistant and durable than CRTs.
Because the new instruments are DRT-based, they minimize the aliasing effects so common in DSOs of the past. The TDS 200 instruments accept full-bandwidth measurements on both channels simultaneously, just like Tektronix' DRT lab instruments.
The physical packaging of the new Tektronix TDS 200 models is a departure from established scope configurations. Although both instruments have full-size displays and front panels, their cubic volume is less than 1/4 that of conventional scopes, either analog or digital. The TDS 200 Series fits well into a congested benchtop. Their light weight (only 2.9 kg) makes them easy to move from bench to bench as required.
The TDS 200 Series instruments (left) occupy less than 1/4th the benchtop space of conventional DSOs (Top View).
The all-important price issue is also settled by the TDS 200 Series. With low-cost integrated DRT components at their heart, plus cost-effective display and packaging technologies, the TDS 200 Series instruments are a first in their field. They are uncompromised digital tools priced like low-cost analog tools, and priced far below competitors' DSO products.
With the traditional objections to digital scopes resolved, analog scope users can now realize the many advantages of digital measurement technology. Following is a partial list of measurement attributes unique to digital tools:
- "Freeze" (stop) the signal
- Store/recall waveforms
- Make automatic measurements
- Make hardcopy records
- Capture one-time transient events
- View pre-trigger waveform details
- Use automatic setups
- View flicker-free waveform display
Apart from their measurement features, the TDS 200 Series instruments include a unique 10-language user interface for all menus and front panel controls. The multi-language interface is a feature unavailable on any analog scope. The languages included are English, German, Spanish, Korean, Chinese (simplified and traditional), Portuguese, Italian, French, and Japanese. Clearly the TDS 200 Series scopes are suitable for use anywhere electronic products are manufactured.
Summary
DRT acquisition technology has provided a platform for high-performance digital scopes at all price points. It overcomes the traditional limitations of digital storage scopes, and therefore is ideally suited for applications in the manufacturing, education, and service fields-applications traditionally claimed by analog scopes. The TDS 210 and TDS 220 are low-cost DRT-based oscilloscopes optimized for these applications. Real-world users will find the scopes' performance better than analog scopes. Moreover, the TDS 210 and TDS 220 provide a host of automated digital functions that will enhance productivity wherever they are used. These new products will hasten the final transition to digital scopes.