Measurement of high
amplitude impulsive signals, such as ESD, can incur errors due to
radiated effects. The characteristics of these effects are discussed
and ways to spot such errors in a measurement are given.
: Figure 1 shows an example of an impulse accompanied by radiated EMI, e
nterference, from the impulse source itself. Note the high frequency
noise in the waveform preceding the main impulse. Although it is
possible the waveform represents the true source, more likely, the high
frequency noise before the pulse represents error in the measurement.
The noise starts about 10 nanoseconds before the impulse. In this case, the EMI
generated by the impulse radiated into the end of the cable at the
scope. The impulse itself traveled down the cable at a slower speed
(~1.5 ns/foot) while the radiated EMI traveled faster in space (~1
ns/ft) and through a more direct route to the scope input, arriving at
the scope about 10 nanoseconds earlier than the impulse itself. Noise like this
on a waveform, occurring before an impulse, often is a sign of direct radiated
interference into the measurement. Figure 2 shows another example where
the noise starts about eight nanoseconds before the impulse.
Another Impulse with Radiated EMI Preceding It
Mechanisms whereby radiated EMI can affect a measurement in this way include:
- Poor shield connection at the oscilloscope either because of the
connector itself (BNC connectors vary widely in quality) or how the
shield interfaces the connector (360 degree, secure connection is best).
- Sometimes an impulsive source like ESD can be powerful enough to
leak through the shielding in a scope and directly affect internal
circuits of the scope.
- The quality of the shielded cable itself can be an issue. I
have seen BNC cables labeled as RG-58/U that provide poor shielding
effectiveness. In fact, those of you who have attended some of my
seminars/classes have seen an experiment that uses just such a
Characteristics of radiated EMI on the scope screen include:
- Noise starts before the impulse being measured.
- The noise, as displayed, often contains frequency components that are higher than the rest of the waveform.
- Subsequent events show similar delays between the start of the higher frequency noise and the impulse to be measured.
- The delay between the start of the noise and the impulse is affected by moving the source and/or cable.
Interference from impulsive sources to measurements can be significant.
There are several characteristics of such interference, the main one is
high frequency noise that starts before the intended pulse to be
Other articles on this website related to interference from impulsive sources are:
tutorial on this subject, covering background as well as more technical details, is available at:
If you like the information in this article and others on this website, much more information is available in my courses.
to see a listing of upcoming courses on design, measurement, and
troubleshooting of chips, circuits, and systems.