Analog or digital scope–which way to go?
As the prices of DSOs continue to drop, the decision is becoming more difficult
BY CHRIS MARTINEZ and ELY SHEMESH
Tektronix, Inc.
Beaverton, OR
In the past, the price differential between analog scopes and digital storage
oscilloscopes operating above 100 MHz has been as great as 200%. And most users
could not justify the higher price for DSOs. Over the last few years, however,
DSO prices have generally declined by as much as two-thirds or more, while
analog scope prices have remained essentially steady (see Fig. 1). Today, most
DSOs are comparably priced with analog scopes offering equivalent bandwidth
performance. Thus, there has been a dramatic shift toward the use of DSOs,
which now outsell analog scopes by almost 10 to 1 for scopes over 100 MHz.
With the price differential between DSOs and analog scopes no longer always the
primary consideration, are DSOs the best choice? The answer is a resounding
“maybe.” It depends on the application and on the type of signals with which
you need to work.
For many applications, DSOs now provide all the necessary functionality. Also,
DSOs continue to offer the unique capabilities that have contributed to their
rapidly increasing popularity. Still, there are applications where an analog
scope may be the best choice. This article takes a look at some of the factors
that enter into the decision of selecting either an analog scope or a DSO.
Viewing complex waveforms
Video, data communications, and various modulated waveforms are complex because
they contain both relatively low-frequency signals and relatively
high-frequency signals (see Fig. 2). Users analyzing these waveforms have
generally preferred an analog scope because it can capture and display both
high and low frequencies simultaneously, making it possible to analyze an
entire waveform at one sweep-speed setting. Analog scopes with sufficient
bandwidth operate without compromising signal representation because they
display an uncorrupted continuous waveform.
DSOs can sometimes fail at this task because they take the analog signal and
digitize it into several points spaced at regular intervals. Valuable data can
be lost between the points. In addition, too few sample points can cause
aliasing if the sampling rate is less than the Nyquist frequency (2.5 times the
frequency of the signal). The result is an incorrectly displayed waveform.
Very high sampling rates and long acquisition memories go a long way toward
solving these problems. Furthermore, some DSOs include a “peak detect” trigger
feature to trigger on glitches between the points. Let's look at these three
separate DSO features individually.
High sampling rates. Very fast sampling rates–often measured up into the
hundreds of Msamples/s range and even into the Gsamples/s range–result in less
information being lost between the sample points so the user can track higher
bandwidth signals more accurately. For single-shot acquisitions, the sampling
rate should be at least five times the bandwidth to accurately reconstruct the
waveform and achieve digital real-time performance. Fast sampling is not as
important with repetitive signals because the waveform can be reconstructed
over several triggered acquisitions.
Longer record lengths. Record length determines either the duration or the
resolution of the signal captured by the scope. A long record length might
provide better representation of more complex waveforms. A long record length
also allows the user to capture an event that occurs over a prolonged period
and sift through the data for problems. Record lengths should be consistent at
all sweep speed settings to ensure maximum resolution and ease of use. Of
course, a long record length usually adds to the DSO's price.
Peak detection. As DSO sampling rate decreases, a waveform is reconstructed
from fewer sample points. Peak detection uses all the available sample points
to display fast events at slow sweep speed settings. It ensures that the scope
will see fast events that would otherwise fall between the samples while
operating the scope at slower times-per-division. It is also useful for
ensuring that the displayed waveform is not aliased.
These three features make it possible for DSOs to measure complex signals.
However, DSOs still lack the simplicity of analog capture because the operator
must understand the effects of sample rate and record length on the signals.
And since DSO price is directly related to these performance parameters, it is
also important to know how much of each you will need if you are specifying a
scope for purchase.
Viewing intensity data
Viewing intensity data can be particularly important, especially if you need to
know how often a metastable condition or a glitch occurs in a circuit.
Intensity data also enables easy viewing of overlaying signals such as video
color bars. Analog scopes convey intensity information through variations in
brightness–the more frequent the signal, the brighter it is (see Fig. 3). The
brightness of the displayed signal indicates both that an event occurred and
how often it did so.
DSOs, however, cannot show how often an event occurs by varying display
brightness. That's because each time a new waveform is acquired, a DSO clears
the screen before displaying it. Thus, the brightness level is uniform for all
captured data.
While uniform display brightness enables easier, more precise viewing of
waveforms, the feature lacks the extra dimension of showing how often events
occur. A DSO's infinite persistence display mode operates by accumulating
acquired waveforms over time. However, since the data is pixelated, this
display mode cannot convey intensity information.
As shown in Fig. 3, some DSOs have a color-graded, variable-persistence display
mode that uses different colors to show how often events occur. For example,
red could show frequent events and violet infrequent events. This display mode
also offers the user flexibility because the degree of persistence can be
varied.
Viewing data with high update rates
DSOs have matched most of the advantages of analog scopes. However, analog
scopes still retain an edge when it comes to monitoring fast signal changes and
displaying rarely occurring events. Rare glitches can be frustratingly fast and
brief, occurring at irregular intervals. It may take hours or days to capture
random instances of noise, crosstalk, or switching pulses using a DSO in its
infinite persistence mode, if these instances can be captured at all (see Fig.
4). Also, the update rate of a DSO is often not fast enough to respond to
circuit changes caused by thermal changes, signal drift, jitter, or signal
recalibration. An analog scope can do a much better job spotting these problems
because it can capture up to about 500,000 waveforms a second. DSOs, on the
other hand, are limited to capturing only about 200 waveforms a second. Thus,
an analog scope is 2,500 times more likely to catch rapid, fleeting signal
changes!
DSO “holdoff” periods, the time during which DSOs process an image, may allow
crucial information to be lost. This must be considered when the time between
signal firings is less than the duration of the holdoff. The result can be a
tremendous loss of productivity for the user; glitches that an analog scope can
capture in a second may take more than an hour for the digital scope to detect.
Finding brief events may take longer with a DSO, but once found, it can be
easier to find root causes of a problem with the DSO than with an analog scope.
First the user can employ the DSO's persistence mode to find out what the event
looks like. Then the user can use the DSO's advanced triggering capabilities to
capture the specific event for further analysis. For example, if the user finds
a randomly occurring spike in the signal, he can use glitch triggers to isolate
the spike once he knows its approximate width. Then the user can employ the
DSO's built-in FFT capability to determine the origin of the spike. DSOs have a
clear advantage here because events that occur infrequently are only displayed
momentarily on an analog scope (see Fig. 5). With an analog scope, the user may
be lucky enough to capture the event with a camera, but he will probably waste
plenty of film trying! Even if he is lucky enough to get it on film, very
little analysis can be performed.
Exclusive advantages of DSOs
Beyond the ability of DSOs to match most of the traditional advantages of their
analog counterparts, users have turned to them for their distinct advantages:
* Automatic measurements and analysis. DSOs allow users to make many common
measurements, including rise time and peak-to-peak voltage, simply by pushing a
button. The built-in intelligence of microprocessor-based DSOs allows them to
automate these functions for tremendous gains in productivity, while ensuring
precise and accurate measurements. Automation is especially useful in
manufacturing test applications where either technicians or automated systems
must perform a continual series of highly repetitive measurements. Some
measurements can be derived from using cursors or counting grid scales.
However, many of today's DSOs can make measurements, such as phase and RMS,
that aren't possible using cursors.
The DSO ability to capture data and analyze it quickly and reliably is
extremely useful. Formerly, to analyze waveforms, data from analog signals had
to be recaptured in digital form and somehow introduced into a computer, an
expensive and difficult operation to set up. But because DSOs, by definition,
convert incoming signals into digital data, they can process the data
themselves for in-depth analysis. Some DSOs offer signal analysis capabilities
such as FFT, integration, and differentiation. Furthermore, DSOs with on-board
digital signal processors perform these operations in real time, allowing the
user to analyze signal changes as they occur. The FFT function has been
especially popular in scopes because it can convert time-domain signals into
frequency domain information used, for example, to find circuit noise sources.
* Storing and documenting data. The waveform on an analog scope is a fleeting
image on a CRT display. Photography can preserve this image, but it is
generally a cumbersome process with limited functionality. Furthermore,
photographs cannot be subjected to further data analysis.
DSOs excel in this area because the signal information they generate is
computer-readable and -storable binary data. Users can store waveform data in
the scope's memory or on disk for later recall to the screen or transfer to a
PC for software analysis. Also, waveforms can be saved in graphics formats for
insertion into word processing programs. This is an easy way to create
sophisticated documentation. For example, Tektronix TDS scopes can store data
in PCX, BMP, TIFF, EPS, RLE, and Interleaf formats.
These storage benefits have become increasingly important as portable scopes
become more popular. Waveforms collected on a floppy disk in the field or lab
can be brought back to the office for analysis. Also, “golden” waveforms can be
stored in portable scopes, allowing a technician to compare field readings to
benchmark waveforms.
Of course, DSOs can also generate copies of waveforms on a printer or plotter
connected directly to the scope's GPIB or Centronics port. And color DSOs can
make full-color WYSIWYG copies on a color printer.
* Ease of use. DSOs offer many more ease-of-use features to users than do
analog scopes. The challenge for digital scope manufacturers has been to add
these features without adding complexity and confusion for the user. Some DSOs
meet this challenge by using graphical icons on their display screens to
describe their features. This makes using a DSO as simple as operating a PC.
Furthermore, the added use of color on some scopes, which intuitively conveys
additional information, makes DSOs even easier to use because they can clearly
display several waveforms and corresponding measurements at once.
CAPTIONS:
Frontispiece (no caption.)
Fig. 1. DSO prices have generally declined while analog scope prices have
remained steady.
Fig. 2. Analog scopes are usually more accurate than DSOs when capturing
complex waveforms such as this video signal.
Fig. 3. The analog scope shows intensity data for this metastable condition
through variations in brightness, while the DSO shows intensity data through
color gradations.
Fig. 4. Tek's 24678 analog scope provides a visual writing speed of 4 div/ns,
allowing it to capture and display a very fast glitch in the signal. The DSO,
operating in its persistence mode, cannot identify this event.
Fig. 5. DSOs provide waveform analysis capabilities like FFT analysis that are
not available in analog scopes.
OVERLINE:
DSO or analog scope?
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