Radiated Emissions and CRT Displays Radiated Emissions

Radiated Emissions and CRT Displays Radiated Emissions
National Semiconductor
Application Note 990
Len Stencel
May 1995
Electromagnetic compatibility (EMC) is a vital concern for
anyone who produces or uses electronic equipment. As the
performance of computer systems continues to improve,
designing for EMC will become more challenging. This
makes it very important to address EMC right at the start of
the product design. Doing this will minimize the product development costs attributable to EMC and avoid unnecessary
delays in product release. This application note addresses
the fundamental EMC concerns for the CRT display. It provides information to help the engineer design for EMC and
bypass easily avoidable emissions problems. The primary
focus will be on radiated emissions.
There are two types of emissions that must be kept under
control; radiated and conducted. They can be described as
Radiated EmissionsÐUnwanted electromagnetic energy that is sourced from an electronic system or a part of
that system by way of an electric or magnetic field.
Conducted EmissionsÐUnwanted energy that is
sourced from one electronic circuit to another through a
common impedance such as power or ground.
Regulations are needed to avoid the malfunctioning of a
piece of electronic equipment due to the electromagnetic
interference caused by another piece of electronic equipment. There are two ways to solve this problem. The first is
to limit the emissions of electronic equipment and the second is to make a piece of electronic equipment immune to
the interference. You can do one or both. Presently, most
regulations are for emissions rather than immunity. The European Community is planning to require immunity testing
and certification starting in 1996.
In 1934, the International Special Committee on Radio Interference (CISPR) was formed to determine measurement
methods and limits for radio-frequency (RF) emissions.
CISPR which is part of the International Electrotechnical
Commission (IEC) is not a regulating agency, but its standards have either been adopted by or used as a guide to
establish regulations by many countries. In 1985, CISPR
adopted Publication 22 for Information Technology Equipment. This standard applies to computers and display devices.
Nations that endorse CISPR regulations include the United
States, Germany, France, Canada, Sweden and the United
Kingdom. Each of these nations has released an emissions
standard for computer products. Table I below summarizes
their standards.
TABLE I. Regulating Agencies and Computer
Products Standards by Country
CSA C 108.8
SEN 471010
BS 6527
FCC Part 15J
C1995 National Semiconductor Corporation
The challenges of today’s CRT display designs include
meeting the performance requirements of the end user,
meeting the cost objectives of the end user (which translates to a set of design and production cost objectives), and
meeting the regulatory agency requirements of the intended
With the advent of more sophisticated uses for the CRT
display, the performance requirements are becoming more
demanding. Applications such as desktop publishing, computer aided design and multi-tasking have demanded higher
resolution as well as higher overall picture quality. Supplying
the needed resolution, brightness, stability, aspect ratio, distortion limits and convergence (for color displays) provide a
tough challenge for all the designers involved. User adjustments are being added beyond the standard contrast and
brightness controls. These include controls for size, pincushion, trapezoid, as well as color temperature. Often
these controls may be exercised using an on-screen display
Design for EMC adds to this challenge. We have previously
mentioned some regulatory agencies. Meeting the requirements of these agencies requires up front design methodology in order to keep test time and production costs to a
minimum. The graph in Figure 1 shows that the cost for
EMC solutions increase exponentially as the EMC design is
delayed with respect to the start of the product design. Using well established design guidelines as well as beginning
EMC testing early in the design cycle will minimize cost and
avoid last minute EMC fire drills.
The law of supply and demand primarily determines what
the CRT display producer may sell his product for on the
open market. This price will set the cost goals for the design
and production of the display. The CRT display market is
very competitive which tells us that keeping EMC costs at a
minimum may give a particular manufacturer a market advantage. Addressing the EMC issues as early in the design
phase as possible will allow the designers more time to implement the most cost effective solution.
Radiated Emissions and CRT Displays
Radiated Emissions and
CRT Displays
RRD-B30M75/Printed in U. S. A.
monitor takes these high frequency video signals and amplifies them by as much as 60 V/V which produces a signal
with an amplitude as large as 50 VP-P at the cathode of the
CRT. All of these signals are rich in high frequency harmonics (this will be discussed further later). The large amplitude
at the CRT cathode provides for some interesting emission
control challenges. When designing a computer system, a
general rule of thumb to use is to divide the allowable emissions equally between the PC (with a properly terminated
video cable attached) and the CRT display. For example, if
you wanted to pass FCC class B testing with 6 dB margin,
the PC and video cable by themselves should have 12 dB
margin. We will look more closely at the CRT display monitor later in this article.
Most of the high frequency energy is contained in waveforms that the monitor design engineer will look at in the
time domain. Open site tests are performed in the frequency
domain, therefore, the EMI engineer will need to convert
these time domain signals to their frequency equivalents in
order to more effectively do his job. This leads us to our
next topic.
FIGURE 1. EMC Solution Cost as EMC Design is
Delayed in Monitor Design Cycle
In general, the display would be tested with the rest of the
system that it would be marketed with. In the case where
the monitor is sold as a separate item, there are minimum
test configurations specified by the regulatory agencies. For
example, the minimum configuration for FCC Class B testing
is as follows:
Inside a display monitor we have synchronization pulses,
video pulses, pulses that drive the switching power supplies,
as well as blanking and clamp pulses. Attention is given to
rise and fall times, pulse widths, ringing and flatness. These
are all time domain parameters. When we go out to the
open site for emissions testing everything is measured in
the frequency domain. How do we make this transformation? The answer is to perform a Fourier analysis of the time
domain waveforms.
A rigorous Fourier analysis is not necessary. It is simpler to
construct a Fourier envelope of our time domain signals.
The Fourier series for a periodic pulse waveform will consist
of a series of discrete sine waves comprised of the fundamental (F0) and integer multiples of it. The Fourier envelope
for the periodic waveform shown in Figure 2 is shown in
Figure 3. The frequency components of the time domain
waveform will be contained inside this envelope. Figure 3
shows that between the first and second break points (F1
and F2), the envelope rolls off at 20 dB/Dec. After the second break point, the envelope rolls off at 40 dB/Dec.
The value of the envelope at a specific frequency (FX) can
be calculated using one of the following equations:
Computer with Graphics Card
Serial port device
Parallel port device
Examples of serial and parallel port devices are printers,
modems or a mouse. The monitor is tested at a maximum of
three resolutions (and therefore scan rates). Table I lists the
documents for various regulatory agencies that should be
referenced for specific test configuration information.
The configuration of the computer system that would be
tested was discussed in the previous section. Most high frequency emissions will emanate from the computer (and
graphics card) and the display monitor . The computer contains high frequency clock signals and other signals generated from this clock. These signals are routed over a large
area on printed circuit boards (PCBs) and ribbon or other
cables. As you already know, long PCB traces and cables
are the primary source of emissions because they are more
efficient transmitters at the frequencies present in these
The graphics card generates the video signal that drives the
CRT display monitor (in our case). The high frequency pixel
clock as well as the video signal itself may contain harmonics that can cause emission problems out to 1 GHz. The
0.64 A
for F1 k FX k F2, V e
T c FX
0.2 A
for FX l F2, V e
T c Tr c F X
(If Tf k Tr, use Tf)
for Fx k F1, V e
where T e the period of the repetitive waveform
u e pulsewidth at the 50% points
A e Pulse amplitude
Tr/Tf e the 10% – 90% rise and fall times
TL/H/12418 – 2
FIGURE 2. Generic Periodic Pulse
Where F0 e
F1 e
F2 e
(If Tr k Tr, use Tf)
TL/H/12418 – 3
FIGURE 3. Generic Graph of The Fourier Envelope of a Periodic Pulse
Figure 4 shows the Fourier envelopes for a 50 MHz square
wave with a 2 ns, 4 ns and 6 ns rise time. The first breakpoint (F1 e 32 MHz) is the same for all three envelopes.
Note the significant increase in high frequency signal content as the rise time is reduced (the second breakpoints are
at different frequencies).
TL/H/12418 – 4
FIGURE 4. Spectral Envelope for a 1 VP-P, 50 MHz Square Wave with Various Rise Times
It is also interesting to compare the Fourier envelopes of
two square waves with the same rise and fall times but of
different frequencies. Figure 5 shows the comparison of
1 VP-P 25 MHz and 50 MHz square waves each with 4 ns
rise and fall times. In this case the first break points are at
different frequencies and the second break points are at the
same frequency. This is representative of the effect that the
Video format (pixel rate) has on the high frequency energy
that is fed to the monitor by the PC, given that the same
graphics card is used.
TABLE II. Emissions at 3m for a
1V Signal from a 6 cm Signal Trace
Far field emissions of conductors that carry high frequency
signals can be estimated using the following equation:
c A c F 2MHz
where E
The electromagnetic field in dB mV/m
D e Observation distance in meters
A e Circuit area in cm2 (A e I x s, where I e trace
length and s e average distance to ground
V e Signal amplitude in volts
ZC e Impedance of the circuit
F e Frequency of interest in MHz
Table II shows the calculated emissions for a 6 cm length
signal trace on a PCB that carries a 1 VP-P signal for various
frequencies. This demonstrates that it doesn’t take a very
large loop area to cause a significant emission at high frequencies. Frequency components whose amplitudes are
greater than 1 VP-P are guaranteed at XGA and higher resolution video formats.
E(dB mV/m) e 20 log
Avg dist to
ground (cm)
(dB mV)
The Fourier envelope along with equation 2 can be used to
estimate the emissions of a monitor in the design stage.
These estimates can be used to help define the requirements for circuit boards and shielding needs (for cables,
boards or even the complete monitor) of a monitor system
early in the design process. For additional information on
calculating emissions, please refer to Reference 1.
We have presented a simple method that can be used to
predict and also to understand the sources and causes of
high frequency emissions. Now let’s move on and discuss
areas that relate directly to the CRT monitor. Figure 6 shows
a simplified block diagram of a CRT monitor. The main areas where HF signals are present are the video channel,
CRT and the switching power supplies.
Before we start it is important to note that the presence of
high frequency signals in itself does not cause high frequency emissions. It is the lack of control of the paths (and the
length of the paths) that these signal voltages and currents
follow that creates the problem. We will discuss the following topics with respect to the CRT monitor: grounding, cabling, PCB layout, shielding and the role of ferrite .
TL/H/12418 – 5
FIGURE 5. Fourier Envelopes of 4 ns Rise Time, 1 VP-P Square Waves
FIGURE 6. Simplified Block Diagram of a CRT Monitor
TL/H/12418 – 6
4. Provide a solid 0V reference for all signals (especially
low level analog signals). The availability of a 0V reference at the input signal connectors establishes a good
It is important that grounding be addressed in all areas of
design and it will be a part of the discussion in all the following sections.
Grounding for Arc Protection
The CRT requires high voltages to perform the beam acceleration and focus functions. These voltages range from
1 KV to 30 KV which present the possibility of an arc. Therefore, safeguards must be implemented to protect the monitor electronics against damage that can be caused by the
energy present in the arc. One common protection scheme
is to use a ground for the CRT that is isolated from the video
electronics circuit ground as shown in Figure 7 . The CRT
ground would be connected to the Heater return as well as
the ground pin for the focus arc protection (which may be
built into the CRT connector). A ground strap is usually connected to the CRT DAG coating (which is connected to the
monitor chassis ground).
This technique works well to protect the electronics against
damage that can be produced by an arc because it provides
a controlled path for the arc current to return back to its
source, which is internal to the CRT. However, this technique elongates the return path of the video signal current
which consists of the electron beam, currents to charge the
cathode capacitance and any stray capacitance in the CRT
connector and PCB. These longer and uncontrolled current
paths may and usually do cause a worse scenario for the
EMI engineer.
Inside a monitor, cables/wires are used to distribute DC
power, Ground, RGB video signals, synchronization (sync)
signals, deflection currents and various control signals
which depend upon the complexity of the monitor. Looking
at this list, the only intended high frequency signal carriers
are the video cables and possibly the cable that carries the
sync signals (due to the edge speed). There are many types
of cables and wires that can be used to distribute these
signals. They include single wire, twisted pair, shielded
twisted pair, coax, triax, ribbon cable and various versions of
these types. Which type is used depends on EMC design
practice as well as cost and performance requirements.
The three grounding practices that are used are single
point, multi-point and hybrid. These practices can be summarized as follows:
# Single PointÐThe 0V references of all circuits/PCB’s/
assemblies are tied together at only one point on the
# Multi-PointÐThe 0V references of all circuits/PCB’s/assemblies are tied together at various points along a common reference plane or chassis.
# HybridÐA combination of both of the above approaches
is used.
In general, the single point grounding practice works well at
low frequencies because the inductance of the wire used for
the ground connections will not be significant. At high frequencies (l 10 MHz) the multi-point approach is needed to
minimize the effects of the wire inductance. In addition to
connections by wire or straps, inductors can be used to
make low frequency ground connections (and high frequency opens), while capacitors can be used to make HF ground
connections (and low frequency opens). In the CRT monitor, the final ground configuration will most likely be hybrid
due to the wide frequency range of signals used. It is also
important to note that parasitic capacitance will cause HF
ground connections that are not intended in the design.
For more information on these grounding approaches
please consult the references at the end of this article. The
grounding scheme used or options available in a monitor will
vary depending on such factors as cost, performance and
even the system it will be used with. Remember that the
control of signal currents and voltages is important to optimize performance for picture quality as well as EMI. Therefore, the most important thing is to know and understand
how the system ground is set up so that you may tailor your
circuit design to best use the available resources in that
Some of the important attributes of a good ground scheme
1. Low impedance at frequencies of interest.
2. Shortest possible return path to minimize the size of the
current loop.
3. Controlled paths of return currents.
TL/H/12418 – 7
FIGURE 7. Grounding for CRT Arc Protection
Substitution of the knowns leads to:
Due to their long length, cables are one of the primary
sources of EMI. Table III below lists the wavelength, (/4
wavelength and (/20 wavelength of a wire or cable at various
frequencies. The (/20 wavelength is the point where the conductor makes the transition from electrically short to electrically long. At this point the conductor can no longer be
treated as a simple wire, but transmission line effects must
be considered. Notice how short the (/20 wavelength numbers get at the higher frequencies. The (/4 wavelength is
important because that is the point where a conductor is a
good RF transmitter or receiver. When we look at the (/4
wavelength numbers, it is apparent that we really need to
keep high frequency signals and noise from undesired cables. This is not always a simple task. It is of no surprise
then that cables are one of our primary sources of emissions.
TABLE III. Conductor Wavelengths
at Various Frequencies
c A c F 2MHz
c 62.5 c 3002
& 30 mV
This calculation neglects the lengths of the signal trace that
is on the assemblies that this wire connects. As we can see,
this is not very much noise. If we would like 6 dB margin, this
noise level would have to be reduced to 15 mV. I think this
example makes it clear that a small noise level in the wrong
place can cause trouble. One way to keep cables and wiring
clean is to use filters at the I/O’s of the assembly or PCB. If
that is not possible, a twisted pair (only effective for differential noise) or a shielded cable can be used.
This leads to another question; should the shield be grounded at one or both ends? For low frequency shielding,
grounding at one end will do the job. For high frequencies,
grounding at both ends is necessary. If grounding at both
ends causes a low frequency problem, then one end can be
connected to ground through a capacitor to eliminate the
low frequency ground loop.
How about cables that are intended to carry high frequency
signals? The video input cable is one of the major concerns
when addressing this category. Coaxial cable, at a minimum, should be used for both internal and external video
signals. l have observed two basic interconnect schemes.
The first is a shielded video cable (group of coax cables with
a shield over them) that is an integral part of the monitor
and is routed directly to the video board. This method
makes it easy to avoid discontinuities in the shield. The second uses a separate set of cables for the inside and the
outside of the monitor. The connector interface to the monitor can be one of several types (e.g., a single connector (D
type) or a group of coax connectors) that are located at the
back panel of the monitor. An external video cable(s) which
is usually shielded or of the coax type is used to interface
the video source to the monitor. Another set of cables
would then be used to route the signals from these connectors to the video board. With either approach the important
thing is to get the video and sync signals to their destinations without imposing high frequency signals on other conductors or circuits and not radiate a significant amount of
energy by themselves. Therefore it is important to:
1. Avoid discontinuities in the shield.
2. Terminate the cables in their characteristic impedance.
3. The cable shield should be connected to chassis, assembly shield or other destination around the full circumference of the cable.
If we refer back to equation 2, we see that emissions are
directly proportional to the area of the path that a signal will
follow. Cable/wire lengths are usually quite long (15 cm to
45 cm). When we add to this that many are not near a
ground wire or ground plane because they are intended to
carry only low frequency or DC signals , the loop areas can
be very large. Cables that penetrate an EMI shield can pick
up noise, carry it outside the shield and radiate this energy
to the outside world. Let’s take a look at how a cable can
cause a problem. Suppose we wanted to know the noise
level at 300 MHz that would potentially cause an FCC Class
B emissions problem (if l 46 dB mV/m) for a 10 inch long
cable whose average distance from ground is 1 inch. Rearranging equation 2 to solve for V yields:
# 20 J
where AdB
Another question that often comes up is: what type of cable
shield or shielded cable should be used? There are two
types of shields used in commercial designs (unless of
course you want to use a rigid conduit). They are braids and
foil. Cable shields must be able to tolerate movement and
flexing without compromising their effectiveness. The main
advantage of a braid is it can meet these mechanical requirements. The main disadvantage is that it does not provide 100% coverage of the cable you want to shield. On the
contrary, foil can provide 100% coverage but is not as tolerant of the movement and flexing that is required. A good
alternative is to use both. This way if the foil is compromised, you will still have the protection of the braid. Another
alternative would be to use a double braid which will increase coverage of the cable.
The Use of Ferrites
Ferrite beads are often used by the EMI Engineer as the last
minute fix for an emission problem. The fact is that they can
be designed in to absorb high frequency (HF) energy that is
headed towards your low frequency and DC circuits and
eliminate potential EMI problems before the equipment gets
out to the test site.
The impedance of a ferrite bead can be expressed using
equation 4.
Z e 0R2 a 02 L2
Insertion loss in dB
The impedance of the source
The impedance of the load
The impedance of the bead
Ferrite beads can be used to control differential mode (DM)
and common mode (CM) noise. Figures 8(a) through 8(c)
show how this can be done. It is important to know what
type of noise you are working with in order to effectively use
a ferrite bead.
TL/H/12418 – 8
FIGURE 8(a). CM Noise Suppression
where R e equivalent resistance of the bead
L e equivalent reactance of the bead
0 e 2qf where f is frequency
Both R and L are dependent on frequency. This equation
does not specifically show the impedance roll off at high
frequency. Presently available ferrite beads have a maximum impedance of 100X –600X which occurs between
10 MHz and 300 MHz. The peak impedance and the frequency at which it occurs depends on the composition of
and the size and shape of the ferrite bead. Since the insertion loss provided by a ferrite bead is what matters most, it
is important to realize that they work best in low impedance
circuits such as power supplies.
The insertion loss can be calculated using equation 5.
ZG a ZL a ZB
AdB e 20 log
TL/H/12418 – 9
FIGURE 8(b). DM Noise Suppression
TL/H/12418 – 10
FIGURE 8(c). CM and DM Noise Suppression
I will note that it is important to have a good ground for the
video circuit to prevent noise from coupling through the
common ground.
Let’s take a look at some ways that ferrite beads will help
reduce noise and emissions in a display monitor. Figure 9
shows a schematic diagram of how a ferrite bead can be
used to filter a DC power supply for a video CRT driver.
Filter capacitors C3 and C4 should be chosen to supply the
required HF energy for the CRT Driver. The ferrite bead
along with C1 and C2 will then function to isolate and filter
any transients caused by the operation of the CRT Driver.
Another example would be using a ferrite bead on the G2
lead at the video board end. It is not uncommon for the HF
video signal to couple unto this wire due to stray capacitance between the video signal paths and the G2 supply
path. Figure 10 shows this. Multiple turns can be used to
increase the effectiveness of the ferrite.
TL/H/12418 – 11
FIGURE 9. A Ferrite Bead Used for Power Supply Filtering
TL/H/12418 – 12
FIGURE 10. A Ferrite Bead Used on the G2 Wire
closer the barrier is to the source of the electric field the
higher the reflective loss will be since Zw gets larger. As you
move up to and beyond a distance of l/2q from the source,
where l is the wavelength of the signal of interest, we make
the transition from the near field to the far field. At this point
ZW e 377X which is the impedance of free space. Therefore, to maximize the electric field losses due to reflection,
the shield should be as close as possible to the energy
The absorption loss can be approximated by the following
AdB e 131 t0mrsrF
After we have estimated the potential emissions or measured the actual emissions of our monitor or its sub-assemblies, we can define the trouble areas and look at our shielding requirements. For most commercial monitor applications
the concern is for radiated emissions from 30 MHz to
1 GHz. Materials such as aluminum, copper and steel all
have good high frequency (HF) shielding characteristics.
Aluminum is probably the most common in commercial
monitors. Let’s quickly review how a shield works and then
we can discuss the two important considerations that will
control the effectiveness of a shield design. For a more in
depth study of shielding refer to references 1 and 2.
There are two basic shielding mechanisms: reflection and
absorption . Shielding effectiveness (SE) can be described
by the following equation:
SEdB e RdB a AdB
where mr is the relative permeability of the material, sr is
the relative conductivity of the material, t is the thickness of
the material in mm and F is the frequency of interest in MHz.
This equation tells us two things: 1) Absorption loss is directly proportional to the thickness of the shield, and 2) Absorption loss will increase as frequency increases. Note that
absorption loss is not dependent on the distance from the
energy source.
One topic I haven’t discussed is re-reflection. If we refer to
Figure 11 we can see that as the field moves through the
shield it is also reflected at the second boundary. This will
occur over and over as the field moves back and forth
through the thickness of the shield, some of the energy escaping at each point of reflection. In our application, this
effect is usually negligible.
If there were no openings in a shield, it would be easy for us
to pass emissions testing. However, that is not possible in
the real world. Since we have cables to interface with,
CRT’s to connect to and we need openings for ventilation,
maximizing the effectiveness of our shield is more challenging. Let’s discuss the two categories that will have the greatest effect on the effectiveness of our shield: 1) Openings in
the shield and 2) Penetrations by cables and wires.
Openings in the shield may be intended or not intended. We
intentionally put openings in to interface with the outside
where R is the loss due to reflection and A is the loss due to
absorption. Figure 11 is a simple model that shows these
basic mechanisms. The reflective loss is due to the part of
the field that does not penetrate (is reflected by) the shield
and is a function of the impedance difference between the
field and the shield (often called the barrier). The reflective
loss for highly conductive shields can be approximated by
RdB e 20 log
#4 Z J
where ZW is the wave (electric or magnetic) impedance in
ohms and ZB is the impedance of the barrier in ohms/
square. ZB can be calculated using the following equation:
ZB e 3.68 c 10b7
where mr is the relative permeability of the material, r is the
relative coductivity of the material and F is the frequency of
Reflection is the dominant effect for electric fields because
there is a large impedance mismatch (ZW l 377X). The
TL/H/12418 – 13
FIGURE 11. Simple Shielding Model
world. Unintentional openings are usually due to a poor connection between two mating surfaces. In either case the
effectiveness of the shield will degrade as a function of the
maximum length of the opening. The shielding effectiveness
for an opening in a box shield is given by the following equation:
SEdB e 20 log
# FL J
The shield requires some openings for interconnect and
ventilation. Our list of openings is as follows:
1. 3 cable connectors - connector size is 0.8× c 0.25×
1× c 0.5× opening
2. 3 0.3× holes for the G2, focus and a ground wire
3. Ventilation holes at the top and bottom - 25 0.25× diameter holes on each surface.
4. Opening for CRT socket to connect to CRT - 1× diameter
Let’s assume that we will build our shield with two mil thick
aluminum (sr e 0.61 and mr e 1). Since the openings in
the shield will have the greatest control over its effectiveness, we will start there. First let’s decide where the openings will go. Items 2 and 4 will be on the front or CRT side of
the shield. The cable connector openings will be on the
sides. Let’s use equations 10 and 11 and make some calculations. Table V summarizes the results.
Table V shows that if we put all three cable connectors on
the same side of the shield we will not meet our objectives.
If we move one cable connector to the other side we will
improve our result by 1.8 dB and meet our objective. The
three wires and the CRT connector opening are on the
same side of the box shield, therefore we must integrate
their effect. Having two one inch holes on the same side of
a shield would yield SE’s of 32.4 dB, 22.9 dB, and 18.4 dB at
100 MHz, 300 MHz and 500 MHz respectively. Since the
three holes for the wires together have a smaller max length
(0.9 inches) than a 1 inch hole we can assume that the total
result for the three wire holes and the CRT connector hole
will be slightly better than this. Therefore, if we properly locate the necessary openings we can meet our objectives.
As a check on our shield material, let’s take a look at the
effectiveness of our shield with no openings. We can do this
using equations 6, 7, 8 and 9. The lowest impedance in the
video signal path will be at the output of the CRT driver
section. Lets estimate it to be about 10 pF. At 500 MHz, the
impedance of 10 pF is approximately 30X [1/(2qfC)]. For
simplicity we will use ZW e 30, which will give us a conservative estimate. Table VI summarizes the results. These
numbers are much higher than the calculations for the
openings. All that we have to do is to pay careful attention
to the penetrations by the cables and wires that interface
with the video board.
where F is frequency in MHz and L is the maximum length of
the opening in meters. Keeping the maximum length less
than l/20 will yield 20 dB minimum shielding effectiveness.
Table III shows the maximum length opening (l/20 lengths)
which will yield 20 dB of shielding effectiveness for various
frequencies. When there are multiple openings on the same
surface of the box shield the reduction in SE can be approximated by the following equation:
SEdB e b20 log 0n
where n is the number of openings.
Cables and wires that penetrate a shield not only require an
opening in the shield to do so, but they are prime candidates
to pick up noise and carry it outside of the shield. This is the
second consideration that must be addressed in order to
maintain the effectiveness of the shield. Due to their long
length they are also effective transmitters of this noise. Filtering of the input and output signals (this may not always
be desired) as close as possible to the point of entry into the
shield can be very effective in controlling EMI. Cables, such
as the video cable, are required to carry HF signals. Depending on the speed of the video signal, these cables may
need to be shielded. See the section on cabling for more
Example 1
Design a box shield for a video board that will mount directly
to the neck of the CRT. The emissions have been calculated and measured and the shielding requirements are defined as in Table IV below.
TABLE IV. Video Board Box Shield Requirements
Frequency Range
Required SE
Required SE (dB)
for 6 dB Margin
30 – 100
100 – 300
300 – 500
TABLE V. SE Calculations for Openings in the Box Shield
SE (dB) for
One Opening
at F (MHz)
Max Length
Openings for
SE (dB) with
Multiple Opening
at F (MHz)
Cable Conn.
b 4.8
b 4.8
b 14.0
CRT Conn.
Component Placement and Interconnection
TABLE VI. SE for a 2 mil Thick Aluminum
Box Shield with No Openings
ZB (X/Sq.)
R (dB)
A (dB)
SE (dB)
The component placement on a video board will be a function of the size and shape of the PCB as well as its interconnect scheme in the monitor. Optimal parts placement will be
different for a PCB that mounts directly to the CRT versus
one that is wired to a CRT connector board. However, the
rules for optimal parts placement will be the same. Mechanical and thermal considerations will also affect the size and
shape of the PCB. Following is a list for the objectives of the
parts placement:
1. Allow minimum lengths for all HF signal traces and return
2. Allow low impedance ground and power connections
over the frequency range of interest.
3. Allow signal flow that supports adequate separation or
isolation between HF signal paths and other signal paths
in order to minimize crosstalk.
Once the system design is completed and the size and
shape of the video board are defined, what procedure or
strategy should be followed to optimize placement? It
makes sense to start with the portion of the circuit which
poses the greatest EMI threat (processes the most HF energy) and then work our way down to the less threatening
circuits as we go along. Fortunately, this also is in the best
interest for performance. The following basic guideline is
1. First locate the CRT driver circuit as close as possible to
the video output connector. Be sure to leave room for arc
protection, AC coupling/clamp or other support circuitry
that is required. It is important to keep the video output
nodes as short as possible. Therefore, any components
that connect to them should be as close as possible.
2. Then locate the video preamp as close to the driver as
possible with the video output signal points as close as
possible to the driver video inputs.
3. Then locate the video input connector as close as possible to the video input section of the preamp.
In summary, once we have measured or estimated the
shielding requirements we can design a shield that will meet
them. For our application (30 MHz–1 GHz), the openings in
the shield and the penetrations into the shield will have the
greatest control over the effectiveness of the box shield.
Printed Circuit Board (PCB) Design
Commercial monitors will contain at least two PCB’s (one
for power supplies and deflection control and one for video).
Since the main HF threat is from the video signal itself, our
discussion will be focused there. However, all the principles
discussed are applicable to any board design. As you move
from the lower to the higher resolution monitors, you will
usually find additional PCB’s. Careful attention to PCB design is a very significant part of controlling EMI. I will discuss
PCB type, component placement and interconnection, minimizing loop areas, power and ground, crosstalk and power
supply filtering . All of these categories are dependent upon
each other, so the choice made in one area will affect your
design in another area. For example if you choose to use a
single sided (SS) PCB, you will also choose larger loop areas (for signals and their returns and power supply filters)
and a higher impedance ground.
PCB Type
Inside the video monitor the PCB’s will usually be either
single sided (SS) or double sided (DS). SS PCB’s offer a
significant cost advantage but make the control of EMI a
greater challenge. Based on some monitor experience, SS
PCB’s are usually used for the power supply and deflection
circuits (often called the main board) and for the video
board in monitors with a maximum display resolution of
1024 c 768. DS PCB’s allow the designer greater control in
minimizing loop areas, lower impedance power and ground
distribution, as well as the capability of designing 75X microstrip for the video input signal trace (for cases where the
preamp is not in close proximity to the video input connector). The ability to have a large ground plane is an important
attribute to help control EMI.
Figure 12 shows the ideal parts placement for a video channel that is comprised of a monolithic preamp and a hybrid
(or monolithic) CRT driver that is part of a video board that
would mount directly to the CRT. In an actual application,
this parts placement would probably not be possible. The
important thing is to keep the video signal flow over the
smallest area possible. The closer we can come to the ideal
parts placement, the better the performance will be and the
lower the emissions of the board will be.
TL/H/12418 – 14
FIGURE 12. Ideal Video Channel Parts Placement
After the parts are placed, the trace routing should be done
using a prioritized list with the HF signals, power and ground
at the top of the list. Following should be the low frequency
signals and then finally any DC signals. Attention must also
be paid to avoid coupling the HF signals unto the low frequency signal traces (crosstalk). Crosstalk will be discussed
later in this article.
Minimizing Loop Areas
Looking at equation 2 we can see that emissions are proportional to the log of the loop area. Therefore, if the loop
area is doubled, we should expect emissions from that particular loop to be doubled (6 dB higher). Looking at it from
the other side, if we cut the loop area in half we should
expect 6 dB lower emissions. Kirchoff’s current law says
that any current that is sourced from a circuit must return to
it (the net current through any closed area must equal zero).
Therefore we must pay attention to the complete current
loop. In order to keep loop areas as small as possible, the
following two guidelines should be followed:
1. All HF signal traces should be as short as possible.
2. All HF signals should have a return trace. On DS boards
this can be through the ground plane. The ground plane
should cover the complete trace length. On SS boards a
return trace should be routed right next to the signal
An area of exception is at the CRT driver outputs. Having a
ground plane underneath these traces usually adds a significant amount of capacitance to the load. A 10 mil wide trace
over a ground plane on a 60 mil thick PCB has about
1.25 pF of capacitance per inch in length. If the performance requirements do not allow this extra capacitance, the
EMI design may require some additional shielding. The arc
protection design may also require some isolation between
the CRT ground and circuit ground. This was discussed in
the section on grounding.
Power Supply Filtering
Proper power supply filtering (also called bypassing) on
PCB’s plays a vital role in the control of EMI. HF power
supply currents supplied to fast circuits must be treated the
same as HF signal currents. The HF currents can also
cause voltage spikes which can be introduced as noise to
other circuits. If these noise transients find their way past
the I/O connector to a cable or wire, we can have an emissions disaster. Remember, we saw in our section on cabling
that noise levels in the tens of millivolts can cause problems . The purpose of filtering the DC power on the PCB is to
provide a local AC energy source where it is needed most
and keep HF power transients from traveling to/from other
circuits on the PCB or I/O cabling.
Figure 13 shows a PCB that contains a high speed circuit
(such as a video amplifier) that is supplied with DC power
from another assembly or PCB. The power is connected
through a wire bundle or cable. The PCB requires filtering at
two points. The first place to filter is right at the power input
connector. A large electrolytic capacitor (10 mF – 100 mF
typically) and one or more small ceramic capacitors
(0.001 mF – 0.1 mF) for high frequency filtering usually will do
the job. The function of these capacitors is to replenish the
energy drawn from the local bypassing located over the rest
of the PCB which will relieve the power supply from doing
this over a long distance (and generating some undesired
emissions). The second place to filter is right next to the
power and ground pins of high speed IC’s or in areas of
discrete circuits where HF transient currents are drawn. Ceramic capacitors (0.001 mF – 0.1 mF) will do the job here. If
the local IC or circuit also draws a significant amount of
average current an electrolytic may also be required. This is
typically the case for video preamps and CRT drivers. The
power supply will also have its own filter caps to keep noise
and ripple below the required levels.
TL/H/12418 – 15
FIGURE 13. DC Power Supply Filtering Scheme
One way to determine if the power supply filtering is adequate is to measure the noise voltage across our filters or
between the power and ground pins of the critical IC. This
can be done using a probe and oscilloscope. Be sure that
the measurement system has enough bandwidth to take
measurements over the frequency range of interest. If the
noise level at specific frequencies is of interest (and it may
be), a spectrum analyzer can also be used. Many oscilloscopes have a 50X output that can be connected directly to
the spectrum analyzer input. Before taking the actual noise
measurement it is important to determine how much noise
the probe is picking up from near field radiation. This can be
done by connecting both the signal and ground of the probe
to the ground side of the filter cap across which the noise
measurement will be taken (hopefully this is a low impedance ground plane). This measurement can then become
our ‘‘noise floor’’ which the noise across the filter can be
compared to. It is also very important to keep the ground
lead of the probe as short as possible.
Other considerations for filtering are:
HF signals can be coupled from one trace (source trace) to
another (victim trace) on a PCB due to the capacitance between them. This is known as crosstalk. We can avoid
crosstalk by not routing a second trace next to a high speed
signal trace for any significant distance. As the length of the
parallel run increases so will the capacitance between the
traces and hence, the crosstalk. Crosstalk can be reduced
by increasing the distance between the traces. Inserting a
ground trace between the traces will significantly reduce
On DS PCB’s, the close proximity of the victim trace to the
ground plane will reduce crosstalk in comparison to a SS
board due to the capacitance from the victim trace to
ground. This capacitance reduces the impedance of the victim trace at high frequencies which will reduce crosstalk.
For a more in depth study of crosstalk please refer to the
references listed at the end of this application note.
We have discussed many topics that are part of the EMI
engineers tool kit which is used to predict, minimize and
control emissions. We know from the relationships that we
have discussed that the emissions potential is directly related to the performance of the display. In other words, as
resolution, picture quality and edge speeds increase it will
take a better EMI design to meet emissions requirements. In
order to design for EMC (lack of EMI), emissions should be
considered from the start of the product design cycle. In
order to do this the complete system design must be considered. This includes the PC and graphics card at a minimum. The following list of guidelines has been put together
for use throughout the product design cycle. It is not exhaustive, but hopefully the reader will find it of interest.
1. The speed (rise and fall times) of the input video signal(s)
should not be any faster than necessary . This will minimize the HF energy in the input signals. Then the CRT
display will have less HF signal to amplify and radiate.
The speed of this signal is dependent on the PC graphics
card design and therefore not under the control of the
display designer. However, in the cases where the whole
computer system is being designed together, we can
work with the graphics card designer to target the desired video speed. If the monitor will be a stand alone
product, then we can choose a graphics card that is designed to meet our performance criteria but is not over
designed for the application.
2. Don’t use a higher bandwidth amplifier than necessary .
The video channel bandwidth should be adequate to provide the desired signal speed at the CRT cathode when
driven by the selected graphics card. Having too much
bandwidth will allow unnecessary HF signals (including
noise) to be amplified, which will produce higher emission levels at these frequencies.
# Component leads should be as short as possible to minimize lead inductance.
# Local HF filter capacitors should be as close as possible
to the power and ground leads of the IC it is supplying
energy to in order to minimize the loop size of the circuit
and the effects of trace inductance.
# Use the lowest value HF filter capacitor that provides
enough energy for the circuit. This will move the self resonance of the circuit up in frequency.
# Ferrite can also be used for isolation between filters on a
PCB. Please see the section on the use of ferrites.
The main goals for the power and Ground distribution on a
PCB are to keep loop areas as small as possible (for intended and unintended signals) and to provide low impedance
distribution of power and ground so that it does not cause
and distribute noise. For DS PCB’s this is somewhat easier
because we can put a ground plane on one side. It is important to remember that power supply current transients will
radiate just like signal currents. I recommend the following
guidelines for DS and SS boards.
For DS boards:
1. The power routing should be over the ground plane over
its full length. Avoid as many discontinuities as possible
(unfortunately we can’t have a solid plane unless we
could use all surface mount components).
2. Avoid routing power etch that feeds high speed circuits
near the edge of the PCB where there won’t be significant overlap of the ground plane. You may even consider routing a ground trace to the outside of the power
3. Use as wide of etch as possible for power distribution in
order to minimize trace inductance.
For SS boards:
1. Route power and ground right next to each other right
from the input connector to the destination. Because
space will be limited give priority to the high speed circuits. Remember that power supply current will radiate
just like signal currents.
2. Use as wide of etch as possible for both the power and
ground distribution in order to minimize trace inductance.
3. Test printed circuit boards as soon as they are available .
There is no need to wait until the first monitor is completed. Testing early can flag a problem that can be corrected easier (and less expensively) earlier in the design cycle.
4. Weigh the tradeoffs between EMI Design and product
volume . For high volume products fine tuning the EMI
design will be worth the cost of the testing and troubleshooting necessary to minimize the unit cost of the EMI
solution. For low volume products, it may cost more for
testing and troubleshooting than will be saved in the unit
cost over the life of the product. The length of the product design cycle must also be considered. There is always a limit to how much time can be spent reducing the
unit cost of an EMI solution.
5. It is easier to remove than add . When doing your up front
EMI design, if you think you need a shield, filter, ground
connection or anything else, design it in or at least put in
the provisions for it so you can easily add it. It can be
very costly and painful to try to add a fix that the product
won’t accommodate. If you end up not needing the
items, simply don’t install them and remove the provisions for them during the next design update. No one will
argue with you when you take things out.
6. Before you test the monitor, know the emissions of the
PC, graphics card and cable . I recommend dividing the
emission level allotment between the PC (with the graphics card and cable) and the CRT Display. Therefore, if
our overall goal is 6 dB margin, the PC with the graphics
card and a properly terminated video cable should have
12 dB margin when tested alone. If the PC system
doesn’t have much margin at video signal frequencies,
passing the test with the monitor will be quite a challenge.
EMC Ð Electromagnetic Compatibility
Ð Electromagnetic Interference
Ð High frequency
Ð Single-sided
Ð Double-sided
Ð Cathode ray tube
Ð Printed circuit board
Ð Shielding Effectiveness
1. Mardiguian, M. 1992, Controlling Radiated Emissions by
Design, Van Nostrand Reinhold.
2. Ott, H.W. 1988, Noise Reduction Techniques in Electronic Systems, second edition, John Wiley & Sons.
3. EDN EMC Special Issue, EDN’s Designer’s Guide to
Electromagnetic Compatibility, January 20, 1994.
4. Cocovich, J., EMI/RFI Board Design, National Semiconductor Application Note 643, December 1989.
5. Osburn, J.D.M. and White, D.R.J., Grounding, 1987 IEEE
EMC Symposium.
6. Cowdell, R.B., Don’t Experiment with Ferrite Beads,
Electronic Design, vol. 17, June 7, 1969.
7. Blood, W. R., MECL System Design Handbook, Motorola
Semiconductor Products, 1983
Table I.
Regulating Agencies and Computer Products
Table II.
Table III.
Emissions at 3m for a 1V Signal from a 6 cm
Conductor Wavelengths at Various
Video Board Box Shield Requirements ÀÀÀÀÀÀ10
SE Calculations for Openings in the Box
SE for a 2 mil Thick Aluminum Box Shield with
Table IV.
Table V.
Table VI.
EMI Solution Cost Versus Time in Monitor
Figure 2.
Figure 3.
Generic Graph of The Fourier Envelope of a
Figure 4.
Spectral Envelope for a 1 VP-P, 50 MHz
Square Wave with Various Rise Times ÀÀÀÀÀÀ3
Figure 5.
Fourier Envelopes of 4 ns Rise Time, 1 VP-P
Figure 6.
Simplified Block Diagram of a CRT MonitorÀÀ5
Figure 7.
Grounding for CRT Arc ProtectionÀÀÀÀÀÀÀÀÀÀ6
Figure 8(a). CM Noise Suppression ÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀ7
Figure 8(b). DM Noise Suppression ÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀ7
Figure 8(c). CM and DM Noise SuppressionÀÀÀÀÀÀÀÀÀÀÀÀ7
Figure 9.
A Ferrite Bead Used for Power Supply
Figure 10.
A Ferrite Bead Used On The G2 WireÀÀÀÀÀÀÀ8
Figure 11.
Figure 12.
Ideal Video Channel Parts Placement ÀÀÀÀÀ11
Figure 13.
DC Power Supply Filtering Scheme ÀÀÀÀÀÀÀ12
Figure 1.
Grounding for Arc Protection ÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀ5
Printed Circuit Board (PCB) Design ÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀ11
Component Placement and InterconnectionÀÀÀÀÀÀÀ11
Radiated Emissions and CRT Displays
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