If you still hesitate to rely on digital storage oscilloscopes (DSOs) because signal capture seems to require more setup skills than demanded by analog scopes, here’s some good news. Today’s DSOs can perform the task for you automatically—at least for the majority of signals you encounter. They also provide a range of capabilities to simplify capturing the other elusive events or signal anomalies.
Subsequent to data capture, you want the signal to be displayed in a readily comprehensible manner, requiring that a meaningful portion of the signal fills the screen. Preferably, it should be scaled and annotated showing engineering units commensurate with the signal’s nature.
Automatic display management can be performed by built-in auto-setup and auto-scaling circuits. Custom annotations and features may be provided by internal programs or personality cards and in some cases by application-oriented DSOs.
Auto-Scaling, Auto-Setup, Auto-Trigger
The auto-scale and auto-setup functions of oscilloscopes automatically adjust the vertical sensitivity and time-base settings to best match the characteristics of the applied signal. The process analyzers sample waveform segments, adjusts scaling and timing parameters, repeats acquisitions and readjusts settings until a sensible presentation is achieved.
“These automatic features are extremely beneficial considering the wide ranges in voltage (typically from 1 mV to 10 V/div) as well as time (200 ps to 10 s/div) over which oscilloscopes acquire signals,” commented Kim Hartman, Tektronix business development manager for oscilloscopes. Consequently, most DSOs and some analog scopes provide these functions.
To achieve auto-setup in the absence of a meaningful external trigger signal, an auto-trigger feature is also needed. Triggering tells the oscilloscope to start the horizontal sweep across the CRT at a speed determined by the time-base setting. With appropriate trigger and sweep-speed settings, a stable waveform display representing either the analog scope’s amplifier output or the DSO’s digitized values automatically appears on the screen.
Many, but not all, DSOs contain extensive auto-triggering facilities. “All of LeCroy’s DSOs offer both auto-setup and auto-trigger,” said Michael Lauterbach, director of product management at LeCroy. “These features work on any signal with amplitudes ranging from 2 mV to 40 V, frequencies above 50 Hz and duty cycles greater than 0.1%.”
To increase the likelihood of automatically capturing and presenting an overview of the applied signal, most DSOs only invoke edge triggering with elementary voltage-threshold or timing qualifiers. Auto-triggering is mainly appropriate when dealing with relatively noncomplex repetitive signals or as a starting point for a more detailed evaluation. In the latter case, the auto-trigger feature is turned off after initial acquisition.
Capturing Single Events and Anomalies
“Given the vast number of ways a signal may vary from any given norm, it’s not desirable to require the oscilloscope in the auto-setup mode to figure out what is wrong or where an aberration occurred,” commented Ms. Hartman. “Instead, the oscilloscope should give you a tool that depicts the greatest amount of information plus a set of advanced trigger functions to explore the subtle details.
“An example of scope technology that locates signal abnormalities is Tektronix’ InstaVu™ acquisition implementation,” Ms. Hartman continued. “It allows you to see the signal in minute detail and to pinpoint the problem, and then directs you to the correct triggering mode.”
Today’s DSOs offer a wide choice of triggering modes, including slew rate, runt, glitch, pulse and dual-threshold triggering. “In addition to conventional edge and auto-level triggering features, some scopes include triggering capabilities normally found only in logic analyzers. The HP54645D, for instance, has pattern, glitch, time qualification and sequential triggering,” commented Johnnie Hancock, program manager at Hewlett-Packard.
“In this microprocessor age, logic state and pattern triggering are very useful,” added Hilton Hammond, product specialist at Fluke. “For example, in a four-channel DSO, you can set up selected channels to wait for specific conditions to occur on the microprocessor’s interrupt request lines before drawing the trace on the CRT. Time- qualified triggering (signal period being
Signal aberrations that occur infrequently, such as those caused by intermittent problems or by multi-interrelation-dependent malfunctions, are among the most difficult to capture. Since we are looking for an event that is relatively unpredictable in time as well as in magnitude, setting up suitable triggering conditions can be difficult and time- consuming.
But even for this difficult problem, a solution has recently been implemented by LeCroy as the exclusion trigger function. This feature operates on the principle that you first define the normal signal characteristics, such as pulse width or pulse-period range. The scope will then not trigger on normal signals and will keep its trigger circuit actively hunting for the rare abnormalities.
In contrast to transients and single-events, many signals have repetitive characteristics, enabling the built-in intelligence of the DSO to pinpoint any aberrations and to simplify finding their causes. “With video, for instance, certain signal parameters are a given and do not readily change, except between standards such as NTSC or PAL,” said Mr. Hammond.
“The oscilloscope’s built-in intelligence can be used on these types of industry- standard signals with relative ease. Because we know where each sync pulse occurs, the scope may be factory-programmed to automatically set up the hold-off, main and delayed time-base settings to provide the signal detail required for troubleshooting or measurement,” Mr. Hammond concluded.
Application-Specific DSOs
Many of today’s oscilloscopes address evolving applications. “For instance, the proliferation of electronic components in what used to be primarily electrical equipment is causing a change in the type of test equipment required by manufacturing test, facility service, repair and installation personnel,” said Roman Petrovitch, business development manager for hand-held instruments at Tektronix.
“These professionals must make highly sophisticated and accurate scope-type, measurements on the factory floor and in the field. The result has been an increasing demand for hand-held, battery-operated oscilloscope/DMM products, a trend which spurred the introduction of the TekScope THS700 Series of hand-held oscilloscope/DMMs,” he continued.
For instance, the THS720, a two-channel, 100-MHz, 500-MS/s DSO and true rms DMM, captures and measures short-duration (8 ns) impulses and triggers on motor-start as well as on video signals. The unit also incorporates a patented isolated-channel architecture allowing you to make up to 600-Vrms measurements safely. This is particularly useful for installing and troubleshooting variable-speed drives, motors, controllers and electronic ballasts.
Similarly, Fluke offers the 90 Series ScopeMeters which accept a series of attachments and provide built-in intelligence. The Fluke 99B, for example, accepts a variety of probes including thermocouples and infrared noncontact temperature probes and pressure/vacuum modules. With these attachments, the instrument is task specific and easy to use. Series 90 instruments are now used extensively in the electronic, electrical, automotive and HVAC markets.
The Gould 770A Synchroscope is an example of a general-purpose/application-specific scope that facilitates detailed investigations of electro and mechanical phenomena. “Not only do you get a full-function DSO with four channels, a color
display, a built-in floppy disk and an optional thermal plotter, the instrument can be synchronized to the rotation of an external shaft,” explained Tom Lecklider, product marketing manager at Gould.
“The most frequent application for this scope is in automotive engine development, but some units are attached to gyroscopes and others to jet engines and large diesel generators. Engineers in these industries think in degrees of rotation rather than in microseconds. As a result, it is fundamental to have an instrument that acquires and measures signals relative to shaft angular position rather than as a function of time,” Mr. Lecklider concluded.
Other general-purpose scopes now provide software add-on packages to simplify or speed up repetitive application-oriented data gathering and analysis tasks. “LeCroy makes two kinds of special applications-analysis packages for digital scopes,” said Dr. Lauterbach. “The first is for manufacturers of disk drives and heads, media, chips and controllers who must measure more than 20 different parameters specified by IDEMA. The other is for the optical recording industry such as digital video disks, CD-ROM and magneto-optical disks to measure pit width, pit-to-pit spacing and other industry-specific parameters.”
Rather than being industry-specific, other DSOs enable you to cope better with the challenges posed by recent technological advancements, such as those incorporated in multimedia and digital communications equipment. For instance, the new HP54645D Mixed-Signal Oscilloscope combines analog and digital circuitry to easily capture and correlate real-world input or output analog signals along with higher-speed digital control signals.
Trends
The trend toward a more task-oriented oscilloscope is expected to continue particularly for field-service use and for automotive, power, physical-phenomena-related and mixed-signal applications. “However, there is also a trend toward a wider range of measurement and analysis facilities within a seemingly general-purpose instrument,” observed Mr. Lecklider. “These instruments represent excellent value because they are not only task specific, but also are configurable.”
More oscilloscopes will be preprogrammed or equipped with personality cards to simplify taking repetitive measurements as required by regulatory agencies. They will also make it easier to capture and deal with standardized, but complex, signals encountered in digital communications and multimedia.
Signal capture of elusive events will become even easier than it is today because not only top-of-the-line but also low-cost oscilloscopes will provide more triggering features. But there is no need to worry that these added capabilities will lead to confusion.
“Most engineers don’t need to know all the triggers, just the few that are particularly common for their applications,” said Mr. Lauterbach. “For instance, an engineer performing logic analysis might use the pattern trigger, disk-drive engineers the ‘delay by nth event’ feature to trigger on certain sectors, and communications engineers the dropout and interval triggers.”
Most companies provide extensive educational material on triggering issues. “The idea is to read the application note, find the types of triggers that are useful for your application then learn just those and ignore the rest. We have even put an abbreviated version of such application notes in our 1997 catalog,” added Dr. Lauterbach.
Copyright 1997 Nelson Publishing Inc.
January 1997