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The lumped circuit model has always been the basis of most calculations
used to predict signal behavior in a circuit. But when edge speeds are
more than four to six times faster than the signal path delay, the simple
lumped model no longer applies.
Circuit board traces just six inches long become transmission lines
when driven with signals exhibiting edge rates below four to six
nanoseconds, irrespective of the cycle rate. In effect, new signal paths
are created. These intangible connections aren’t on the schematics, but
nevertheless provide a means for signals to influence one another in
unpredictable ways.
At the same time, the intended signal paths don’t work the way they
are supposed to. Ground planes and power planes, like the signal
traces described above, become inductive and act like transmission
lines; power supply decoupling is far less effective. EMI goes up as
faster edge speeds produce shorter wavelengths relative to the bus
length. Crosstalk increases.
In addition, fast edge speeds require generally higher currents to produce
them. Higher currents tend to cause ground bounce, especially on wide
buses in which many signals switch at once. Moreover, higher current
increases the amount of radiated magnetic energy and with it, crosstalk.
Viewing the Analog Origins of Digital Signals
What do all these characteristics have in common? They are classic
analog
phenomena. To solve signal integrity problems, digital designers
need to step into the analog domain. And to take that step, they need
tools that can show them how digital and analog signals interact.
Digital errors often have their roots in analog signal integrity problems.
To track down the cause of the digital fault, it’s often necessary to turn
to an oscilloscope, which can display waveform details, edges and noise;
can detect and display transients; and can help you precisely measure
timing relationships such as setup and hold times.
Understanding each of the systems within your oscilloscope and how to
apply them will contribute to the effective application of the oscilloscope
to tackle your specific measurement challenge.
The Oscilloscope
What is an
oscilloscope
and how does it work? This section answers
these fundamental questions.
The oscilloscope is basically a graph-displaying device – it draws a
graph of an electrical signal. In most applications, the graph shows how
signals change over time: the vertical (Y) axis represents
voltage
and the
horizontal (X) axis represents
time
. The
intensity
or brightness of the
display is sometimes called the Z axis. (See Figure 2.)
This simple graph can tell you many things about a signal, such as:
The time and voltage values of a signal
The frequency of an oscillating signal
The “moving parts” of a circuit represented by the signal
The frequency with which a particular portion of the signal is occurring relative to
other portions
Whether or not a malfunctioning component is distorting the signal
How much of a signal is direct current (DC) or alternating current (AC)
How much of the signal is noise and whether the noise is changing with time
XYZs of Oscilloscopes
Primer
Z (intensity)
Y (voltage)
X (time)
Y (voltage)
X (time)
Z (intensity)
Figure 2.
X, Y, and Z components of a displayed waveform.
Summary of Contents for XYZs
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