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Dif ferential and
Common-Mode
Signals
Before digging too deep
into cause and effect, we
have to understand the two
types of signals we’ll be
dealing with: differential-mode (DM) and common-mode (CM). Figure 1 shows
DM and CM signals for
typical unshielded paired
wires at A, and a coaxial
cable at B. Remedies for
DM signals probably won’t
work for CM and vice versa.
DM signals are what we often
think of as “balanced” in that the
signal consists of identical currents
flowing in opposite directions along
two closely-matched paths — neither
of which is connected to a ground or
grounded enclosure. CM signals flow
equally on all conductors of a multiconductor cable or on the common
(shield) conductor of a coaxial or
shielded cable.
Taking a close look at Figure 1B,
you can see that a single cable can
support both DM and CM signals at
the same time. In fact, at RF, the
outside of a braided or foil shield is
electrically independent from the
inside! That’s why we use shielded
cables — to keep noise and radiated
signals from being picked up by the
conductor inside the shield.
Why does this happen? The skin
effect restricts AC current flow to the
outside of a conductor. As shown in
Figure 2, at frequencies above 1
MHz, current penetrates copper or
aluminum less than 0.1 mm!
What Causes RFI?
Interference itself can take a
variety of forms. If you are a radio
user — as hams are — interference
may just be higher noise levels or it
can be an actual spurious signal or
spur that obscures a desired signal or
upsets receiver operation. For
example, the harmonic of an
intentionally generated signal at an
integer multiple of a fundamental
frequency can fall on the same
frequency as a desired signal thereby
interfering with reception.
Spurious signals are a common
problem when RF leaks out of
equipment due to improper shielding
or cabling. Digital signals that
transition very quickly between
voltage levels are composed of
fundamental and many harmonics to
generate the sharp edges. The
harmonics appear all across the
RF spectrum into the microwave
region for high speed data.
Once radiated, they propagate
just like any other intentionally
radiated signal.
Switchmode or switching
power supplies or power
converters are another very
common source of spurious
signals. These power converters
work by “charging up” inductors
with magnetic energy, then
suddenly interrupting the
current to transfer the energy to
an output filter capacitor where
it is changed to steady DC.
The interruption of current
creates a wide spectrum of
spurious signals spaced at
intervals of the supply’s
switching frequency. If output
filtering is not designed properly
or — in a growing number of
instances for imported
supplies — not installed at
all, the strong spurious
signals can disrupt normal
communication throughout
an entire neighborhood.
Common uses of
switching converters include
lightweight wall wart DC
supplies, battery chargers,
electronic lighting ballasts,
and low voltage lighting.
Really strong signals can
overload a receiver to the
point where it causes
distortion of a desired signal
— even if the signal is
completely legal and not in the
frequency range being received. This
is called fundamental overload and is
caused by the receiver’s inability to
reject the out-of-band signal.
Inexpensive receivers such as
wireless phones or portable radios
have limited filtering, so are
susceptible to this type of
interference.
Maybe a receiver isn’t involved
at all and the interfering signal is
simply so strong that it disrupts the
proper operation of an electronic
FIGURE 1.
Common-mode
(CM) and
differential-mode
(DM) signals can
flow on parallel
conductor and
coaxial cables at
the same time
without mixing.
None of the
conductors need
to be grounded
for RF to pick up
and/or radiate RF
common-mode
signals.
FIGURE 2. Skin depth vs. frequency for some
materials. Red vertical line denotes 50 Hz
frequency: Mn-Zn, magnetically soft ferrite; Al,
metallic aluminum; Cu, metallic copper; steel
410, magnetic stainless steel; Fe-Si, grain-oriented electrical steel; Fe-Ni, high-permeability permalloy (80%Ni-20%Fe).
September 2015 63
