The situation was summed up by Jorg Hesser, vice president of marketing at B&K Precision, “Key scope components such as FPGAs, DSPs, and A/D chips continue to increase in performance and decrease in price, which will further improve DSOs and reduce their cost. DSOs also benefit from trends in the IT industry. Adding USB, LXI, and LAN interfaces to low-cost instrumentation used to be cost prohibitive. With increasingly integrated and lower cost solutions available, this is no longer the case.
“Features such as digital filters and FFT are now widely implemented in software,” he continued, “thus minimizing the manufacturing cost. Consequently, features that were only typical in mid- to high-end scopes in the past are now increasingly migrating to low-cost instruments.”
Traditionally, there has been a clear distinction among types of scopes with benchtop for development, hand-held for troubleshooting in the field, and PC-based better suited to automated measurements. On the other hand, the case is made by Pico Technology that field techs and engineers already have a PC, so an add-on scope is very handy for them and an easily justified expense. Also, PC-based measurement systems aren't limited to only four channels, and they lend themselves better to special custom measurements and analysis.
Phil Stearns, product manager-economy oscilloscopes at Agilent Technologies, elaborated, “We have a good perspective because we offer both stand-alone and PC-hosted scopes. There was a time when budget-constrained R&D engineers embraced PC-hosted scopes because they had the lowest price for a given bandwidth. But,” he explained, “as displays and computing resources become less expensive, allowing a decrease in economy scope prices, more of the PC-hosted scope early adopters are returning to the traditional benchtop instrument.
“PC-hosted scopes definitely have a place: They are the lowest-cost option and take less space in a rack or test fixture. This makes them well suited for automated test applications where knobs and a display don't add value. Also, it's much easier to develop software in a PC environment than with an embedded operating system, so a PC-hosted scope offers the possibility of developing a feature-rich analysis scope at a low price.”
B&K's Mr. Hesser said, “We currently do not offer PC-based oscilloscopes. Our main market is in benchtop and, to a lesser degree, hand-held instrumentation. Most of our customers prefer benchtop DSOs because they are primarily interested in waveform viewing and less concerned about remote control, system integration, and high data throughput.
“A benchtop DSO can be used out of the box and is easier to operate, especially for users who are familiar with analog oscilloscopes,” he continued. “Bench instruments also are more flexible in terms of signal conditioning and tend to offer more complex triggering options. In the case of plug-in DSOs, many of our customers are concerned that the inherently noisy PC environment or PC software processing might adversely affect the signal integrity and measurement accuracy.”
Mr. Stearns' and Mr. Hesser's comments notwithstanding, Pico Technology's Jeff Bronks said, “As the capabilities of PC oscilloscopes continue to improve, the attractions of a traditional benchtop model will continue to decline. Furthermore, as laptop PCs replace desktop PCs, the large size of benchtop scopes will increasingly be seen as a handicap.”
Pico Technology manufactures only PC-based instruments. Extech Instruments, a FLIR company, makes many types of instruments, but all the scopes are hand-held. Not surprisingly, this company has yet another viewpoint. “Hand-held scopes are no longer glorified multimeters with wave-graphing capabilities,” according to André Rebelo at Extech. “Customers expect core bench scope capabilities in a hand-held form factor.
“The idea of a PC-hosted scope is impractical for many professionals that use hand-held scopes primarily for their portability, moving from one installation or job to another. Nevertheless,” he concluded, “adopting PC-based scopes is the most logical way to provide increasingly customized or frequently changing routines and software. Extech's focus emphasizes a mix of capabilities that are available via the scope or the PC.”
A Core of Common Features
Although scope manufacturers may have different views of the market and a need to differentiate their products from the competition, as Extech's Mr. Rebelo said, users still expect a core set of features. The large comparison chart that accompanies this article shows that all scopes have to provide inputs, a time base, triggering, memory, and a means of displaying the acquired waveforms.
A scope-function block diagram is shown in Figure 1. The solid lines indicate hardware connections that are found in all scopes: the analog input must connect to the ADC, and the memory must receive the digitized signal data. The position of the data within the memory is controlled by the trigger.
Dashed lines show that a great deal of architectural variability is possible. Some scopes only have digital triggering; some only have analog. Other scopes support both an external trigger source as well as an external clock, or only one or neither of these inputs may be present. High-end scopes may have dual time bases controlled by the trigger. The processing and display boxes represent functionality included in a benchtop or hand-held scope but done by a PC with a PC-based scope.
Scope feature sets have steadily increased over the years to the extent that most data sheets now resemble lists of headline capabilities. Unfortunately, especially for low-cost scopes, headlines may be all that's available: Detailed specifications often are not published. An instrument's performance could be very good, but without detailed, guaranteed specifications, you can't assume that data sheet values apply across a wide range of operating conditions.
Nevertheless, like Windows PC applications, many features have a common look and feel regardless of the manufacturer, form factor, or cost. Measurements are a good example of this, with broad adoption of automated approaches defined in IEEE 181-2003: IEEE Standard on Transitions, Pulses, and Waveforms as well as relatively uniform manual V and t cursor systems. If you have made these kinds of measurements on your current scope, you probably can make them on just about any scope, assuming the particular measurement you need is supported.
One way in which PC-based instruments save money is by performing as many functions as possible in the PC. So make sure that the functionality you need can be achieved with the basic scope and its included software. As the comparison chart shows, PC-based scopes can provide a large number of automatic measurements, but additional software sometimes is needed. This is the case, for example, with the National Instruments (NI) USB-5133 Scope that supports 40 automatic measurements by itself but more than 750 as well as mask testing when combined with LabVIEW.
Auto setup is found in all the scopes listed in the comparison chart. Digital filtering, FFT, and trace math capabilities are a close second in popularity, being supported by many scopes. These are good examples of functions that software provides at low cost but that enable entirely new classes of applications to be addressed.
Actually, there are quite a few common aspects to low-cost scopes, regardless of the form factor. Obviously, PC-based scopes use the PC's display so the resolution is set by the PC's monitor. Most benchtop and hand-held scopes are limited to quarter VGA (QVGA) resolution, primarily because of cost. A VGA display provides 480 pixels vertically and 640 horizontally. A QVGA has only 240 x 320 although several scopes use a color QVGA panel to help distinguish among traces.
The Tektronix DPO2012 features a 7″ diagonal wide QVGA (WQVGA) color panel with 234 x 480 pixels compared to many other manufacturers' scopes with a 5.7″ QVGA. However, the 100-MHz Tek product lists for $2,580 and is the lowest cost model in the series.
Part of what you get for this price is simplicity. Because both channels are sampled at 1 GS/s and each has a 1-Msample memory, there's no need to interleave samples to boost single-channel performance. And, 10x oversampling ensures waveforms are captured with high fidelity. In addition, digital phosphor oscilloscope technology, the DPO in the model number, provides intensity grading, and with the Wave Inspector pan and zoom feature, searching through the 1-M memory is much easier.
LeCroy's WaveAce WA232 is typical of a scope that interleaves acquisitions. It samples at 1 GS/s in dual-channel mode, doubling to 2 GS/s in single-channel operation. The memory is only 9k per channel, and it too doubles, becoming 18k in the single-channel mode. The 300-MHz WA232 is the highest performance model in the series and costs $2,390. A 60-MHz bandwidth model with 4k memory per channel is only $950. The Tek TDS1000B and TDS2000B Series directly compete with the WaveAce, the 40-MHz TDS1001B with 2.5-kS memory costing $900.
Generally, the lowest cost scopes have limited acquisition memory size. However, as memory cost has reduced, it's become less of a restriction. GW Instek, Link Instruments, NI, and Pico Technology all offer scopes with several megasamples of memory for less than $1,800.
Another area that affects cost is the scope's network interface. In the past, RS-232 was a popular but slow option while GPIB was much faster but also more expensive. USB 2.0 appears to have displaced both of these standards and in some cases even powers the scope.
The input attenuator, also a common scope circuit, performs the same function in all scopes. Nevertheless, specifications vary. Typical high-end scope ranges are from 2 mV/div to 5 V/div in a 1-2-5 sequence. Many low-cost scopes provide these ranges, but several extend the less sensitive end of the range to 10 V/div or even 20 V/div. Some models start at 5 mV/div rather than 2 mV/div. If you intend to use your scope on the most sensitive range, check the bandwidth specification. On this range, it's common for the bandwidth to be only 70% of the headline spec—only 70 MHz for a 100-MHz specification.
Generally, input sensitivity is specified in V/div; however, a few data sheets list volts full-scale. And this can be further complicated by vendors that have adopted something other than the standard 10 horizontal x 8 vertical division grid. Also, some data sheets mix direct ranges with ranges further attenuated by a 10:1 or 100:1 probe. You need to determine the attenuation ranges provided directly. Then it's easy to see the effect of a 10:1 or 100:1 probe.
Although these observations are true, hand-held scopes often have evolved from DMMs or at least combine scope and DMM functions. Accordingly, the input attenuation may extend well beyond the usual 5 V/div, even without scope probes. An example is Fluke's Model 125 ScopeMeter with 2 mV/div to 50 V/div and 500 V/div with the company's 10:1 VPS40 Probe.
This rating looks impressive, but the probe's maximum working voltage is specified to be 600-V CAT III or 1,000-V CAT II. The scope inputs also have a 600-V CAT III rating so the basic 50-V/div sensitivity could be quite useful for work with 120-V or even 240-V AC equipment suitably downstream in the power distribution system.
The time base is yet another common scope function. Unfortunately, there has been a trend to enhance specifications at the expense of clarity. Many scope data sheets combine expansion with sweep speeds under the heading of horizontal. Again, just as happens with vertical attenuation specifications, different types of quantities have been mixed together. If a scope's basic time base runs from 5 s/div to 50 ns/div, x10 expansion would allow the numbers to be changed to 5 s/div to 5 ns/div.
This implies a very fast scope, but in reality, it's only another way to indicate an expansion capability. This is easy to show by considering a scope's fastest sampling rate, time base, memory length, and display resolution. Agilent's Model DSO1024A is a good example because it does not offer equivalent time sampling (ETS), a mode that further complicates the calculations.
This scope specifies 1 ns/div as the fastest time-base rate but samples at 1 GS/s. A 1-ns/div horizontal rate is equivalent to only one sample per division, the rest of the displayed points being interpolated. The fastest time base really is 32 ns/div with an expansion of x32.
Agilent's Model DSO1024A was used as an example, but combining horizontal expansion and time base speeds is widespread. So too is the tendency to highlight a scope's ETS sampling rate because it generally is much faster than the single-shot acquisition capability. You really must be very careful when comparing data sheet specifications and even more so when determining if a scope actually will address your ETS application.
In ETS operation, successive cycles of a repetitive signal are sampled at slightly different times relative to the trigger, and these acquisitions are reassembled to form a complete waveform. This only is possible with sufficiently low trigger jitter. Each acquisition is referenced to the trigger, and if its position changes significantly compared to the sample interval, the resulting waveform will appear to be very noisy.
Innovations Make the Difference
When compiling the comparison chart, several innovative features stood out. Some of these may be more retro than novel, but in certain applications they are invaluable.
Alternate trigger is an old analog scope feature that rapidly switches the trigger source from one channel to the other so that both waveforms are stationary regardless of their actual time relationship. Most digital scopes allow channel 1 or 2 trigger source selection, but this means that if one signal repeats every 100 ms and the other takes only 99.9, one will be seen sliding through the display in relation to the other. Not so with alternate trigger.
Roll mode was an innovation on the earliest DSOs but mimics chart recorders, not scopes. Analog scopes don't store waveforms, and neither are they very useful at very slow sweep speeds, showing only a vertically bouncing dot progressing from left to right. In contrast, roll mode continuously appends new data at one end of the acquisition memory while discarding old data from the other end. The data is displayed starting from the last memory location written to, treating the memory like a recirculating shift register, so the waveform appears to slide from right to left with the newest samples at the right edge. If you are a medical researcher working with ECG waveforms, this is the scope mode you will be using.
Perhaps the most imaginative trend has been for PC-based scopes to encompass the functionality of several instruments, not just scopes. As an example, Link Instruments' MSO-9212 is a 200-MHz bandwidth, two-channel PC-based DSO with 2-Mword memory per channel, a 1-GS/s sampling rate, 12 additional cross-triggered logic channels, and serial bus trigger/decode. It also provides 40 types of automatic measurements, waveform averaging, simple math, and FFT as well as pass/fail testing. Serial trigger and decode, FFT spectral analysis, and pass/fail testing are via software. The software also includes four separately zoomable, scrollable windows and windows for I2C and SPI decoding. The MSO-9212 sells for $1,799.
Link Instruments also has the lowest cost PC-based scope in the chart: the MSO-19 at $249. This instrument combines a single-channel 60-MHz scope with eight logic channels and an 8-b pattern generator. In addition, the MSO-19 triggers on and analyzes I2C and SPI data and supports spectrum analysis through its FFT function.
You can spend tens of thousands of dollars on an expensive DSO to address high-speed applications. But for your money, you also get the usual 8-b resolution and no better than 3% amplitude accuracy. Pico Technology's Model 4424 boasts an impressive 12-b resolution with 1% accuracy and an enhanced acquisition mode with 16-b performance. This long-memory, four-channel DSO features a 1-Mbin FFT with several window types and magnitude, peak hold, and average display modes.
Also providing high-end functionality, GW Instek's GDS-2000 Series Scopes include a recall image capability that allows users to save X-Y displays so they can compare live hysteresis or safe operating area plots against a reference. Four-channel models in this series simultaneously display up to three different Y channels against one X.
By all means, buy a low-cost scope and save money. But, don't buy a cheap scope. As performance improves, more and more applications can be addressed by scopes like those in the comparison chart. No, they won't handle extremely fast signals because $2,500 will only buy 200- or 300-MHz bandwidth. However, there aren't many other general statements about low-cost scope limitations that you can make.
Your particular needs and preferences will determine whether benchtop, hand-held, or PC-based is the best form factor for your work. Of course, the process of identifying suitable scopes includes looking at prices as well as kicking tires. Some of the PC-based scopes have very impressive feature sets at a cost that's hard to ignore. On the other hand, if you find a scope awkward to use or if its long list of features is somewhat mutually exclusive, maybe it's not such a bargain after all.