November 2015 21
THE LATEST IN NETWORKING AND WIRELESS TECHNOLOGIES
Just how does EMI get into the
“victim” device anyway? Actually,
there are two ways: conducted and
radiated. Conducted EMI is sent
to a device via some wire or cable
or other direct connection. One
common way is via the AC power
line. Switching spikes from loads are
a nuisance and other noise on the
line. Even power line communications
(PLC) systems that use frequencies up
to 30 MHz to transmit data on the AC
mains can become EMI to someone.
EMI in the form of power supply
ripple and noise — especially from
switch-mode power supplies (SMPS)
— can be passed to another circuit or
unit by way of the DC power bus.
Radiated EMI gets into a
device by way of coupling via
mutual capacitance, induction, or
radio waves. Remember that any
current-carrying conductor creates
a magnetic field that can induce a
voltage into any nearby conductor. In
addition, signals in one conductor can
be coupled to another conductor by
Finally, conductors or
components in one circuit can act as
a transmitter that radiates a wireless
signal to nearby devices. Such
couplings mean that practically any
circuit can potentially interfere with
any other close circuit or device.
Keep in mind that any wire, printed
circuit board (PCB) trace, or other
conductor can be an antenna to send
or receive radiated EMI.
Preventing or Minimizing
EMI is becoming a huge
problem with the growing number
of electronic devices we use every
day. For that reason, steps have been
taken over the years to reduce EMI
to acceptable levels. For instance, the
Federal Communications Commission
(FCC) has rules and regulations
related to how much EMI must be
reduced in all electronic products.
Check out the extensive guidelines
outlined in Parts 15 and 18 of the
FCC’s Code of Federal Regulations
(CFR) 47 if you’d like to learn more.
All new products are designed with
the idea that EMI must be reduced.
Here’s some of the ways EMI is
minimized or controlled.
Isolate Circuits or Products. EMI
signals are usually weak but even so,
sensitive circuits like some with high
gain can still be compromised. Since
coupled signals decrease in amplitude
by a factor of the square or cube
of the distance between circuits or
equipment, the fastest and easiest
way to reduce EMI is to separate the
interfering units as much as possible.
When designing products, that
means isolating offending generators
like digital clocks from analog
circuits as much as possible. Good
PCB layout is critical. Keep copper
connections short and spaced
widely. Wiring should be short and
not parallel. As for radio equipment,
keep it as far away as possible from
one another and from TV sets, audio
devices, or anything else that may get
EMI (like a cordless phone).
Cabling. One way EMI is coupled
from one device to another is by
interconnecting cables. If cables are
run parallel to one another, you can
bet that some coupling will occur.
This is especially true of long cables.
Transferred signals by inductive
or capacitive coupling are called
crosstalk. To suppress EMI, separate
cables from one another as much as
possible, or run them at right angles
to each other.
Another solution is to use twisted
pair cable that minimizes coupling
and cancels common mode signals.
Shielded twisted pair is even better.
Coax cable is self-shielded, so is good
at minimizing noise and EMI pickup.
Even with this beneficial cabling, keep
the cables short and prevent them
from running in parallel with one
If giving products space does
not work and cabling is not an issue,
try one or more of the “big three”
solutions to EMI: filtering, grounding,
Filtering. If you experience
EMI, you can usually get rid of it
with a filter. Power line noise can be
suppressed with an AC line version.
This is a low pass filter that lets in only
the 60 Hz from the power line and
greatly attenuates all high frequency
signals. These are very effective and
most new products have such a filter
built in. If not, you can buy external
AC line filters to do the job.
Another form of filtering is DC
power bus decoupling. This is the
process of bypassing the DC line
with one or more capacitors to filter
out power supply ripple and noise.
In most cases, such decoupling
capacitors are also placed near each
IC power connection to suppress
In wireless applications, all
sorts of low and high pass filters are
available to attenuate undesirable
signals. In some cases, a notch
or band pass filter may be more
appropriate depending on the nature
of the offending signal.
Filters at the inputs or outputs of
equipment work great, but remember
that they also introduce attenuation
to desired signals and may cause
distortion of some digital signals that
may be unacceptable.
Grounding. Most electronic
circuits use a ground for common
connections. Poor grounds can
introduce EMI. Long ground
connections can be the cause of
ground loops that pick up signals or
introduce ground voltage drops that
couple one signal to another. Large
low resistance grounds are part of the
solution. Keeping ground connections
short is another requirement. Best of
all is to run all ground connections
to one common point to eliminate
Shielding. One way to stop one
signal from reaching a circuit is to use
a shield. A shield is a metallic barrier
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