February 22, 2005
An oscilloscope is sometimes called "the electronic
engineer's screwdriver," meaning it is a basic tool that
can be used for many different jobs. In the past, you could
use a small blade screwdriver on a wide range of screws. In
today's world, however, specialized screws like Torx, spline
and hex are commonplace and the blade screwdriver can no longer
get a wide variety of jobs done.
The same is true of the engineer's screwdriver, the oscilloscope.
Today's hardware-development engineer has specialized requirements
that are difficult, if not impossible, to meet with a "good-old"
traditional digital storage oscilloscope (DSO). Today's engineers
typically need an oscilloscope with:
- more channels to be able to view all the signals that
are controlling the system
- higher bandwidth to keep up with advances in memory and
processor technologies
- sustained sampling speed across time spans required for
serial data capture
- triggering to isolate critical information
- high-resolution displays that clearly show the fine details
of critical signals
- high-speed display systems that will capture infrequent
events
- connectivity to share information across a diverse work
group
This paper focuses on the first three of these needs - more
channels, higher bandwidth and sustained sampling. (Learn
more about oscilloscope displays from Agilent Application
Notes 1552, Oscilloscope Display Quality Impacts Ability
to Uncover Signal Anomalies publication number 5989-2003
and 1551, Improve Your Ability to Capture Elusive Events:
Why Oscilloscope Waveform Update Rates are Important publication
number 5989-2002.)
Changing channel-count requirements
When circuits were comprised of discrete components, the
traditional oscilloscope was a very powerful troubleshooting
tool. In Figure 1, we see a filter built up of operational
amplifiers and discrete components. A traditional dual-channel
oscilloscope can fully characterize the operation of this
filter by comparing the input to the output signals.
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Figure 1: Filter using 1980's
technology
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As digital technology became more pervasive, measurement
problems changed. With the move to a digital architecture,
electronic device performance increased and costs dropped.
The filter in Figure 2 can still be evaluated with a simple
dual-channel scope, but if the system has a problem, a traditional
scope cannot provide the requisite insight to resolve the
problem.
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Figure 2: Filter in current
digital technology
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The eight data lines driving the input to the digital signal
processor (DSP) and the DSP's eight output lines need to be
monitored to determine if the DSP is receiving correct information;
a dual-channel scope lacks sufficient channels for the task.
In this simple example, we can see that the traditional oscilloscope
- like the old screwdriver - is no longer adequate. Figure
3 shows how a mixed signal oscilloscope can give you additional
insight into the problem by displaying the eight data lines
input to the DSP, as well as both the input and output analog
signals.
In this example, the input (top yellow trace) and the output
(second purple trace) are displayed aligned in time with the
eight data lines driving the DSP. Notice how the relative
position of the data indicates the shape of the input signal.
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Figure 3: The mixed signal
oscilloscope shows the entire problem.
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Changing bandwidth requirements
The rapid development of PC technology has had a major effect
on the technology available to engineers developing embedded
systems for non-computing applications. Engineers are adopting
technology that recently was expensive and complex for use
in a wide variety of digital hardware designs. This trend
has created new requirements for "the engineer's screwdriver"
that were unheard of a few years ago. With the adoption of
PC technology advances, engineers need the ability to debug
serial data buses, and they need increased scope bandwidth.
The need for increased bandwidth is driven by increasing speeds
of the embedded processor's memory system. Memory systems
such as 200 MHz SDRAMs that were seen only in the highest-performance
computer systems a few years ago are now being applied to
embedded systems.
Figure 4 shows the memory used in embedded systems
as reported in the 2004 Embedded Market Survey conducted by
Embedded Systems Programming and EE Times. The bi-modal distribution
of this data aligns almost exactly with the host processor
used in the system. Engineers designing 8- and 16-bit processor-based
systems used 511k memory or less. Engineers designing 32-bit
processor and FPGA-based systems used memories of 32 M and
greater.
To troubleshoot these large memories, engineers need a scope
with bandwidth greater than 500 MHz. A 200 MHz clock will
be outside the range for good waveform reproduction by a 500
MHz scope. The old rule of thumb that the oscilloscope's bandwidth
needs to be four times the fundamental frequency of the signal
you measure, still holds true. Therefore, engineers need a
scope with a bandwidth of 800 MHz to make accurate measurements
on the 200 MHz clock of current SDRAM technology.
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Figure 4: Memory systems in
embedded system applications
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The need for sustained sampling speeds
These higher-performance systems also make use of serial
data communication technologies derived from PC technology.
These serial buses are being used because they have been proven
in the PC world and their cost is falling. Figure 5 shows
that more than 70 percent of all embedded systems developers
would consider using, or have used, some form of a serial
bus to interface between system components.
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Figure 5: Serial buses used
or under consideration for embedded systems
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To observe the flow of critical information in the design
of these serial buses, engineers need to be able to view wider
time spans. Additionally, users need to be able to isolate
specific events of interest, which requires the ability to
trigger on a specific address or data within the serial data
traffic.
Traditional shallow-memory DSOs might have the bandwidth
to capture these signals, but they fail to maintain their
sampling speeds when the time base is adjusted to a setting
that lets engineers observe an entire data packet. Figure
6 is a plot of the sampling speeds of the Tektronix TDS 3054B
oscilloscope and the Agilent DSO6054A oscilloscopes as a function
of sweep speed.
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Figure 6: Because of lack of
memory depth, the shallow-memory scope (TDS3054B) is
forced to decimate its sampling speed as time windows
are expanded.
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The drop in sampling speed, as sweep speed increases, directly
affects the oscilloscope's ability to accurately display the
information contained in serial data packets. A 10Base-T LAN
bus signal must be captured at 10 us/div, so a shallow-memory
scope will not be able to provide detailed information on
the content of the signal because it will be under sampled.
The higher-speed PCI bus requires a sweep speed of 1 us/div,
so the bus signal also will not be captured with the TDS3052
scope shown above, even though it appears to have an advantage
of higher basic sampling speed.
Conclusion
Compared to their predecessors, today's hardware development
engineers need higher bandwidth to keep up with advances in
memory and processor technologies; more channels to be able
to view all the signals that are controlling their systems;
and sustained sampling speed across time spans required for
serial data capture.
Agilent's new 6000 Series oscilloscopes provide the bandwidth
to enable accurate waveform viewing of high-speed signals
found in FPGA and 32-bit-based systems. The MSO versions of
these oscilloscopes provide the additional channels required
for triggering and viewing the complex operations of embedded
systems. With memory depths of up to 8 Mpts, these oscilloscopes
allow the capture and analysis of the serial buses in these
systems.
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Contacts:
Janet Smith, Agilent
+1 970 679 5397
janet_smith@agilent.com
Annie Lennon
Weber Shandwick, for Agilent
+1 503 552 3747
alennon@webershandwick.com
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