EMC For Dummies

EMC For Dummies
Electromagnetic Compatibility
Engineering
by Henry W. Ott
Designing for EMC

The system approach considers EMC throughout the design. EMC is
designed into– and not added onto– the product. 90% or more of the
potential problems can be eliminated prior to initial testing.
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US EMC Regulations

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US EMC Regulations
FCC A
FCC B
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US EMC Regulations
LISN: Line Impedance Stabilization
Network
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US EMC Regulations
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Typical Noise Path

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Methods of Noise Coupling

Conductively Coupled Noise – one of the most obvious, but often
overlooked, ways to couple noise into a circuit is on a conductor.
1. The solution is to prevent the wire from picking up the noise or to
remove the noise from it by filtering before it interferes with the
susceptible circuit.
2. The noise conducted into a circuit on the power supply leads.
3. The noise coupled into or out of a shielded enclosure by the wires
that pass through the shield.

Common Impedance Coupling – occurs when currents from two
different circuits flow through a common impedance.
1. This type of coupling usually occurs in the power and/or ground
system.
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Methods of Noise Coupling
2. In the power
distribution circuit,
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Use of Network Theory
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Signal Grounds

To reduce Vg ----
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Zg
Ig
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Signal Grounds

Chassis Grounds




Chassis ground is any conductor that is connected to the equipment’s
metal enclosure.
The key to minimizing noise and interference is to determine where
and how to connect the signal ground to the chassis.
It is important to establish a low-impedance connection between the
chassis and the circuit ground in the I/O area of the board.
Establishing a low-impedance connection between the circuit ground
and the chassis in the I/O area is also advantageous with respect to
radio frequency immunity.
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Balancing

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Power Supply Decoupling
2.
The capacitor should serve as a short circuit across the frequency
range over which the amplifier is capable of producing gain.
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Ferrites

Eliminate the
interference of lowlevel circuits by the
high-f
communication
noise.
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Ferrites

Impedance data for a typical ferrite core.
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Ferrites
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Ferrites

Ferrite core used as common-mode chokes on a USB cable to
suppress radiated emission
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Apertures

The amount of leakage from an aperture depends mainly on
the following 3 items:
1.
The maximum linear dimension, not area, of the aperture.
2.
The wave impedance of the EM field.
3.
The f of the field.
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Internal Noise Sources

Ground noise is created when output of gate 1 switches from high
to low.
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Digital Circuit Ground Noise


A grid spacing of 0.5 in. or less should be used in order to obtain the
most significant reduction of ground noise.
Ground grids
have been
used
successfully on
double-sided
boards at
frequencies up
to a few tens
of MHz.
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Forward

Differential-mode radiation from PCB

Although these signal loops are necessary for circuit operation, their
size and area must be controlled during the design process to
minimize the radiation.
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Forward

Common-mode radiation from system cables

Common-mode radiation is often harder to understand and control.
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Differential-Mode Radiation
Assuming the rise time equals
the fall time

n tr 

sin(
)
 sin(n d )  
T
I n  2 Id 


 n d   n tr 


T

For a 50% duty cycle (d=0.5), the first harmonic has an amplitude
I1  0.64 I and only odd harmonics are present.
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Differential-Mode Radiation

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Differential-mode radiated emission envelope vs. frequency
To minimize the emission, it is desirable to slow down the rise time
of the signal as much as functionally possible.
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Differential-Mode Radiation

The spectrum for a 6-MHz, 4-ns rise time, 35-mA clock signal in a 10cm2 loop.
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Controlling Differential-Mode Radiation

Board Layout


The most critical loops should be individually analyzed; however, most
other noncritical loops can be controlled by just using good PCB layout
practices.
The most critical loops are those that operate at the highest frequency
and where the signal is periodic.
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Controlling Differential-Mode Radiation


Frequency spectrum of the third harmonic of a 60-MHz clock with
and without dithering
Reducing the loop area, or providing canceling loops, only controls
the differential-mode emission and has no effect on the commonmode emission.
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Common-Mode Radiation

For a short dipole antenna:




4  107 ( f  I cm )sin 
E
r
(V/m)
For a real dipole antenna, the current goes to zero at the open
ends of the wire.
In practice, a more
uniform current
distribution can be
achieved if capacitor
loaded or top-hat
antenna:
This configuration is
approximated when the
antenna (cable)
connects to another
piece of equipment.
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Common-Mode Radiation



The common-mode radiation can be controlled by:
1.
Reducing the magnitude of the common-mode current
2.
Reducing the frequency or harmonic content of the current
3.
Reducing the antenna (cable) length
The primary method of minimizing the common-mode
radiation is to limit the common-mode current.
Common-mode radiated
emission envelope vs.
frequency
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Controlling Common-Mode Radiation


Filtering of the I/O cables can be accomplished by adding a high
impedance in series with the common-mode noise (e.g., a
common-mode choke or ferrite core), or by providing a lowimpedance shunt (a capacitor) to divert the common-mode
noise to “ground.”
Separate I/O Grounds

The I/O ground
plane should have
multiple connections
to the enclosure to
minimize its
inductance and
provide a low
impedance
connection.
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