Scopes Shorten Time to Insight
by Tom Lecklider, Senior Technical Editor
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?
Distinguishing Characteristics
Visual Capabilities
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.
Sampling Modes
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).
Figure 1. 12-GHz PicoScope 9201 With Captured Eye Diagram
Courtesy of Pico Technology
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.
Time Quantization
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
Low Cost
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
Schröder, 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."
Small Size
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.
Schröder 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.
Large Display
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.
PC Environment
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.
Figure 2. Screen Shot Showing Virtual Controls and Persistence Display
Courtesy of ZTEC Instruments
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).
Figure 3. Application Areas of Digitizers and Oscilloscopes
Courtesy of National Instruments
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."
Figure 4. Octopus CompuScope 8389 PC-Based Digitizer
Courtesy of GaGe/KineticSystems
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.
Conclusion
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 |