Signals and noise coupled
onto conductors via capacitive or inductive mechanisms have different
characteristics that can be used to advantage in design and
troubleshooting. Both coupling modes are explored using circuit models and examples of uses are given.
: Figure 1 shows capacitive coupling from a source onto a conductor terminated in loads ZL
The injected signal is a current coupled across the capacitance resulting in
current flowing in opposite directions on the conductor. For the purposes of
this discussion we will assume that the dimensions of the coupling
mechanism are a small fraction of a wavelength at the highest
frequencies of interest so that time and phase delays may be ignored
for simplicity. The two currents may be different if the circuit is not
symmetrical about the coupling point (for example if the ZL
loads are the not the same).
Current probes placed on the cable on opposite sides of the
coupling point with the same orientation would display waveforms on a
scope having inverted phase. If a ferrite core were placed to the
right of the coupling point, the impedance inserted into the wire by
the ferrite core would reduce the current to the right and in most
cases, increase the current to the left. This effect can be used in
troubleshooting equipment design problems and is covered in detail in
my new seminar "Failure Analysis and Prevention in Electronic Circuits (Design Troubleshooting for the Lab and Field)
Figure 2 shows the equivalent case for inductive coupling (mutual inductance). Instead
of injecting a current that splits, as in capacitive coupling, a series
voltage is induced in the coupled conductor that pushes a current along
the conductor. In Figure 2, the source current entering the inductor at
the "dot" side causes a voltage drop from left (+) to right (-). This
induces a source
voltage from left (+) to right (-) in the coupled conductor.
The coupled voltage in the (secondary) conductor is the same polarity as the drop in the primary,
but since it is a source instead of a drop, the current direction is
Note that the current is the same on both sides of the coupling point,
even if the two loads are not equal and the circuit is not symmetrical
(as long as the capacitive coupling between the inductors is
If a ferrite core is placed on either side of the coupling point, the
current will be reduced on both sides. With or without the ferrite
core, a pair of current probes on each side of the coupling point will
show the same current in the same direction. Although Figure 2 shows a
"transformer," inductive coupling can sometimes take the form of two
parallel conductors at frequencies of few MHz and higher.
Inductive Coupling Circuit
Most coupling I have encountered is either mostly capacitive or
inductive although there are significant cases where both types of
coupling come into play such as with SWR (s
atio) bridges on RF transmission lines.
Figures 3 and 4 show capacitive coupled current and inductively coupled current, respectively, from the March 2002
and the February 2002
Technical Tidbits on this website. In those articles, it was shown that
enough current can be coupled by either means from an IC onto a cable
to potentially cause EMC emissions problems. The opposite phase of
capacitively coupled current and the same phase of inductively coupled
current are clearly seen in Figures 3 and 4.
Capacitively Coupled Current
Inductively Coupled Current
If the circuit is symmetrical (ZL
loads are the same and the rest of the circuit is a mirror image) then the
capacitively coupled currents will be equal on both sides. The capacitively and inductively coupled currents are then easily
separated from each other and measured with current probes. If a pair of matched current probes are
placed on the coupled conductor on each side of the coupling mechanism in the same orientation and their outputs are
subtracted (either in the scope or by reversing one probe and adding in
the scope), the inductive component will be canceled leaving only the
total capacitively coupled current displayed. If the output of the probes are
added, the capacitive component of currents will be canceled leaving
only the inductive components displayed (with 2X amplitude).
For the non-symmetrical case, the situation gets complicated. Inductive
currents may flow in the capacitive coupling as well as along the wire
between the loads so simple addition or subtraction of the current
probes cannot be used to separate the components of the current.
Capacitive and inductive coupling modes produce different current
characteristics in the receiving circuit. The difference can affect how
a design performs. Capacitive or inductive coupling can also be used for troubleshooting,
allowing one to direct noise currents towards or away from a circuit
port, for example.
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.
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