What is a scope? This is a difficult question to answer because the definition of an oscilloscope continues to change. To a purist, the only real oscilloscope is one that displays a waveform in direct response to the input signal: in other words, an analog cathode-ray oscilloscope with electrostatic vertical deflection plates driven by an amplified version of the input.
In contrast, all digital storage oscilloscopes (DSOs) manipulate a digitized version of the analog input in various ways before producing the displayed waveform. Some combined analog/digital scopes retain an electrostatic CRT but can store a waveform digitally and display that data via a DAC. However, most DSOs today use a magnetically deflected raster-scan CRT or a flat-panel LCD as the visual display device, and these are driven digitally.
As DSOs developed, memories became longer, more channels were added, bandwidths increased, and disk storage was provided so captured waveforms could be archived. Mathematical operations could be performed on data allowing rise time and pulse width measurements and comparison between live and reference waveforms. And new data could be created such as an instantaneous power waveform produced by multiplying acquired voltage and current waveforms.
When PCs became sufficiently fast and their displays good enough, data acquisition boards were coupled with scope application software to make PC-based scopes. Certainly, such a system displays waveforms much like a conventional scope does, and it is easier to manipulate files within the PC rather than have them stored in a stand-alone scope. But, exactly what makes an oscilloscope different from a data acquisition system?
Typically, scopes deal with much faster signals than most data acquisition systems can handle, and scope applications require waveform display. The emphasis on a displayed waveform differentiates a scope from other instruments. Many writers have commented that an oscilloscope acts as an electronic engineer's eyes, and this remains true regardless of how the displayed image is formed.
How can you tell a scope from a data acquisition instrument? The short answer is that many times you can't. A scope might be used for high-speed data acquisition with no one observing the displayed images. And, software is sufficiently refined today that signals captured by a data acquisition system can be made to look just like waveforms from a scope. As the CEO of one PC-based scope company commented, the distinctions among data acquisition, digitizer, and scope products are becoming blurred.
High Bandwidth and Sample Rate
If you are looking at single-shot events spaced far apart in time, there's little difference in the results these instruments might produce. However, one factor becomes apparent if the events occur close together. A scope tends to have a short re-arm time, which means that almost immediately after one waveform has been displayed you can capture and display another. Many digitizers also have short re-arm times, but accompanying software may not emphasize display responsiveness.
This capability is important because the way a waveform changes in time can convey as much information as the signal's basic shape. The triggering system and data acquisition architecture must be designed to cope with a high event repetition rate and the large amount of data that results from it.
Many terms are used to describe different sampling modes: real-time, direct, equivalent-time, and random-interleaved being common. Most scope users understand that varying waveforms must be sampled at least twice as fast as the signal's highest frequency component.
This restriction, the Nyquist criterion, must be observed to allow reconstruction of the original waveform without aliasing. Theoretical signal processing work done by Shannon and others showed that if the Nyquist criterion is satisfied, the original signal could be reconstructed exactly, not just approximately.
In contrast, random-interleaved sampling (RIS) or equivalent-time sampling (ETS) refers to the process of assembling a waveform based on samples taken from several different signals widely spaced in time. This sampling mode requires the input signal to exactly repeat at each trigger. By taking samples at progressively later times from successive signals, a composite waveform can be developed equivalent to the original signal sampled at a very high rate.
Datasheets highlight ETS sample rates when describing a scope's performance. Indeed, ETS rates can be impressive, but they only apply to repetitive signals. Nevertheless, the bandwidth of a very high-frequency sampling scope must be genuine even if the displayed trace is a composite of waveforms acquired at a slower sampling rate. The PicoScope 9201 is an example of a 12-GHz bandwidth, 16-b, two-channel, PC-based sampling scope with a 5-TS/s maximum sequential ETS rate and 50-ps transient response (Figure 1).
Sometimes, interpolation is described as a means of achieving a higher sampling rate. This is not correct. A scope's highest sampling rate determines the shortest achievable time between real samples. Interpolation only adds artificial samples between the real samples.
Some scopes even intensify the real samples to distinguish them from the interpolated points. Interpolation facilitates zoom expansion beyond the highest available sampling rate, but it doesn't increase the rate at which the signal originally was sampled.
In general, DSOs are much more complex than traditional analog scopes partly because the waveform acquisition rate and the display rate are only indirectly related. After an analog scope is triggered, the beam sweeps linearly in the horizontal direction at the speed of the selected time-base setting.
The sweep circuit must be reset or re-armed before it can be triggered again, but many analog scopes accomplish this in microseconds. Typically, a few hundred thousand waveforms/s can be displayed, one at a time, corresponding to each trigger. Naturally, there must be a sufficiently high trigger rate that the sweep re-arm time becomes the limiting factor at the fastest time-base settings, not the trigger rate.
Compared to this type of operation, a DSO may capture thousands of waveforms before the display is redrawn. Today's raster-scan electromagnetic and flat-panel displays have typical update rates not much greater than 100 Hz. The consequence of the data acquisition and display systems running at different rates is quantization in time.
In other words, the best that can be done in a modern digital scope is to combine the waveforms that have been acquired since the last display update and present their composite effect at the next update. How multiple waveforms should be combined has been the subject of intense innovation for a long time. Not all PC-based scopes support variable persistence and similar enhanced display modes that resulted from this work.
Which brings the discussion back to the original question: What is a scope? To the extent that an instrument supports highly responsive waveform acquisition and display of fast electronic signals, it's a scope and can be used for the jobs traditionally filled by analog scopes. However, if the waveforms/s acquisition rate is significantly limited and the bandwidth only appropriate for electrical or mechanical signals, you may instead have a data acquisition product that displays a graphical output.
PC-Based Scopes Today
If PC-based scopes really are scopes, then a comparison with stand-alone instruments is relevant. We asked several PC-based scope manufacturers to rank characteristics that distinguish their products from stand-alone scopes. They gave a wide range of responses, some providing thoughtful insight into the basic what is a scope question.
“Currently, we see speed and functionality as a competitive advantage of stand-alone scopes,” commented Bart Schr der, technical director at Cleverscope. “However, PC-based scopes will improve over time. For example, we will soon introduce a 500-MS/s, 12-b scope. PC-based scopes have a lower cost of materials and the advantage of capabilities such as better signal analysis.
“Nevertheless, many PC-based scope user interfaces are far inferior to those of standard scopes, and functionality tends to be limited. To counter these deficiencies, we offer anti-alias filters, input offsetting, peak capture and display, and up to 14-b resolution,” he explained. “In addition, PC-based math functions can be used to provide filtering, integration, differentiation, and conditional expressions.”
Interestingly, one of the traditional objections to PC-based scopes may be changing. We asked if stand-alone scopes were better suited for troubleshooting, especially in applications requiring a portable scope. Mr. Schr der agreed that this had been the case. However, he added, many Cleverscope customers needed to take a PC on-site to record customer details, job requirements, and other information relevant to their company's quality programs. Because they already had a PC with them, it was convenient also to record waveforms on-site for fault reporting or archiving.
Other indications that the PC has virtually a permanent place on most benches were provided by Todd Schreibman, vice president for sales at Link Instruments. He said that small size is one of the reasons customers buy Link's PC-based scopes. They can be placed closer to the DUT and require less bench space than stand-alone scopes. Also, you can easily take the scope with you in your laptop case when traveling.
The PC's large color display is much easier to read than most small scope screens, and you can increase the number of channels being acquired by plugging more DSOs into the same PC. And, while mentioning multiple DSOs, the price of a PC-based scope is low enough that every engineer in a lab can have one.
Another benefit Mr. Schreibman cited was the capability to simultaneously view the DSO display window next to the output from a design simulation program. This kind of direct comparison simply can't be done on most stand-alone scopes. Of course, once the data is in the PC, analysis, sharing, and archiving are simplified.
Some stand-alone high-end scopes offer multiple display windows, but this is commonplace among PC-based scopes. Often, a PC-based scope actually is an integrated instrument with more than just scope capabilities. Once digitized, an input signal may be characterized by its frequency-domain behavior with a spectrum analyzer type of display or simply as a DMM value. Multiple windows can be used to display the same signal in different domains.
PC-based scopes often have very deep acquisition memories that complement a high sampling rate. For example, Pico Technology's PicoScope 5204 has a maximum 1-GS/s sampling rate and 128 MS of memory. Transferring such a huge amount of data to the PC isn't practical if anything approaching a live display is needed. Instead, the PicoScope 5204 uses internal hardware to intelligently decimate the data, providing the important detail but only as many points as the PC actually can display.
If you change the zoom or time-base setting, the PC requests that the PicoScope recalculate the points to be displayed. Communication with the PC is via USB 2.0, so a high display update rate can be maintained by minimizing the amount of data transferred. Nevertheless, if the purpose is to archive and analyze acquired waveforms in detail, then much more data must be transferred to the PC, and the update rate will drop accordingly.
According to Alan Tong, the company's managing director, aside from a price/performance advantage, a PC-based scope offers these capabilities:
• ??Powerful software that uses the processing power and familiar user interface of a modern PC.
• ??PC connectivity saving, printing, e-mailing waveforms, connecting to video projectors; great for education and training.
• ??Upgrades with new features and functions, unlike most scopes that have a fixed function set at the time of purchase.
• ??Software drivers and examples so customers can write their own applications.
In addition, PC-based scopes are available with up to 16-b resolution rather than the basic 8-b found in most stand-alone scopes. And, high-speed streaming effectively eliminates the memory-size limitation in a PC-based scope, allowing users to transfer very long, gap-free records to the PC.
Mr. Tong emphasized the need to satisfy customer requirements, placing customer satisfaction ahead of simple performance metrics such as sampling rate or bandwidth. “Competing with the performance of high-end benchtop oscilloscopes is not really a factor driving our design,” he explained. “It is more important to listen to what customers want.”
PC-Based vs. Benchtop
“To be honest,” Mr. Tong concluded, “it would be very difficult for an existing PC oscilloscope manufacturer to suddenly leapfrog ahead of Tektronix, LeCroy, or Agilent Technologies in terms of sampling rate or bandwidth. One of the key advantages of PC oscilloscopes is value for money, and this comes from being PC-based and using low-cost, mass-produced, off-the-shelf components.”
ZTEC Instruments' Director of Marketing and Product Strategy Boyd Shaw agreed. “PC-based scopes available today have performance that meets or, in many cases, exceeds that of benchtop scopes up to about 1-GHz bandwidth. PC-based scopes top out at about 1 GHz while a number of benchtop oscilloscopes are available with bandwidths above 1 GHz,” he concluded.
On-board signal processing is important in a PC-based scope, regardless of the host PC's capabilities. With a built-in 64-b microprocessor, ZTEC's scopes maintain their responsiveness even while determining waveform parameters from a range of more than 40 measurement types. These scopes feature auto-decimation that minimizes the amount of data that must be transferred to the PC and improves the screen refresh rate.
Figure 2 shows the virtual control panel of a ZTEC scope. The traces are being displayed with persistence.
In addition to cost and size, Mr. Shaw listed higher channel density and greater vertical resolution as PC-based scope advantages. Also, PC-based scopes can be combined with other types of test instrument functionality, making small size even more important. Compared to the inconvenience of lugging around multiple separate instruments, a PC-based scope combined with extra functionality in a single chassis is very attractive.
ZTEC makes PC-based scopes in VXI and PXI plug-in formats. Mr. Shaw's comments about multiple instruments relate to a single PXI chassis with separate plug-in instruments. In contrast, USB-connected PC-based scopes are compact, low-power modules that do not require a separate chassis. Although these scopes provide limited multiple instrument functionality, the capability is not as extensive as the large selection of instrument types available in the PXI format.
National Instruments (NI) manufactures a wide range of PC-based instruments, including PCI/PXI digitizers. Like ZTEC, several types of instruments and one or more digitizers can be housed in a single PXI chassis with the benefit of a very fast PXI or PXIe data transfer rate.
John Hottenroth, digitizers/oscilloscopes product manager at NI, said, “The most important characteristic of deciding between a stand-alone oscilloscope and a modular digitizer is whether it will be used in an interactive or automated application.
“Modular digitizers are ideal for automated use because they focus on features such as measurement and data throughput, synchronization for higher channel-count and mixed-signal applications, higher resolution, and ease of automation.” He continued, “Stand-alone scopes are better for interactive measurements where quick visualization, ease of probing, and very high bandwidth are necessary. Of course, there is some overlap where a customized measurement system is needed in the design lab or very high bandwidth automated testing in production” (Figure 3).
NI recently introduced the NI USB-5133 100-MS/s, USB-connected PC-based digitizer. It does not require a separate chassis, is about the size of a paperback book, and weighs 8.6 oz. The form factor, similar to other USB-connected PC-based scopes, addresses the need for good performance along with easy portability.
An Application Example
Whether or not a particular oscilloscope, data acquisition system, or waveform digitizer has the smallest size or lowest cost doesn't matter if it can't address a given application. Nicole Faubert, marketing manager at GaGe/KineticSystems, described a complex nuclear physics experiment recently instrumented with GaGe equipment:
“In this nuclear magnetic resonance (NMR) spin-echo experiment, 24 simultaneously applied RF pulses cause multiple atomic moments to precess in phase. About 20 ms after the pulses are applied, the spin-echo response signals appear, which must be captured using at least a 100-MS/s sampling rate and 1-ns clock edge resolution. Because the echo response signals may require a high dynamic range, 12-b resolution is needed on the 24 digitizing channels.”
The speed and resolution requirements were handled by three Octopus CompuScope 8389 PC-based digitizer cards (Figure 4), each capable of 125-MS/s sampling at 14-b resolution on eight channels. If the application did not require a very precise time delay before acquisition, the cards could easily have been triggered using the built-in trigger-delay functionality. Instead, because trigger timing was exceptionally tight and the delay relatively long, an external trigger was generated by a separate PC-based arbitrary waveform generator.
Was this a scope application? No scope has 24 channels, but on the other hand, most data acquisition systems don't run at 125-MS/s with 14-b resolution. GaGeScope software works with the CompuScope digitizers to provide a PC-based scope display.
The captured data is acquired from many separate experiments run sequentially. This means that the capabilities to segment the digitizer memory and accurately control trigger timing are very important. Initially, the experiments' outputs must be characterized by the displayed signals from each of the digitizers. Because so many channels and separate sets of data are involved, it is planned that a custom software application will analyze the results.
Given the many conflicting factors that exist in real applications, trying to answer the what is a scope question may not be a useful exercise. Perhaps, a better way to spend your time is in understanding your test requirements thoroughly so that you can address them with the most appropriate instrument. If the product name includes oscilloscope, whether benchtop or PC-based, you can be relatively certain that it will provide a useful waveform display and a range of triggering capabilities. The bandwidth and sampling rate in a scope are high enough to acquire electronic signals.
In contrast, single- and multichannel digitizers are available with a wide range of speeds and resolutions. Recently, the term has been used to describe a general-purpose function in relation to synthetic instruments and, in this case, implies high-speed operation. GaGe's Ms. Faubert commented that her company's PC-based digitizers offered high resolution and up to 4 GB of memory in contrast to conventional scopes that are optimized for visualization.
Scopes have only voltage inputs, and most scopes may accept a much wider range of signal amplitudes than PC-based digitizers. Furthermore, a portfolio of probe technologies is available. With such scope probes, users can measure signals hundreds of volts in amplitude or with bandwidths exceeding 10 GHz.
Data acquisition systems typically have many channels, often with high resolution and very flexible signal conditioning but not blinding speed. Of course, the term data acquisition system has a general meaning that only adds to the confusion.
Throughout this discussion of PC-based scopes and related instruments, one factor continues to distinguish an oscilloscope: a highly responsive waveform display. If you don't need to display a signal's excursions in real time, any digitizing instrument with sufficient channels, accuracy, and speed can address your application.
If you do need to see how the input signal changes in time, buy a scope. And, if a PC-based scope provides the number of channels, speed, triggering, and memory depth you need, it can be a very cost-effective solution.
|FOR MORE INFORMATION||Click below|
|Cleverscope||CS328A 100-MHz Scope||Click here|
|GaGe/KineticSystems||Octopus CompuScope 8389 Digitizer||Click here|
|Link Instruments||DSO-8502 250-MHz Scope||Click here|
|National Instruments||NI USB-5133 100-MS/s Digitizer||Click here|
|Pico Technology||9201 12-GHz Sampling Scope||Click here|
|ZTEC Instruments||ZT4611 4-GS/s PXI Digitizer/Oscilloscope||Click here|