EMI/RFI Board Design

EMI/RFI Board Design
National Semiconductor
Application Note 643
Joe Cocovich
December 1989
INTRODUCTION
The control and minimization of Electro-Magnetic Interference (EMI) is a technology that is, out of necessity, growing
rapidly. EMI will be defined shortly but, for now, you might be
more familiar with the terms Radio Noise, Electrical Noise, or
Radio Frequency Interference (RFI). The technology’s explorations include a wide frequency spectrum, from dc to
40 GHz. It also deals with susceptibility to EMI as well as the
emissions of EMI by equipment or components. Emission
corresponds to that potential EMI which comes out of a piece
of equipment or component. Susceptibility, on the other
hand, is that which couples from the outside to the inside.
In HPC designs to date, we have looked at noise situations
ranging from 2 MHz to 102 MHz. EMI, in some cases, can
affect radio reception, TV reception, accuracy of navigation
equipment, etc. In severe cases, EMI might even affect
medical equipment, radar equipment, and automotive systems.
This Application Note will define ElectroMagnetic Interference and describe how it relates to the performance of a system. We will look at examples of Inter-system noise and
Intra-system noise and present techniques that can be used
to ensure ElectroMagnetic Compatibility throughout a system and between systems.
We will investigate and study the sources of noise between
systems through wire-harness and backplane cables and
connectors. Active circuit components can be contributors of
noise and be susceptible to it. The fast switching times of
CMOS devices fabricated in today’s technology can cause
incredible noise in a system. This noise typically is made up
of crosstalk, power supply spiking, transient noise, and
ground bounce.
The minimization and suppression of EMI can be obtained
by utilizing proper control techniques. Intra-system noise,
noise within a single module, sometimes can be controlled
with methods such as filtering, shielding, careful selection of
components, and following good wiring and grounding procedures. Controlling noise between systems, Inter-system
noise, uses subtler techniques such as frequency management and time management, etc.
DESCRIPTION OF NOISE
Appropriate time and resources should be spent during the
design of a system or systems to insure that no problems will
be encountered due to effects of EMI. Design guidelines will
be presented that can be used to increase ElectroMagnetic
Compatibility between systems by reducing the effects of
noise between them. Above all, don’t forget that the development tools used are also systems and are important to consider in your planning.
ElectroMagnetic Interference
EMI is a form of electrical-noise pollution. Think of the time
when an electric drill or some other power tool jammed a
nearby radio with buzzing or crackling noises. Sometimes it
got so bad that it prevented you from listening to the radio
while the tool was in use. Or the ignition of an automobile
idling outside your house caused interference to your TV picture making lines across the screen or even losing sync altogether making the picture flip. These examples are quite annoying but not catastrophic.
EMI/RFI Board Design
EMI/RFI Board Design
More serious, how about a sudden loss in telephone communication caused by electrical interference or noise while
you are negotiating an important business deal? Now EMI
can be economically damaging.
The results of EMI incidences can be even farther reaching
than these examples. Aircraft navigation errors resulting
from EMI or interruption of air traffic controller service and
maybe even computer memory loss due to noise could
cause two aircraft to collide resulting in the loss of lives and
property.
These were just a few examples to help you identify the results of EMI in a familiar context. To help understand an
ElectroMagnetic Interference situation, the problem can be
divided into three categories. They are the source, the victim, and the coupling path. Secondary categories involve the
coupling path itself. It the source and victim are separated by
space with no hard wire connection, then the coupling path is
a radiated path and we are dealing with radiated noise. If the
source and victim are connected together through wires,
cables, or connectors, then the coupling path is a conducted
path and we are dealing with conducted noise. Incidentally,
both types of noise can exist at the same time.
ElectroMagnetic Interference Situation
AN010562-1
A brief look will be taken at the environment and tools required for different levels of noise testing. Relative risk-costs
between preparing for EMC or excluding EMI concerns from
the project will be listed.
AN-643
© 1998 National Semiconductor Corporation
AN010562
www.national.com
nals causing “glitches” on adjacent steady state signal lines
(crosstalk) causing erratic operation, mutiple simultaneously
switching logic outputs propagating ground bounce noise
throughout system, etc.
ElectroMagnetic Compatibility
If you think about the examples given, one can understand
that EMI or electrical noise is of national concern. The Government and certain industry bodies have issued specifications with which all electrical, electromechanical, and electronic equipment must comply. These specifications and
limitations are an attempt to ensure that proper EMC techniques are followed by manufactures during the design and
fabrication of their products. When these techniques are
properly applied, the product can then operate and perform
with other equipment in a common environment such that no
degradation of performance exists due to internally or externally conducted or radiated electromagnetic emissions. This
is defined as ElectroMagnetic Compatibility or EMC.
Coupling Paths
The modes of coupling an emitter source to a receptor victim
can become very complicated. Remember, each EMI situation can be classified into two categories of coupling, conducted and radiated. Coupling can also result from a combination of paths. Noise can be conducted from an emitter to a
point of radiation at the source antenna, then picked up at
the receptor antenna by induction, and re-conducted to the
victim. A further complication that multiple coupling paths
presents is that it makes it difficult to determine if eliminating
a suspected path has actually done any good. If two or more
paths contribute equally to the problem, eliminating only one
path may provide little apparent improvement.
Inter-System EMI
For the purpose of this Application Note, when the source of
noise is a module, board, or system and the victim is a different and separate module, board, or system under the control
of a different user, that is considered to be an inter-system
interference situation. Examples of inter-system interference
situations could be a Personal Computer interfereing with
the operation of a TV or an anti-lock brake module in a car
causing interference in the radio. This type of interference is
more difficult to contain because, as mentioned earlier, the
systems are generally not under the control of a single user.
However, design methods and control techniques used to
contain the intra-system form of EMI, which are almost always under the control of a single user, will inherently help
reduce the inter-system noise.
Conducted Interference
In order to discuss the various ways in which EMI can couple
from one system to another, it is necessary to define a few
terms. When dealing with conducted interference, there are
two varieties that we are concerned with. The first variety is
differential-mode interference. That is an interference signal
that appears between the input terminals of a circuit. The
other variety of conducted interference is called
common-mode interference. A common-mode interference
signal appears between each input terminal and a third
point; that third point is called the common-mode reference.
That reference may be the equipment chassis, an earth
ground, or some other point.
Let’s look at each type of interference individually. In Figure
1 we show a simple circuit consisting of a signal source, VS,
and a load, RL. In Figure 2 we show what happens when
differential-mode interference is introduced into the circuit by
an outside source. As is shown, an interference voltage, VD,
appears between the two input terminals, and an interference current, ID, flows in the circuit. The result is noise at the
load. If, for instance, the load is a logic gate in a computer,
and the amplitude of VD is sufficiently high, it is possible for
the gate to incorrectly change states.
Intra-System EMI Manifestations
AN010562-2
This Application Note will address problems and solutions in
the area of intra-system noise. Intra-system interference
situations are when the sources, victims, and coupling paths
are entirely within one system or module or PC board. Systems may provide emissions that are conducted out power
lines or be susceptible to emissions conducted in through
them. Systems may radiate emissions through space as well
as be susceptible to radiated noise. Noise conducted out antenna leads turns into radiated noise. By the same token, radiated noise picked up by the antenna is turned into conducted noise within the system. A perfect example is ground
loops on a printed circuit board. These loops make excellent
antennas. The system itself is capable of degrading performance due to its own internal generation of conducted and
radiated noise and its susceptibility to it.
AN010562-3
FIGURE 1.
Some results of EMI within a system: Noise on power line
causing false triggering of logic circuits, rapidly changing sig-
www.national.com
2
tenna in the presence of this field. As such, an interference
current, IN, and an interference voltage, VN, are induced in
the circuit. The magnitude of the induced interference signal
is roughly proportional to the frequency of the incoming field,
the size of the loop, and the total impedance of the loop.
Cable-to-cable coupling occurs when two wires or cables are
run close to one another. Figure 5 shows how cable-to-cable
coupling works. Figure 5a shows two lengths of cable (or
other conductors) that are running side-by-side. Because
any two conducting bodies have capacitance between them,
called stray capacitance, a time-varying signal in one wire
can couple via that capacitance into the other wire. That is
referred to as capacitive coupling. This stray capacitance, as
shown in Figure 5c makes the two cables behave as if there
were a coupling capacitor between them. Another mechanism of cable-to-cable coupling is mutual inductance. Any
wire carrying a time-varying current will develop a magnetic
field around it. If a second conductor is placed near enough
to that wire, that magnetic field will induce a similar current in
the second conductor. That type of coupling is called inductive coupling. Mutual inductance, as shown in Figure 5b,
makes the cables behave as if a poorly wound transformer
were connected between them. In cable-to-cable coupling,
either or both of those mechanisms may be responsible for
the existance of an interference condition. Though there is
no physical connection between the two cables, the properties we have just described make it possible for the signal on
one cable to be coupled to the other.
AN010562-4
FIGURE 2. Differential-Mode Interference
Figure 3 shows what happens when a ground loop is added
to our circuit. Ground loops, which are undesirable current
paths through a grounded body (such as a chassis), are usually caused by poor design or by the failure of some component. In the presence of an interference source,
common-mode currents, IC, and a common-mode voltage,
VC, can develop, with the ground loop acting as the
common-mode reference. The common-mode current flows
on both input lines, and has the same instantaneous polarity
and direction (the current and voltage are in phase), and returns through the common-mode reference. The
common-mode voltage between each input and the
common-mode reference is identical.
AN010562-5
FIGURE 3. Common-Mode Interference
AN010562-7
FIGURE 5. Cable-to-Cable Coupling
Either or both of the above-mentioned properties cause the
cables to be electromagnetically coupled such that a
time-varying signal present on one will cause a portion of
that signal to appear on the other. The “efficiency” of the coupling increases with frequency and inversely with the distance between the two cables. One example of
cable-to-cable coupling is telephone “crosstalk”, in which
several phone conversations can be overheard at once. The
term crosstalk is now commonly used to describe all types of
cable-to-cable coupling.
AN010562-6
FIGURE 4. Field-to-Cable Coupling
Radiated Interference
Radiated coupling itself can take place in one of several
ways. Some of those include field-to-cable coupling,
cable-to-cable coupling, and common-mode impedance
coupling. Let’s look at those types of coupling one at a time.
The principle behind field-to-cable coupling is the same as
that behind the receiving antenna. That is, when a conductor
is placed in a time-varying electromagnetic field, a current is
induced in that conductor. That is shown in Figure 4. In this
figure, we see a signal source, VS, driving a load, RL. Nearby
there is a current carrying wire (or other conductor). Surrounding the wire is an electromagnetic field induced by the
current flowing in the wire. The circuit acts like a loop an-
Common-mode impedance coupling occurs when two circuits share a common bus or wire. In Figure 6 we show a circuit that is susceptible to that type of coupling. In that figure
a TL092 op-amp and a 555 timer share a common return or
ground. Since any conductor (including a printed circuit
board trace) is not ideal, that ground will have a non-zero impedance, Z. Because of that, the current, I, from pin 1 of the
555 will cause a noise voltage, VN, to develop; that voltage is
equal to I x Z. That noise voltage will appear in series with
the input to the op-amp. If that voltage is of sufficient amplitude, a noise condition will result.
3
www.national.com
While not all inclusive, these coupling paths account for, perhaps, 98% of all intra-system EMI situations.
AN010562-9
FIGURE 7. Cables and Connectors
Connectors usually are needed to terminate cables. When
no cable shields or connector filters or absorbers are used,
connectors play essentially no role in controlling EMI. The influence of connector types, however, can play a major role in
the control of EMI above a few MHz. This applies especially
when connectors must terminate a cable shield and/or contain lossy ferrites or filter-pins.
Connectors and cables should be viewed as a system to
cost-effectively control EMI rather than to consider the role of
each separately, even though each offers specific interference control opportunities.
AN010562-8
FIGURE 6. Common-Mode Impedance Coupling
NOISE SOURCES
In this Application Note, we will look at sources of EMI which
involve components that may conduct or radiate electromagnetic energy. These sources, component emitters, are different from the equipment and subsystems we have been talking about. Component emitters are sources of EMI which
emanate from a single element rather than a combination of
components such as was previously described. Actually,
these component emitters require energy and connecting
wires from other sources to function. Therefore, they are not
true sources of EMI, but are EMI Transducers. They convert
electrical energy to electrical noise.
Components
Under conditions of forward bias, a semiconductor stores a
certain amount of charge in the depletion region. If the diode
is then reverse-biased, it conducts heavily in the reverse direction until all of the stored charge has been removed as
shown in Figure 8. The duration, amplitude, and configuration of the recovery-time pulse (also called switching time or
period) is a function of the diode characteristics and circuit
parameters. These current spikes generate a broad spectrum of conducted transient emissions. Diodes with mechanical imperfections may generate noise when physically
agitated. Such diodes may not cause trouble if used in a
vibration-free environment.
Cables and Connectors
The three main concerns regarding the EMI role of cables
are conceptualized in Figure 7. They act as (1) radiated
emission antennas, (2) radiated susceptibility antennas, and
(3) cable-to-cable or crosstalk couplers. Usually, whatever is
done to harden a cable against radiated emission will also
work in reverse for controlling EMI radiated susceptibility.
The reason for the word usually, is that when
differential-mode radiated emission or susceptibility is the
failure mode, twisting leads and shielding cables reduces
EMI. If the failure mechanism is due to common-mode currents circulating in the cable, twisting leads has essentially
no effect on the relationship between each conductor and
the common-mode reference. Also cable shields may help or
aggravate EMI depending upon the value of the transfer impedance of the cable shield. Transfer impedance is a figure
of merit of the quality of cable shield performance defined as
the ratio of coupled voltage to surface current in ohms/meter.
A good cable shield will have a low transfer impedance. The
effectiveness of the shield also depends on whether or not
the shield is terminated and, if so, how it is terminated.
www.national.com
4
AN010562-12
AN010562-10
FIGURE 10.
FIGURE 8. Diode Recovery Periods and Spikes
Power Supply Noise
Power-supply spiking is perhaps the most important contributor to system noise. When any element switches logic
states, it generates a current spike that produces a voltage
transient. It these transients become too large, they can
cause logic errors because the supply voltage drop upsets
internal logic, or because a supply spike on one circuit’s output feeds an extraneous noise voltage into the next device’s
input.
With CMOS logic in its quiescent state, essentially no current
flows between VCC and ground. But when an internal gate or
an output buffer switches state, a momentary current flows
from VCC to ground. The switching transient caused by an
unloaded output changing state typically equals 20 mA peak.
Using the circuit shown in Figure 9, you can measure and
display these switching transients under different load
conditions.
AN010562-13
FIGURE 11.
AN010562-14
FIGURE 12.
AN010562-11
FIGURE 9.
Figure 10 shows the current and voltage spikes resulting
from switching a single unloaded (CL = 0 in Figure 9) NAND
gate. These current spikes, seen at the switching edges of
the signal on VIN, increase when the output is loaded. Figure
11, Figure 12, and Figure 13 show the switching transients
when the load capacitance, CL, is 15 pF, 50 pF, and 100 pF,
respectively. The large amount of ringing results from the
test circuit’s transmission line effects. This ringing occurs
partly because the CMOS gate switches from a very high impedance to a very low one and back again. Even for
medium-size loads, load capacitance current becomes a
major current contributor.
AN010562-15
FIGURE 13.
Although internal logic generates current spikes when
switching, the bulk of a spike’s current comes from output
circuit transitions. Figure 14 shows the ICC current for a
NAND gate, as shown in the test circuit, with one input
5
www.national.com
switching and the other at ground resulting in no output transitions. Note the very small power-supply glitches provoked
by the input-circuit transitions.
Logic Family Comparisions
CharacLS/ALS
Symbol
HCMOS
teristic
TTL
AC
ACT
Input
Voltage
(Limits)
VIH
(Min)
2.0V
3.15V
3.15V
2.0V
VIL
(Max)
0.8V
0.9V
1.35V
0.8V
Output
Voltage
(Limits)
VOH
(Min)
2.7V
VOL
(Max)
0.5V
VCC−0.1 VCC−0.1 VCC−0.1
0.1V
0.1V
0.1V
FIGURE 15.
AN010562-16
To illustrate noise margin and immunity, Figure 16 shows the
output that results when you apply several types of simulated noise to a 74HC00’s input. Typically, even 2V or more
input noise produces little change in the output. The top
trace shows noise induced on the high logic level signal and
the bottom trace shows noise induced on the low logic level
signal.
AN010562-17
FIGURE 14.
High-Speed CMOS Logic Switching
The magnitude of noise which can be tolerated in a system
relates directly to the worst case noise immunity specified for
the logic family. Noise immunity can be described as a device’s ability to prevent noise on its input from being transferred to its output. It is the difference between the worst
case output levels (VOH and VOL) of the driving circuit and
the worst case input voltage requirements (VIH and VIL, respectively) of the receiving circuit.
Using Figure 15 as a guide, it can be seen that for TTL (LS
or ALS) devices the worst case noise immunity is typically
700 mV for the high logic level and 300 mV for the low logic
level. For HCMOS devices the worst case noise immunity is
typically 1.75V for high logic levels and 800 mV for low logic
levels. AC high speed CMOS logic families have noise immunity of 1.75V for high logic levels and 1.25V for low logic
levels. ACT CMOS logic families have noise immunity of
2.9V for high logic levels and 700 mV for low logic levels.
www.national.com
6
AN010562-21
AN010562-18
AN010562-22
FIGURE 17.
AN010562-19
Signal Crosstalk
The problem of crosstalk and how to deal with it is becoming
more important as system performance and board densities
increase. Our discussion on cable-to-cable coupling described crosstalk as appearing due to the distributed capacitive coupling and the distributed inductive coupling between
two signal lines. When crosstalk is measured on an undriven
sense line next to a driven line (both terminated at their characteristic impedances), the near end crosstalk and the far
end crosstalk have quite distinct features, as shown in Figure 18. It should be noted that the near end component reduces to zero at the far end and vice versa. At any point in
between, the crosstalk is a fractional sum of the near and far
end crosstalk waveforms as shown in the figure. It also can
be noted that the far end crosstalk can have either polarity
whereas the near end crosstalk always has the same polarity as the signal causing it.
The amplitude of the noise generated on the undriven sense
line is directly related to the edge rates of the signal on the
driven line. The amplitude is also directly related to the proximity of the two lines. This is factored into the coupling constants KNE and KFEby terms that include the distributed capacitance per unit length, the distributed inductance per unit
length, and the length of the line. The lead-to-lead capacitance and mutual inductance thus created causes “noise”
voltages to appear when adjacent signal paths switch.
AN010562-20
FIGURE 16.
Figure 17 shows how noise affects a 74HC74’s clock input.
Again, no logic errors occur with 2V or more of noise on the
clock input.
When using high speed CMOS, even with its greater noise
immunity, crosstalk, induced supply noise and noise transients become factors. Higher speeds allow the device to respond more quickly to externally induced noise transients
and accentuate the parasitic interconnection inductances
and capacitances that increase self-induced noise and
crosstalk.
7
www.national.com
AN010562-23
FIGURE 18. Crosstalk
terminated equally; if one line requires termination, all lines
in the bus should be terminated. This will ensure similar signals on all of the lines.
Several useful observations that apply to a general case can
then be made:
•
•
•
•
The crosstalk always scales with the signal amplitude V I.
Absolute crosstalk amplitude is proportional to slew rate
V I/tr, not just 1/tr.
Far end crosstalk width is always tr.
For tr < 2 TL, where tris the transition time of the signal on
the driven line and TLis the propagation or bus delay
down the line, the near end crosstalk amplitude VNE expressed as a fraction of signal amplitude VI is KNE which
is a function of physical layout only.
Ground Bounce
Ground bounce occurs as a result of the intrinsic characteristics of the leadframes and bondwires of the packages used
to house CMOS devices. As edge rates and drive capability
increase in advanced logic families, the effects of these intrinsic electrical characteristics become more pronounced.
One of these parasitic electrical characteristics is the inductance found in all leadframe materials.
•
The higher the value of “tr”(slower transition times) the
lower the percentage of crosstalk (relative to signal amplitude).
Although all circuit conductors have transmission line properties, these characteristics become significant when the
edge rates of the drivers are equal to or less than about three
times the propagation delay of the line. Significant transmission line properties may be exhibited, for example, where
devices having edge rates of 3 ns are used to drive traces of
8 inches or greater, assuming propagation delays of 1.7 ns/ft
for an unloaded printed circuit trace.
Figure 19 shows a simple circuit model for a CMOS device in
a leadframe driving a standard test load. The inductor L1
represents the parasitic inductance in the ground lead of the
package; inductor L2 represents the parasitic inductance in
the power lead of the package; inductor L3 represents the
parasitic inductance in the output lead of the package; the
resistor R1 represents the output impedance of the device
output, and the capacitor and resistor CLand RL represent
the standard test load on the output of the device.
Signal Interconnects
Of the many properties of transmission lines, two are of major interest to the system designer: Zoe, the effective equivalent impedance of the line, and tpde, the effective propagation
delay down the line. It should be noted that the intrinsic values of line impedance and propagation delay, Zo and tpd, are
geometry-dependent. Once the intrinsic values are known,
the effects of gate loading can be calculated. The loaded values for Zoe and tpde can be calculated with:
Zoe = Zo/(1 + Ct /Ci)**0.5
tpde = tpd * (1 + Ct/Ci)**0.5
where Ci = intrinsic line capacitance
Ct = additional capacitance due to gate loading.
These formulas indicate that the loading of lines decreases
the effective impedance of the line and increases the propagation delay. As was mentioned earlier, lines that have a
propagation delay greater than one third the rise time of the
signal driver should be evaluated for transmission line effects. When performing transmission line analysis on a bus,
only the longest, most heavily loaded and the shortest, least
loaded lines need to be analyzed. All lines in a bus should be
www.national.com
AN010562-24
FIGURE 19. Ground Bounce
The three waveforms shown represent how ground bounce
is generated. The top waveform shows the voltage (V)
across the load as it is switched from a logic HIGH to a logic
LOW. The output slew rate is dependent upon the characteristics of the output transistor, and the inductors L1 and L3,
and CL, the load capacitance. In order to change the output
from a HIGH to a LOW, current must flow to discharge the
load capacitance. The second waveform shows the current
8
that is generated as the capacitor discharges [I = −C L * (dV/
dt)]. This current, as it changes, causes a voltage to be generated across the inductances in the circuit. The formula for
the voltage across an inductor is V = L(dI/dt). The third
waveform shows the voltage that is induced across the inductance in the ground lead due to the changing currents
[VGB = L1 * (dI/dt)]. This induced voltage creates what is
known as ground bounce.
Because the inductor is between the external system ground
and the internal device ground, the induced voltage causes
the internal ground to be at a different potential than the external ground. This shift in potential causes the device inputs
and outputs to behave differently than expected because
they are referenced to the internal device ground, while the
devices which are either driving into the inputs or being
driven by the outputs are referenced to the external system
ground. External to the device, ground bounce causes input
thresholds to shift and output levels to change.
ing EMI-control techniques, however, address themselves to
both self-jamming and emission/susceptibility in accordance
with applicable EMI specifications. The techniques that will
be discussed include filtering, shielding, wiring, and grounding.
Inter-system EMI distinguishes itself by interference between two or more discrete and separate systems or platforms which are frequently under independent user control.
Culprit emissions and/or susceptibility situations are divided
into two classes: (1) antenna entry/exit and (2) back-door
entry/exit. More than 95% of inter-system EMI problems involve the antenna entry/exit route of EMI. We can group
inter-system EMI-control techniques by four fundamental
categories: frequency management, time management, location management, and direction management.
Although this discussion is limited to ground bounce generated during HIGH-to-LOW transitions, it should be noted that
the ground bounce is also generated during LOW-to-HIGH
transitions. This ground bounce though, has a much smaller
amplitude and therefore does not present the same concern.
There are many factors which affect the amplitude of the
ground bounce. Included are:
• Number of outputs switching simultaneously: more outputs results in more ground bounce.
• Type of output load: capacitive loads generate two to
three times more ground bounce than typical system
traces. Increasing the capacitive load to approximately
60–70 pF, increases ground bounce. Beyond 70 pF,
ground bounce drops off due to the filtering effect of the
load itself. Moving the load away from the output also reduces the ground bounce.
• Location of the output pin: outputs closer to the ground
pin exhibit less ground bounce than those further away
due to effectively lower L1 and L3.
• Voltage: lowering VCC reduces ground bounce.
Ground bounce produces several symptoms:
• Altered device states.
• Propagation delay degradation.
• Undershoot on active outputs. The worst-case undershoot will be approximately equal to the worst-case quiet
output noise.
AN010562-25
FIGURE 20. Intra-System EMI Manifestations
The first step in locating a solution is to identify the problem
as either an inter-system or intra-system EMI situation. Generally, if the specimen has an antenna and the problem develops from what exits or enters the antenna from another
specimen or ambient, then the problem is identified as an
inter-system EMI one. Otherwise, it is an intra-system EMI
situation which we will discuss now.
Intra-System EMI-Control Techniques
Shielding
Shielding is used to reduce the amount of electromagnetic
radiation reaching a sensitive victim circuit. Shields are
made of metal and work on the principle that electromagnetic fields are reflected and/or attenuated by a metal surface. Different types of shielding are needed for different
types of fields. Thus, the type of metal used in the shield and
the shield’s construction must be considered carefully if the
shield is to function properly. The ideal shield has no holes or
voids, and, in order to accommodate cooling vents, buttons,
lamps, and access panels, special meshes and
“EMI-hardened” components are needed.
Once a printed-circuit board design has been optimized for
minimal EMI, residual interference can be further reduced if
the board is placed in a shielded enclosure. A box’s shielding
effectiveness in decibels depends on three main factors: its
skin, the control of radiation leakage through the box’s apertures or open areas (like cooling holes), and the use of filters
or shields at entry or exit spots of cables.
NOISE SUPPRESSION TECHNIQUES
EMI control techniques involve both hardware implementations and methods and procedures. They may also be divided into intra-system and inter-system EMI control. Our
major concern in this Application Note is intra-system EMI
control, however, an overview of each may be appropriate at
this time.
Figure 20 illustrates the basic elements of concern in an
intra-system EMI problem. The test specimen may be a
single box, an equipment, subsystem, or system (an ensemble of boxes with interconnecting cables). From a strictly
near-sighted or selfish point-of-view, the only EMI concern
would appear to be degradation of performance due to self
jamming such as suggested at the top of the figure. While
this might be the primary emphasis, the potential problems
associated with either (1) susceptibility to outside conducted
and/or radiated emissions or (2) tendency to pollute the outside world from its own undesired emissions, come under
the primary classification of intra-system EMI. Correspond-
A box skin is typically fabricated from sheet metal or metallized plastic. Normally sheet metal skin that is 1 mm thick is
more than adequate; it has a shielding effectiveness of more
than 100 dB throughout the high-frequency spectrum from
1 MHz to 20 GHz. Conductive coatings on plastic boxes are
another matter. Table 1 shows that at 10 MHz the shielding
9
www.national.com
ground. The inductors, L1 and L2, are called common-mode
chokes, and are placed in the circuit to impede any
common-mode currents.
effectiveness can be as low as 27 dB if a carbon composite
is used, or it can run as high as 106 dB for zinc sprayed on
plastic by an electric arc process. Plastic filled materials or
composites having either conductive powder, flakes, or filament are also used in box shielding; they have an effectiveness similar to that of metallized plastics.
TABLE 1
Surface
Shielding
Shielding
Resistance,
Effectiveness, dB
Material
(Note 1)
At
At
At
Ohms/Square
10
MHz
100
MHz
1
GHz
0.004
67
93
97
0.1
59
81
87
0.05
57
82
89
3.0
35
47
57
10.0
27
35
41
0.002
106
92
98
N.A.
86
66
48
Silver Acrylic
AN010562-26
FIGURE 21. Filtering
Wiring
Now that the equipment in each box can be successfully designed to combat EMI emission and susceptibility separately,
the boxes may be connected together to form a system.
Here the input and output cables and, to a lesser extent, the
power cable form an “antenna farm” that greatly threatens
the overall electromagnetic compatibility of the system. Most
field remedies for EMI problems focus on the coupling paths
created by the wiring that interconnects systems. By this
time most changes to the individual equipment circuits are
out of the question.
Let us address five coupling paths that are encountered in
typical systems comprised of two or more pieces of equipment connected by cables. These should adequately cover
most EMI susceptibility problems. They are:
Paint
Silver Epoxy
Paint
Silver
Deposition
Nickel
Composite
Carbon
Composite
Arc-Sprayed
•
A common ground impedance coupling — a conducting
path in which a common impedance is shared between
an undesired emission source and the receptor.
•
A common-mode, radiated field-to-cable coupling, in
which electromagnetic fields penetrate a loop formed by
two pieces of equipment, a cable connecting them, and a
ground plane.
•
A differential-mode, radiated field-to-cable coupling, in
which the electromagnetic fields penetrate a loop formed
by two pieces of equipment and an interconnecting transmission line or cable.
•
A crosstalk coupling, in which signals in one transmission
line or cable are capacitively or inductively coupled into
another transmission line.
Zinc
Wire Screen
(0.64 mm
Grid)
Note 1: Effectiveness of shielding materials with 25-µm thickness and for
frequencies for which the largest dimension of the shielding plate is less than
a quarter of a wavelength.
In many cases shielding effectiveness of at least 40 dB is required of plastic housings for microcontroller-based equipment to reduce printed-circuit board radiation to a level that
meets FCC regulations in the United States or those of the
VDE in Europe. Such skin shielding is easy to achieve. The
problem is aperture leakage. The larger the aperture, the
greater its radiation leakage because the shield’s natural attenuation has been reduced. On the other hand, multiple
small holes matching the same area as the single large aperture can attain the same amount of cooling with little or no
loss of attenuation properties.
•
A conductive paththrough power lines feeding the equipment.
The first coupling path is formed when two pieces of equipment are connected to the same ground conductor at different points, an arrangement that normally produces a voltage
difference between the two points. If possible, connecting
both pieces of equipment to a single-point ground eliminates
this voltage. Another remedy is to increase the impedance
along a loop that includes the path between the ground connections of the two boxes. Examples include the isolation of
printed-circuit boards from their cabinet or case, the use of a
shielded isolation transformer in the signal path, or the insertion of an inductor between one or both boxes and the
ground conductor. The use of balanced circuits, differential
line drivers and receivers, and absorbing ferrite beads and
rods on the interconnecting cable can further reduce currents produced by this undesirable coupling path.
Filtering
Filters are used to eliminate conducted interference on
cables and wires, and can be installed at either the source or
the victim. Figure 21 shows an AC power-line filter. The values of the components are not critical; as a guide, the capacitors can be between 0.01 and 0.001 µF, and the inductors are nominally 6.3 µH. Capacitor C1 is designed to shunt
any high-frequency differential-mode currents before they
can enter the equipment to be protected. Capacitors C2 and
C3 are included to shunt any common-mode currents to
www.national.com
10
•
Common Ground Impedance Coupling
Adding a cable shield over either or both transmission
lines.
• Twisting the source’s or receptor’s wire pairs.
• Twisting both wire pairs in opposite directions.
The fifth coupling path conductively produces both
common-mode and differential-mode noise pollution on the
power mains. Among several remedies that can suppress
the EMI here are the filters and isolation transformers.
There are only about 50 common practical remedies that can
be used in most EMI situations. Of these, about 10 suffice in
80 percent of the situations. Most engineers are aware of at
least some of these remedies — for example, twisting wires
to reduce radiation pickup.
In order to attack the EMI problem, one can make use of the
information contained in Table 2. First, decide what coupling
path has the worst EMI interference problem. From the 11
most common coupling paths listed at the top of the table,
find the problem coupling path. Using the numbers found in
that table entry, locate the recommended remedy or remedies from the 12 common EMI fixes identified at the bottom
of the table. This procedure should be repeated until all significant coupling paths have been properly controlled and
the design goal has been met.
AN010562-27
A balanced circuit is configured so its two output signal leads
are electrically symmetrical with respect to ground, as the
signal increases on one output the signal on the other decreases. Differential line drivers produce a signal that is electrically symmetrical with respect to ground from a
single-ended circuit in which only one lead is changing with
respect to ground. Ferrite beads, threaded over electrical
conductors, substantially attentuate electromagnetic interference by turning radio-frequency energy into heat, which is
dissipated in them.
In the second coupling path, a radiated electromagnetic field
is converted into a common-mode voltage in the ground
plane loop containing the interconnect cable and both boxes.
This voltage may be reduced if the loop area is trimmed.
Inter-System EMI Control Techniques
There are many EMI controls that may be carried out to enhance the chances of inter-system EMC. They can be
grouped into four categories which we will discuss briefly.
The following discussion is not intended to be complete but
merely provide an overview of some EMI control techniques
available to the intersystem designer and user.
Frequency management suggests both transmitter emission
control and improvement of receptors against spurious responses. The object is to design and operationally maintain
transmitters so that they occupy the least frequency spectrum possible in order to help control electromagnetic pollution. For example, this implies that long pulse rise and fall
times should be used. Quite often one of the most convenient, economic and rapid solutions to an EMI problem in the
field, is to change frequency of either the victim receiver or
the culprit source.
In those applications where information is passed between
systems, a possible time management technique could be
utilized where the amount of information transferred is kept
to a minimum. This should reduce the amount of time that
the receptor is susceptible to any EMI. In communication
protocols, for example, essential data could be transmitted in
short bursts or control information could be encoded into
fewer bits.
Common-Mode, Radiated Field-to-Cable Coupling
AN010562-28
The third coupling path produces a differential-mode voltage
that appears across the input terminals of the EMI receptor.
One way of controlling this is to cancel or block the pickup of
differential-mode radiation. In a balanced transmission line,
this is done by use of twisted-wire pairs and a shielded
cable.
As for crosstalk, the fourth coupling path — the reduction of
capacitive coupling can be achieved by the implementation
of at least one of these steps:
Location management refers to EMI control by the selection
of location of the potential victim receptor with respect to all
other emitters in the environment. In this regard, separation
distance between transmitters and receivers is one of the
most significant forms of control since interfering source
emissions are reduced greatly with the distance between
them. The relative position of potentially interfering transmitters to the victim receiver are also significant. If the emitting
source and victim receiver are shielded by obstacles, the degree of interference would be substantially reduced.
Direction management refers to the technique of EMI control
by gainfully using the direction and attitude of arrival of electromagnetic signals with respect to the potential victim’s receiving antenna.
• Reducing the spacing between wire pairs in either or both
of the transmission lines.
• Increasing the separation between the two transmission
lines.
• Reducing the frequency of operation of the source, if possible.
11
www.national.com
TABLE 2. Electromagnetic Interference Coupling Paths
Radiated Field to Interconnecting Cable
2, 7, 8, 9, 11
Radiated Field to Box
12, 13
2, 5, 6
Box to Radiated Field
12, 13
1, 3, 9, 11
Box-to-Box Radiation
12, 13
1, 3, 5, 6, 7
Box-to-Box Conduction
1, 2, 7, 8, 9
1, 2, 3, 4, 5, 6, 10, 11
Power Mains to Box Conduction
4, 11
Box to Power Mains Conduction
4
(Common-Mode)
Radiated Field to Interconnecting Cable
(Differential-Mode)
Interconnecting Cable to Radiated Field
(Common-Mode)
Interconnecting Cable to Radiated Field
(Differential-Mode)
Cable-to-Cable Crosstalk
Electromagnetic Interference Fixes
and susceptibility is practical early. Of course, other factors
such as required system performance, speed, and timing
considerations must enter into this decision. If possible, design the circuit with a slow speed logic. The use of slow
speed logic, however, does not guarantee that EMC will exist
when the circuit is built; so proper EMC techniques should
still be implemented consistently during the remainder of the
circuit design.
1. Insert Filter In Signal Source
2. Insert Filter in Signal Receptor
3.
4.
5.
6.
7.
8.
9.
10.
11.
12.
Insert Filter in Power Source
Insert Filter in Power Receptor
Twist Wire Pair
Shield Cable
Use Balanced Circuits
Install Differential Line Drivers and Receivers
Float Printed Circuit Board(s)
Separate Wire Pair
Use Ferrite Beads
Use a Multilayer Instead of a Single-Layer Printed
Circuit Boards
Component Layout
Component layout is the second stage in PC board design.
Schematics tell little or nothing about how systems will perform once the board is etched, stuffed, and powered. A circuit schematic is useful to the design engineer, but an experienced EMC engineer refers to the PC board when
troubleshooting. By controlling the board layout in the design
stage, the designer realizes two benefits: (1) a decrease in
EMI problems when the circuit or system is sent for EMI or
quality assurance testing; and (2) the number of EMI coupling paths is reduced, saving troubleshooting time and effort
later on.
Some layout guidelines for arranging components according
to logic speed, frequency, and function are shown in Figure
22. These guidelines are very general. A particular circuit is
likely to require a combination and/or tradeoffs of the above
arrangements. Isolation of the I/O from digital circuitry is important where emissions or susceptibility may be a problem.
For the case of emissions, a frequently encountered coupling path involves a digital energy coupling through I/O circuitry and signal traces onto I/O cables and wires, where the
latter subsequently radiate. When susceptibility is a problem,
it is common for the EMI energy to couple from I/O circuits
onto sensitive digital lines, even though the I/O lines may be
“opto-coupled” or otherwise supposedly isolated. In both
situations, the solution often lies in the proper electrical and
physical isolation of analog and low speed digital lines from
high speed circuits. When high speed signals are designed
to leave the board, the reduction of EMI is usually performed
via shielding of I/O cables and is not considered here.
Therefore, a major guideline in laying out boards is to isolate
the I/O circuitry from the high speed logic. This method applied even if the logic is being clocked at “only” a few MHz.
Often, the fundamental frequency is of marginal interest,
with the harmonics generated from switching edges of the
clock being the biggest emission culprits. Internal system
DESIGN GUIDELINES
The growth of concern over electromagnetic compatibility
(EMC) in electronic systems continues to rise in the years
since the FCC proclaimed that there shall be no more pollution of the electromagnetic spectrum. Still, designers have
not yet fully come to grips with a major source and victim of
electromagnetic interference — the printed circuit board. The
most critical stage for addressing EMI is during the circuit
board design. Numerous tales of woe can be recounted
about the eleventh hour attempt at solving an EMI problem
by retrofit because EMC was given no attention during design. This retrofit ultimately costs much more than design
stage EMC, holds up production, and generally makes managers unhappy. With these facts in mind, let’s address electromagnetic compatibility considerations in printed circuit
board design.
Logic Selection
Logic selection can ultimately dictate how much attention
must be given to EMC in the total circuit design. The first
guideline should be: use the slowest speed logic that will do
the job. Logic speed refers to transition times of output signals and gate responses to input signals. Many emissions
and susceptibility problems can be minimized if a slow speed
logic is used. For example, a square wave clock or signal
pulse with a 3 ns rise time generates radio frequency
(100 MHz and higher) energy that is gated about on the PC
board. It also means that the logic can respond to comparable radio frequency energy if it gets onto the boards.
The type of logic to be used is normally an early design decision, so that control of edge speeds and, hence, emissions
www.national.com
12
input/output PCB circuity should be mounted as close to the
edge connector as possible and capacitive filtering of these
lines may be necessary to reduce EMI on the lines.
High speed logic components should be grouped together.
Digital interface circuitry and I/O circuitry should be physically isolated from each other and routed on separate connectors, if possible as shown in Figure 22d.
• No High Frequency Signals to the Backplane
AN010562-41
FIGURE 22. Board Layout
13
www.national.com
good decoupling over a large range of frequencies. A low impedance distribution system requires two design features:
(1) proper power supply and return trace layout and (2)
proper use of decoupling capacitors.
At high frequencies, PCB traces and the power supply busses (+VCC and 0V) are viewed as transmission lines with associated characteristic impedance, ZO, as modeled in Figure
23. The goal of the designer is to maximize the capacitance
between the lines and minimize the self-inductance, thus
creating a low ZO. Table 3 shows the characteristic impedance of various two-trace configurations as a function of
trace width, W, and trace separation, h.
Power Supply Bussing
Power supply bussing is the next major concern in the design phase. Isolated digital and analog power supplies must
be used when mixing analog and digital circuitry on a board.
The design preferably should provide for separate power
supply distribution for both the analog and digital circuitry.
Single point common grounding of analog and digital power
supplies should be performed at one point and one point
only — usually at the motherboard power supply input for
multi-card designs, or at the power supply input edge connector on a single card system. The fundamental feature of
good power supply bussing, however, is low impedance and
TABLE 3
*Mylar dielectric assumed: DC = 5.0
D @ nearby ground plane
**Paper base phenolic or glass epoxy assumed: DC = 4.7
Any one of the three configurations may be viewed as a possible method of routing power supply (or signal) traces. The
most important feature of Table 3 is the noticeable difference
in impedance between the parallel strips and strip over
ground plane compared with the side-by-side configurations.
As an example of the amount of voltage that can be generated across the impedance of a power bus, consider TTL
logic which pulls a current of approximately 16 mA from a
supply that has a 25Ω bus impedance (this assumes no decoupling present). The transient voltage is approximately
dV = 0.016 x 25Ω = 400 mV, which is equal to the noise immunity level of the TTL logic. A 25Ω (or higher) impedance is
not uncommon in many designs where the supply and return
traces are routed on the same side of the board in a
AN010562-29
where LO and CO are, respectively, the distributed inductance and
capacitance per unit length of the line
FIGURE 23.
www.national.com
14
50Ω) and more than 750 mA will be required if all eight lines
switch at once. This instantaneous current requirement will
generate a voltage drop across the impedance of the power
lines, causing the actual VCC at the chip to droop. This droop
limits the voltage swing available to the driver. The net effect
of the voltage droop will be to lengthen device rise and fall
times and slow system operation. A local decoupling capacitor is required to act as a low impedance supply for the driver
chip during high current demands. It will maintain the voltage
within acceptable limits and keep rise and fall times to a minimum. The necessary values for decoupling capacitors can
be calculated with the formula given in Figure 25.
In this example, if the VCC droop is to be kept below 0.1V
and the edge rate equals 4 ns, we can calculate the value of
the decoupling capacitor by use of the charge on a capacitor
equation: Q = CV. The capacitor must supply the high demand current during the transition period and is represented
by I = C (dV/dt). Rearranging this somewhat yields C = I
(dt/dV).
side-by-side fashion. In fact, it is not uncommon to find situations where the power supply and return traces are routed
quite a distance from each other, thereby increasing the
overall impedance of the distribution system. This is obviously a poor layout.
Power and ground planes offer the least overall impedance.
The use of these planes leads the designer closer to a
multi-layer board. At the very least, it is recommended that
all open areas on the PC board be “landfilled” with a 0V reference plane so that ground impedance is minimized.
Multi-layer boards offer a considerable reduction in power
supply impedance, as well as other benefits. As shown in
Table 3, the impedance of a multi-layer power/ground plane
bus grows very small (on the order of an ohm or less), assuming a W/h ratio greater than 100. Multi-layer board designs also pay dividends in terms of greatly reduced EMI,
and they provide close control of line impedances where impedance matching is important. In addition, shielding benefits can be realized. For high-density, high-speed logic applications, the use of a multi-layer board is almost
mandatory. The problem with multi-layer boards is the increased cost of design and fabrication and increased difficulty in board repair.
Decoupling
High-speed CMOS has special decoupling and printed circuit board layout requirements. Adhering to these requirements will ensure the maximum advantages are gained with
CMOS devices in system performance and EMC performance.
AN010562-32
Q = CV charge on capacitor
I = C dV/dt
C = I dt/dV = 750 mA x 4 ns / 0.1V = 0.030 µF
Select CB = 0.047 µF or greater
Local high frequency decoupling is required to supply power
to the chip when it is transitioning from a LOW to a HIGH
value. This power is necessary to charge the load capacitance or drive a line impedance.
For most power distribution networks, the typical impedance
can be between 50 and 100Ω. This impedance appears in
series with the load impedance and will cause a droop in the
VCC at the part. This limits the available voltage swing at the
local node, unless some form of decoupling is used. This
drooping of rails will cause the rise and fall times to become
elongated. Consider the example presented in Figure 24
used to help calculate the amount of decoupling necessary.
This circuit utilizes an octal buffer driving a 100Ω bus from a
point somewhere in the middle.
FIGURE 25.
Now, I = 750 mA assuming all 8 outputs switch simultaneously for worst case conditions, dt = switching period or
4 ns, and dV is the specified VCC droop of 0.1V. This yields
a calculated value of 0.030 µF for the decoupling capacitor.
So, a selection of 0.047 µF or greater should be sufficient.
It is good practice to distribute decoupling capacitors evenly
throughout the logic on the board, placing one capacitor for
every package as close to the power and ground pins as
possible. The parasitic induction in the capacitor leads can
be greatly reduced or eliminated by the use of surface mount
chip capacitors soldered directly onto the board at the appropriate locations. Decoupling capacitors need to be of the
high K ceramic type with low equivalent series resistance
(ESR), consisting primarily of series inductance and series
resistance. Capacitors using 5ZU dielectric have suitable
properties and make a good choice for decoupling capacitors; they offer minimum cost and effective performance.
Proper Signal Trace Layout
Although crosstalk cannot be totally eliminated, there are
some design techniques that can reduce system problems
resulting from crosstalk. In any design, the distance that
lines run adjacent to each other should be kept as short as
possible. The best situation is when the lines are perpendicular to each other. Crosstalk problems can also be reduced by moving lines further apart or by inserting ground
lines or planes between them.
AN010562-31
FIGURE 24.
Being in the middle of the bus, the driver will see two 100Ω
loads in parallel, or an effective impedance of 50Ω. To switch
the line from rail to rail, a drive of 94 mA is needed (4.8V/
15
www.national.com
Series terminations are most useful in high-speed applications where most of the loads are at the far end of the line.
Loads that are between the driver and the end of the line will
receive a two-step waveform. The first wave will be the incident wave. The amplitude is dependent upon the output impedance of the driver, the value of the series resistor and the
impedance of the line according to the formula:
VW = V CC * Zoe/ (Zoe + RS + ZS)
For those situations where lines must run parallel as in address and data buses, the effects of crosstalk can be minmized by line termination. Terminating a line in its characteristic impedance reduces the amplitude of an initial crosstalk
pulse by 50%. Terminating the line will also reduce the
amount of ringing.
There are several termination schemes which may be used.
They are series, parallel, AC parallel and Thevenin terminations. AC parallel and series terminations are the most useful
for low power applications since they do not consume any
DC power. Parallel and Thevenin terminations experience
high DC power consumption.
Series Termination
AN010562-33
(Vw = V CC x Zoe/(Zoe + RS + ZS)
where RS is the series resistor
ZS is the output impedance of the driver
Z oe is the equivalent line impedance
The amplitude will be one-half the voltage swing if RS (the
series resistor) plus the output impedance (ZS) of the driver
is equal to the line impedance (Zoe). The second step of the
waveform is the reflection from the end of the line and will
have an amplitude equal to that of the first step. All devices
on the line will receive a valid level only after the wave has
propagated down the line and returned to the driver. Therefore, all inputs will see the full voltage swing within two times
the delay of the line.
Parallel terminations are not generally recommended for
CMOS circuits due to their power consumption, which can
exceed the power consumption of the logic itself. The power
consumption of parallel terminations is a function of the resistor value and the duty cycle of the signal. In addition, parallel termination tends to bias the output levels of the driver
towards either V CC or ground depending on which bus the
resistor is connected to. While this feature is not desirable for
driving CMOS inputs because the trip levels are typically
VCC/2, it can be useful for driving TTL inputs where level
shifting is desirable in order to interface with CMOS devices.
www.national.com
16
•
Use the lowest VCC possible or separate the power supplies.
•
Use board design practices which reduce any additive
noise sources, such as crosstalk, reflections, etc.
Parallel Termination
Components
The interference effect by rectifier diodes, typically found in
power supply sections of PC boards, can be minmized by
one or more of the following measures:
AN010562-34
AC parallel terminations work well for applications where the
increase in bus delays caused by series terminations are undesirable. The effects of AC parallel terminations are similar
to the effects of standard parallel terminations. The major difference is that the capacitor blocks any DC current path and
helps to reduce power consumption.
Thevenin terminations are not generally recommended due
to their power consumption.
•
Placing a bypass capacitor in parallel with each rectifier
diode.
•
•
Placing a resistor in series with each rectifier diode.
Placing an R-F bypass capacitor to ground from one or
both sides of each rectifier diode.
•
Operating the rectifier diodes well below their rated current capability.
AC Parallel Termination
AN010562-35
AN010562-37
Thevenin Termination
Connectors
AN010562-36
AN010562-38
Like parallel terminations, a DC path to ground is created by
the terminating resistors. The power consumption of a Thevenin termination, though, will generally be independent of
the signal duty cycle. Thevenin terminations are more applicable for driving CMOS inputs because they do not bias the
output levels as paralleled terminations do. It should be
noted that output lines with Thevenin terminations should not
be left floating since this will cause the undriven input levels
to float between VCC and ground, increasing power consumption.
Cables and Connectors
Several options are available to reduce EMI from a typical
ribbon cable used to interconnect pieces of equipment.
These include:
• Reduce spacing between conductors (h in the figure) by
reducing the size of wires used and reducing the insulation thickness.
Ground Bounce
Observing either one of the following rules is sufficient to
avoid running into any of the problems associated with
ground bounce:
•
Join alternate signal returns together at the connectors at
each end of the cable.
•
•
Twist parallel wire pairs in ribbon cables.
Shield ribbon cable with metal foil cover (superior to
braid).
•
Replace discrete ribbon cable with stripline flexprint
cable.
In the case of joining alternate signal returns, wire N is carryig the signal current, in, whereas its mates, N−1 and N+1
wires are each carrying one half of the return currents, in−1
and in+1, respectively. Thus, radiation from pair N and N−1 is
out of phase with radiation from pair N and N+1 and will tend
to cancel. In practice, however, the net radiation is reduced
by 20–30 dB with 30 dB being a good default value.
• First, use caution when driving asynchronous TTL-level
inputs from CMOS octal outputs. Ground bounce glitches
may cause spurious inputs that will alter the state of
non-clocked logic.
• Second, use caution when running control lines (set, reset, load, clock, chip select) which are glitch-sensitive
through the same devices that drive data or address
lines.
When it is not possible to avoid the above conditions,
there are simple precautions available which can minimize ground bounce noise. These are:
• Choose package outputs that are as close to the ground
pin as possible to drive asynchronous TTL-level inputs.
Alternating Signal Returns Minimizes Radiation
AN010562-39
17
www.national.com
other. The use of these package types in high speed designs
can facilitate board layout to help reduce the effects of
crosstalk.
The opposite of this is to conserve signal returns by only using one, or two, wires to service N data lines in a ribbon
cable. For data lines farther from the return line, the differential mode radiation becomes so great that this cable tends to
maximize EMI radiation. Another disadvantage of this approach is poor impedance control in the resulting transmission line. This could result in distortion of pulses and cause
reflections, especially for high-speed logic, and common return impedance noise in this single ground wire.
Use extra heavy ground wires between emulator and target
board. Rely on the ground returns in the emulator cable for
reduction of differential-mode noise radiated from the cable
but heavy-duty help is required for reducing power line impedance in the integrated development system.
Unused HPC inputs, most importantly NMI and RDY/HLD,
must be tied to VCC directly or through a pull-up resistor. This
not only tends to reduce power consumption, but will avoid
noise problems triggering an unwanted action.
Single Signal Return Maximizes Radiation
In order to reduce the effects of noise generated by high
speed signal changes, a sort of Frequency Management
technique might be applied. If possible, develop application
hardware and software at a slower crystal operating frequency. If ringing, crosstalk, or other combinations of radiated and conducted noise problems exist, the result may be
to move the problem from one point in the affected signal
waveform to a different point. Thus, apparent “noise glitches”
that caused a latch to erroneously trigger when the input
data was still changing, may now come at a time when they
are non-destructive such as at a point when the input data is
now stable.
Some applications require driving the HPC clock input, CKI,
with an external signal. The emulator tools are all clocked
using a crystal network with the HPC so that the generation
of the system timing is contained on the tool itself. Consequently, there is no connection between the emulator cable
connector on the tool and the CKI pin at the HPC. However,
when the emulator cable is now inserted into the target
board, the target board’s clock signal travelling along the
cable couples noise onto adjacent signal lines causing
symptoms pointing to an apparent failure of the emulator
tool. The recommendation is to disable the clock drive to the
CKI pin at the HPC pad on the target board whenever the
emulator tool is connected. The emulator tools supply the
system clock so there is no need for the clock on the target
and signal crosstalk on the emulator cable can be greatly reduced with minimal implementation. If one insists that the
emulator tool and the target be synchronous, then bring the
clock signal from the target to the emulator tool external to
the emulator cable via twisted wire pair or coax cable. Remove the clock drive connection to CKI at the target to prevent the signal from entering the cable. Finally, remove crystal components on emulator tool to prevent problems with
the signal.
Connecting boards and modules together to make a totally
unique system in which EMC was practiced is necessary to
ensure little problem with the environment. But, connecting
an emulator tool makes it an entirely new and unique system, both in physical and electrical properties. Treat the
emulator tool as part of the system during the design phase
and development phase.
AN010562-40
Ideally, connectors should have negligible resistance for obvious reasons other than EMI control. They should provide
foolproof alignment to minimize the possibility of contact
damage over time and use which would increase the resistance and be prone to vibration and shock. Adequate force to
provide good mating between contacts which will insure low
resistance and limit likelihood of damage. Connectors should
mate with little friction to minimize the effects of continual
disconnections and connections increasing the contact resistance with use as the contacts wear out. A contamination
free design should be used to avoid corrosion and oxidation
increasing resistance and susceptibility to shock and vibration causing intermittent contact.
Special Considerations with Development Tools
The following set of guidelines have been compiled from the
experiences of the Development Systems Group and the Microcontroller Applications Group in Santa Clara. They should
be considered additional techniques and guidelines to be followed concurrently with the standard ones already presented. Some are general and some may be specific to development systems use.
Ground bounce prevention and minimization techniques presented in this Application Note should be strictly adhered to
when using ’373 type transparent latches on the HPC’s external address/data bus. Multiple simultaneously switching
outputs could produce ground bounce significant enough to
cause false latching. Observe good EMI planning by locating
the latches as close to the HPC as possible. The use of
multi-layer printed circuit boards with good ground planes
and following appropriate layout techniques is also essential,
especially if emulation will be done at frequencies above
10 MHz. With the foregoing discussions about “antenna
farms”, radiated noise, and ideal connector characteristics, it
becomes obvious that wire-wrap boards and the use of IC
sockets is absolutely out of the question. The concern here
is not so much EMI affecting the outside world but EMI strangling the operation of the module itself.
The inputs to the buffers in a ’244 type octal buffer package
are placed adjacent or side-by-side outputs of other buffers
in the package. This configuration would tend to maximize
the crosstalk or noise coupling from the inputs to the outputs.
On the other hand, the buffer inputs in a ’544 type package
are on one side of the package and the outputs are on the
www.national.com
NOISE MEASUREMENT
The basic purpose of FCC Part 15J is to minimize the jamming of commercial broadcasting systems by computer devices. Toward this end, the FCC has established test limits,
for both conducted and radiated emissions, which must be
met. These two tests together span the frequency range
18
ments and report results. It automatically marks the computer display with FCC limits for quick comparison with the
amplitude of the emissions signal. This setup is outside the
shielded enclosure and can be used to determine if the
equipment under test is failing any FCC requirements.
If the test sample fails, we can move inside the room and use
near-field probes to help pinpoint the source of emissions.
The spectrum analyzer samples the signal generated by the
source at many different frequencies. The scale across the
bottom of the screen is frequency and the scale along the
side is signal amplitude in dBuV/m. Thus, we can quickly determine where the peak amplitude of the generated noise is
located, read what level that is, and at what frequency it is
being generated.
A little analysis and thought should then allow you to determine what signal could be the culprit. For example, if the
noise problem is at 16 MHz and the system clock is 16 MHz,
then the basic clock signal is causing the problem. If the
noise problem is at even multiples of 16 MHz it could be
caused by rise and fall times on the 16 MHz clock or overshoot and undershoot on that clock. In the case of the HPC,
since it generates a clock output that is the system clock divided by 2 (CK2 = CKI/2), the noise frequency generated at
the multiple of the 16 MHz signal could also be due to CK2
or any device that is clocked by that signal. Unfortunately for
the investigator, everything else inside the part is clocked by
CK2, which includes bus transitions and input sampling.
from 450 kHz to 1000 MHz. To accomplish FCC Part 15J
testing requires the following equipment and associated support items:
• EMC Receivers or Spectrum Analyzers to cover the frequency range from 450 kHz to 1000 MHz.
• Dipole antennas (2) to cover the frequency range from
30 MHz to 1000 MHz.
• Masts or supports which will allow antenna elevation to
be increased to at least 4 meters and also allow the polarization to be changed.
• Line impedance stabilization networks (LISN) built in accordance with CISPR requirements. These are 50Ω,
50 µH devices and are inserted between power mains
and test item to permit making repeatable conducted EMI
measurements.
• Power line filters.
• An appropriate test site.
Environment
The most controversial item on the test requirement list is the
appropriate test site. The FCC required emission limits are
comparable with the ambient RF level. These low limits and
the noisy ambient would indicate that the tests should be
made in a shielded enclosure. Unfortunately, all shielded enclosures introduce significant errors into the radiated measurements because of room reflections, room resonances,
and antenna loading. To reduce the magnitude of these
problems, the FCC has specified that measurements should
be made at an open-field test site. Open-field test sites frequently have high ambient levels especially in the FM broadcast band. They may also have ground reflection variations
as a function of soil moisture.
The FCC will permit the use of anechoic shielded enclosures
which have reduced reflections, provided an error analysis is
made to show correlation of interior RF levels with those of
an open-field test site. The cost of an anechoic enclosure is
its major drawback. For measurements other than for certification, the test site does not have to be in accordance with
government regulations. There are also alternatives where
an agency or private company will perform the tests for you
at their facility for a nominal fee.
Many manufacturers are using shielded enclosures that they
have constructed on site or purchased from one of the
shielded enclosures manufacturers. The measurement requirement is that the RF ambient levels should be 6 dB or
more below the specifications limits. This may require 20 dB
worth of aluminum foil or 160 dB worth of electrical seals.
Only a site survey can provide that answer. In any case,
some margin of safety should be made, 6–10 dB, plus periodic check for reflection problems.
Cost
Basically, the risks of no EMI control will include the following:
• Vehicle/System Performance Degradation
• Degradation to outside world equipment
• Personal Hazards
• Ordinance Hazards
• Acceptance Delays
The sum which can mean anything from a minor system or
equipment performance compromise to the total cancellation
of a project.
The cost of EMI control will vary and include the following:
• Government procurement requirements
• Company proposal preparation
• EMI Control Plan
• Test Plan
• EMI Tests and Reports
A rough guideline that can be used might be:
1%–3% of $100 Million projects
3%–7% of $1 Million to $10 Million projects 7%–12% of
small items
Instrumentation
SUMMARY
After the appropriate test site has been obtained, whether a
room or a quiet open field, then the testing can begin. If the
equipment to be tested is not floor standing, the test sample
is placed on a non-conducting stand 80 cm high and at least
40 cm from the wall of the enclosure. Antennas are then set
up so that radiated emission levels can be measured. The
test sample should be loaded with full electrical and mechanical loads and operated in a manner that closely approximates normal operation. During operation of the equipment under test, the EMI measuring equipment is used to
determine the amplitude of the radiated emission.
The design and construction of an electromagnetically compatible printed circuit board does not necessarily require a
big change in current practices. On the contrary, the implementation of EMC principles during the design process can
fit in with the ongoing design. When EMC is designed into
the board, the requirements to shield circuitry, cables, and
enclosures, as well as other costly eleventh hour surprises,
will be drastically reduced or even eliminated. Without EMC
in the design stage, production can be held up and the cost
of the project increases.
At NSC, we have a spectrum analyzer than can be attached
to a Personal Computer that runs software to control experi19
www.national.com
EMI/RFI Board Design
7.
REFERENCES
1. Atkinson, Kenn-Osburn, John-White, Donald, Taming
EMI in Microprocessor Systems, IEEE Spectrum, December 1985, pp 30–37
8.
Balakrishnan, R. V., Reducing Noise on Microcomputer
Buses, National Semiconductor Application Note
337, May 1983
3. Brewer, Ron W., Test Methodology to Determine Levels
of Conducted and Radiated Emissions from Computer
Systems, EMC Technology, July 1982, p 10
4. Engineering Staff, Don White Consultants, Inc., The
Role of Cables & Connectors in Control of EMI, EMC
Technology, July 1982, pp 16–26
2.
5.
Fairchild Semiconductor, Design Considerations, Fairchild Advanced CMOS Technology Logic Data Book,
1987, pp 4–3 to 4–12
6.
Fairchild Semiconductor, Understanding and Minimizing
Ground Bounce, Application Note DU-6, August 1987
9.
10.
11.
Interference Control Technology, Introduction to EMI/
RFI/EMC, Course Notebook on Electromagnetic
Compatibility, August 1988
Violette, Michael F. and J.L., EMI Control in the Design
and Layout of Printed Circuit Boards, EMC Technology,
March-April 1986, pp 19–32
Violette, Michael F., Curing Electromagnetic Interference, Radio-Electronics, November 1985, pp 50–56
Wakeman, Larry, High-Speed-CMOS Designs Address
Noise and I/O Levels, National Semiconductor Application Note 375, September 1984
White, Donald R. J., EMI Control Methods and Techniques, Electromagnetic Interference and Compatibility — Vol 3, copyright 1988 by Interference Control
Technologies
LIFE SUPPORT POLICY
NATIONAL’S PRODUCTS ARE NOT AUTHORIZED FOR USE AS CRITICAL COMPONENTS IN LIFE SUPPORT DEVICES OR SYSTEMS WITHOUT THE EXPRESS WRITTEN APPROVAL OF THE PRESIDENT OF NATIONAL SEMICONDUCTOR CORPORATION. As used herein:
AN-643
1. Life support devices or systems are devices or systems which, (a) are intended for surgical implant into
the body, or (b) support or sustain life, and whose failure to perform when properly used in accordance
with instructions for use provided in the labeling, can
be reasonably expected to result in a significant injury
to the user.
National Semiconductor
Corporation
Americas
Tel: 1-800-272-9959
Fax: 1-800-737-7018
Email: [email protected]
www.national.com
2. A critical component in any component of a life support
device or system whose failure to perform can be reasonably expected to cause the failure of the life support
device or system, or to affect its safety or effectiveness.
National Semiconductor
Europe
Fax: +49 (0) 1 80-530 85 86
Email: [email protected]
Deutsch Tel: +49 (0) 1 80-530 85 85
English Tel: +49 (0) 1 80-532 78 32
Français Tel: +49 (0) 1 80-532 93 58
Italiano Tel: +49 (0) 1 80-534 16 80
National Semiconductor
Asia Pacific Customer
Response Group
Tel: 65-2544466
Fax: 65-2504466
Email: [email protected]
National Semiconductor
Japan Ltd.
Tel: 81-3-5620-6175
Fax: 81-3-5620-6179
National does not assume any responsibility for use of any circuitry described, no circuit patent licenses are implied and National reserves the right at any time without notice to change said circuitry and specifications.
Was this manual useful for you? yes no
Thank you for your participation!

* Your assessment is very important for improving the work of artificial intelligence, which forms the content of this project

Download PDF

advertisement