Surprisingly, after more than a half century of advancements in oscilloscope architectures and implementations, improvements–even technological breakthroughs–are still being made. Most recently introduced models not only provide more features at lower cost, but some perform tasks that have never before been accomplished.
Almost all of today’s scopes are easier to use and many provide more built-in analysis capabilities. Some digital storage oscilloscopes (DSOs) have also become more analog-like.
The storage feature of the DSO is its most desirable attribute, because stored and quantized captured signal data is available whenever, and for however long, you need it. DSO shortcomings include signal representation limitations due to sampling, possible omission of transient capture between display updates, and lack of signal speed-to-trace intensity correlation.
Many new DSOs include circuitry to circumvent or overcome these problems so they mimic the characteristics of analog scopes. They also provide the analog feel with which many engineers and test professionals are more comfortable.
Easier to Use
Many oscilloscopes have long included auto-setup, a feature that, among other initializing functions, automatically sets the time base and vertical sensitivity to capture and display several cycles of an applied signal. A recent addition is autoranging, a continuous process which constantly adjusts the time base and sensitivity to maintain an optimal display of the waveform(s).
“Autoranging is one of the major oscilloscope technological advances of the last 12 months,” said Charles Holtom, Product Manager at Fluke. “It does much more than the now-common AutoSet.
“Until now, autoranging has been offered only on DMMs, where it greatly enhances their speed and ease of use. Recently, we introduced these same autoranging features to the DSO,” he said.
Another innovation that speeds operation is a new probe equipped with a user-programmable command switch near the tip. When used in conjunction with autoranging, it eliminates the need to repeatedly reach for the controls on the instrument panel. It can initiate different scope functions, such as AutoSet, and on the CombiScopesTM switch between DSO and analog modes, according to Mr. Holtom.
To facilitate examination of minute, not easily definable events in long records, faster processing power is becoming available. The LeCroy 9354L offers 8 million points of memory, and live or stored waveforms can be expanded up to 20,000 times. “To make it easier to examine fine signal details, we also provide a 9″ screen with 810 x 696 pixels,” commented Mike Lauterbach, Director-Product Management at LeCroy.
More measurements are performed automatically today. The HP 54750 High Bandwidth (20 GHz) Digitizing Oscilloscope has built-in measurement capabilities to automatically determine 50 types of signal parameters. Many mid-range scopes perform 25 different measurements individually or in programmable sequences.
Some scopes simplify the measurement task by becoming application-specific when a personality memory card is inserted. Others, such as Nicolet PowerPro, are inherently designed to be application-specific.
“The PowerPro scope is designed for the power industry,” explained Anne Dorn, Product Manager at Nicolet. “It is shielded to withstand hostile electrical fields and has built-in software for automatic power measurements such as voltage-flicker analysis, high-voltage impulse analysis, and real-time measurement of power parameters, such as power factor, apparent power and average power.”
Emulating Desirable Analog Features
Some oscilloscopes, such as the Fluke CombiScope, provide analog and DSO capabilities in the same instrument. You can make use of either set of features, choosing what is best for the type of signals to be observed or measurements to be taken.
Several companies now address the signal slew rate vs trace brightness correlation issue by providing both vector and dot-raster display modes. “Slew-rate intensity causes the scope trace to be brighter when the waveform slew rate is low, a characteristic normally only seen on an analog scope,” said Mark Lombardi, Product Marketing Engineer at Hewlett-Packard. “The HP 54600B series displays the waveform like an analog scope, by updating the screen with 1.5 million points per second and using a real-time vector display.”
To provide the analog feel that many users prefer, the knobs on the HP 54600B are laid out and labeled similarly to analog scopes. Three processors ensure that the scope responds instantly to the knobs.
“Most digital scopes sample a signal, process the data and subsequently display the resulting waveform. While the scope is processing and displaying, the signal is ignored,” added Mr. Lombardi. “The three-processor design of the HP 54600 series allows sampling, processing and displaying to take place in parallel.”
Ignoring incoming data during short intervals is inconsequential if signals are continuous or repetitive, but unacceptable for finding single events. Analog scopes can acquire and refresh signals to the screen several hundred thousand times per second, but still have trouble displaying extremely fast transients since the CRT phosphor is too slow.
DSOs can easily display these fast events once they have been captured. But a major shortcoming of DSOs has been the small fraction of time they actually spend in capturing waveforms.
The human eye gets an impression of rapid waveform capture when a DSO updates its display 60 times per second. For example, if the DSO is set at a sweep speed appropriate for displaying a 10-MHz clock, each display refresh will show about five cycles of this clock, or 500 ns. Observing 500 ns 60 times per second means acquiring data for 30 us out of every second, or 30 parts per million of all real time.1
To avoid such long dead times and ensure that a true representation of signal events is captured, Tektronix achieved a technical breakthrough, according to Richie Faubert, the company’s Instrument Business Unit General Manager. The TDS 700A TruCaptureTM DSOs employ a new architecture with an operational mode called InstaVuTM acquisition that performs more than 400,000 acquisitions per second.
InstaVu technology combines high-speed acquisition memory with high-speed rasterization. Dead time is decreased between acquisitions to 1.7 us, allowing users to find system glitches or observe waveform changes instantly.
Aliasing and modulation appear in their true form. Complex waveforms, such as video and radar signals, can be visually assessed. InstaVu acquisition also shows a true picture of crosstalk, jitter and signal interference that previously was available only with some analog oscilloscopes, according to the company.
Better Analysis
Efficient and accurate analysis requires that records to be examined must be of sufficient length, quantized with adequate resolution, and processed with convenient, appropriate mathematical tools. Extensive capabilities to fulfill all of these requirements, even for the most complex signals, are provided by top-of-the-line and, to varying degrees, by many mid-range oscilloscopes.
But for many applications, one of these characteristics may be more important than the others. A wide selection of models catering to any or all of these needs is available.
As the chart accompanying this article indicates, if you are looking for very long record length, LeCroy, Hitachi and Nicolet scopes capture and store up to 8 M, 2 M and 1 M points, respectively. Many scopes also contain hard or floppy disk drives to capture long records for post-acquisition analysis.
For applications requiring high vertical resolution, Nicolet extends the range of high precision with new products, which include 5-MS/s, 14-bit digitizers; 20-MS/s, 12-bit digitizers; and 75-MS/s, 10-bit digitizers. “Nicolet high-resolution scopes specialize in physical and mechanical measurements where high precision and low noise are very important. They feature differential inputs to eliminate ground loops and make floating measurements safely,” added Ms. Dorn.
The many mathematical facilities available today are exemplified by the new DataSYS 840 from Gould, a scope which also features selectable vertical resolution (12b to 10 MS/s, 8b to 150 MHz). Analysis features include FFT, live math of waveforms, digital filtering, energy calculations and trending of measurements (change of measurement over a period of time).
“Power can be derived by multiplying the rms of voltage by rms of current by the cosine of the phase difference,” said Michael McCorkle, Sales Engineer at Gould. “Measurements are made on any waveform, and multiple measurements can appear on screen or be printed on the built-in plotter. Scaling of measurements gives you the ability to see answers in their units, and view a voltage reading in psi and a time reading in degrees.”
These are only some examples of the many capabilities offered today. The chart lists a wide selection of DSOs. For more information on these instruments, circle the appropriate reader-service number.
Reference
1. InstaVuTM Acquisition: Setting the Benchmark in DSO Performance, A White Paper from Tektronix, Inc.
Copyright 1995 Nelson Publishing Inc.
January 1995