Troubleshooting Radiated Emissions

Troubleshooting Radiated Emissions
5
Troubleshooting Radiated Emissions
—A Practical Approach
Kenneth Wyatt
Abst r act
Because time-to-market and budget factors often drive many of today's high-tech designs, electromagnetic compatibility
(EMO) issues often surface at the last moment in the design cycle, potentially delaying product introductions. Very often,
simple pre-compliance measurements and techniques can identify issues early when the cost of implementation is
substantially lower and design improvements may be made with less impact on schedules. This paper describes a number
of si e techniques and tools useful in characterizin e radiated emissions of a desion at various stages
f simple techniq d tool ful in charact g the radiated (RE) of a design at tages of
development and will better prepare products for a successful radiated emission qualification.
Keywords
radiated emission, field probe, troubleshooting kit
Introduction
There are usually five key threats that comprise most
electromagnetic compatibility (EMC) problems —radiated
emissions, ESD, susceptibility to RF fields, power
disturbances and internal crosstalk. Of these, radiated
emissions (RE) can be the most difficult EMC test for a
product to pass. Because emissions limits are established
worldwide, products that don't meet the limits may not be
placed on the market. The best way to achieve this is
through proper product design, but often these design
techniques are not taught in universities, nor are these
techniques fully understood by many experienced
engineers. In many cases, EMC is considered as "black
magic' and many products must be tested repeatedly
through a system of "trial and error", in order to finally
pass.
PR Le
dl
ne
-
3 LP — a - m i
x ar A SL A vel
Figure 1: Photo of a typical 3m radiated emission test chamber.
32 SAFETY € EMC 2008
This 1s unfortunate, because the emissions a product
may produce 1s easily understood if the designer considers
that it's the high-frequency currents in circuit loops that
tend to broadcast these emissions. These circuit loops may
be in the form of printed circuit traces (differential-mode
currents) or cables connecting two subsystems
(common-mode currents). There may also be combinations
of these phenomenon. The circuit and system design level
of a product usually falls within the domain of the
electronic engineer. The other consideration is the
shielding properties of the product, which typically falls
within the domain of the mechanical engineer. Ideally,
these to must work together as a team to address the whole
product in order to be successful in addressing EMC.
Background Theory
In order to better understand RE and how to troubleshoot
your product, we must review how harmonics are created
and then understand differential-mode (DM) and
common-mode (CM) currents and how they get generated.
General design techniques are mentioned but specific
design practices are a subject for another paper.
A periodic square wave (Figure 2) may actually be
represented by a series of more basic signals called "basis
functions". Assuming the rise and fall times of the square
wave are straight up and down, an infinite number of
harmonically-related basis functions or sine waves are
required. Digital circuitry today uses rise and fall times of
sub-nanoseconds, which can generate harmonics of
several hundreds to thousands of MHz.
Y — yv(1)
1,9 еее Г eer ER EER
0,5 |---0000000ceeeeeeeeecec cc rerecerenee [eecccere rec erere ee fee eee c cernes
0,0
E armee soma |
A Erase. Aina IZ — ——
0,0 10,0 20,0 30,0 40,0
time mS
Figure 2: A periodic square wave digital signal
The building of a square wave: Gibbs' effect
a0 100 120 140 160
Figure 3: À representation of the square wave is comprised of a linear comb
ination of basis functions, or sine waves. Image courtesy of MathWorks.
Difterential-mode (DM) currents are caused by digital
signals (and their harmonics) traveling through circuit
loops. The larger the loop, the stronger the fundamental
and harmonic emissions. We want to minimize the area of
any circuit loops through use of ground planes (typically
by use of multi-layer circuit boards). For low-cost
products, multi-layer boards may not be feasible, so other
design techniques must be used to minimize these loops.
Let's consider a simple circuit loop with a square-wave
source and resistive load. The fundamental (signal)
current, plus all the related harmonic currents, will
circulate around the loop as shown. Let's assume we have
a receiving antenna 3m away from this circuit loop. These
currents are really phasors. When the phasor produced by
the far wire 1s added to the phasor produced by the near
5
wire, the result is the difference in phasor magnitudes,
which produce a small resultant phasor and relatively low
emission as shown in Figure 4. The area of the loop, the
current levels and harmonic frequencies, dictates how
much radiation will be produced.
Source Ip Load
Signal :
, К
Signal Return |
—— |!
lo
: d= 3m
Phasor from far wire —
Phasor from near wire +—
Resultant phasor —
Figure 4: Differential mode currents in a circuit loop. The source is a digital
signal (with harmonics) and we'll assume a resistive load. Because the phasor
current in the far wire is opposite the phasor current in the near wire, the
resultant phasor is relatively small compared to that produced by
commorr mode current phasors. However, reducing the loop area is very
important in limiting radiated emissions.
The equation for calculating the emission level in
volts meter for a DM signal flowing in a loop is shown
below in Equation 1.
7 fLs
В =1.316х10 227
d
D, max
Equation 1: Field level (VAn) due to DM current, where f = frequency (EZ),
L =length of the wires (mm), s =spacing between wires (mm) and d =the
measurement distance (typically 3m or 10m).
So, how do we minimize the emissions froma DM
circuit loop? Note that the area of the loop is L*s. First, we
must realize that I 1s likely fixed by the design. Likewise,
frequency f, is probably fixed. However, length of the loop
L, and distance s, may be reduced. In other words, the
area of the loop may be decreased to reduce emissions.
This is an important point to keep in mind during circuit
layout. Placing a crystal oscillator (one common source of
harmonics and resulting emissions) close to the circuitry
that requires the clock signal is a good design practice.
Likewise, the use of multi-layer boards with full ground
planes serves to reduce the loop area substantially.
Now, let's consider common-mode (CM) currents and
how they are generated, because it is not intuitive as to
how current may travel the same direction through both
the signal and signal-return wires in a system. Referring to
Figure 5, note that due to finite impedance in any
SAFETY & EMC 2008 33
5
grounding system (including circuit board grounds), there
will be a voltage difference between any two points within
that ground. This is denoted by Vp and Vew» in the
figure. This difference in potential will drive CM currents
through common cabling or circuit traces between circuits
or sub-systems. These CM currents may be generated on
circuit boards or within sub-systems inside product
enclosures. Because the current phasors are additive, the
resulting radiated phasor may be quite large compared to
those generated by DM currents. Therefore, CM currents
tend to be more of an issue than DM currents.
Source le Load
Signal
| ‘ К,
Signal Return
—_— =Z+—
У lc У
Venp1 Venp2
d = 3m
Phasor from far wire “
Phasor from near wire “* —————
Resultant phasor +
Figure 5: Common mede currents in a circuit loop. The source is a digital
signal (with harmonics) and we'll assume a resistive load. Because the phasor
current in the far wire is in the same direction as the phasor current in the
near wire, the resultant phasor is relatively large compared to that produced
by diffcrentialmode current phasors. In this case, lowering the harmonic
content (by slowing the digital rise Aall-times) or diverting/blocking the CM
current 1s very important in limiting radiated emissions.
The equation for calculating the emission level in
volts meter for a CM signal is shown below in Equation 2.
A
ГИ.
а
Equation 2: Ficld level (V/A duce to CM current, where £= frequency (Hz),
L =length of the wires (m) and d = the measurement distance (typically 3m or
10m).
—1.257x 107°
E
C,max
Note that the exponent is now -6, rather than -14 —a
much larger number; thus, the emissions from CM
currents are typically much higher than that from DM
currents. So, how do we minimize the emissions from a CM
circuit loop First, we must realize that frequency, fand
the length of the cabling, Lis likely fixed by the design.
However, I, which is an undesirable signal, may be
reduced by either using a lower-impedance ground, or by
blocking (with a ferrite choke or with CM filters) or by
diverting through proper mechanical design. This is an
34 SAFETY € EMC 2008
important point to keep in mind during system design.
The solution to most EMC problems is to Control The
Path of Current!
Troubleshooting Philosophy-Radiated
Emissions
In troubleshooting any radiated emission problem, it's
useful to think of the problem in the form of a
'source-path-receptor' model. See Figure 6 below.
Source Transfer Receptor
(emitter) (coupling path) (receiver)
: | | | 5 , Slot, :
Radiated © TV, Radio
Emission Oscillator
Figure 6: Source—Path—Receptor model. Typically, the source of radiated
emibelons iva Mb femenil exilbloracoter high-Tequeney,
fast—edged, high—current signal. ASICs, FPGAs and A/D or D/A converters
may also generate these high—frequency harmonics. The 'path' is the coupling
mechanisex rorfhemeansbyrahioh fie bigh-Bequensy energy Being
radiated. The "receptor", in most cases is the EMI receiver at the test site with
specified emission limits.
By using various probes, it should be possible to
identify the source or sources. Once the sources are
identified, the path or coupling mechanism must be
identified and fixed. What's difficult is that there may be
multiple sources and coupling mechanisms to identify and
fix, before passing results are achieved. In addition, if a fix
is improperly installed, the emission can actually get
worse! That's probably why the field of EMCis considered
so "mysterious".
By using a structured approach, the troubleshooting
phase should go smoothly. Generally, you'll want to
diagnose the issues first then try various fixes. Leave
these fixes installed as you continue the troubleshooting
process. If you set up an antenna and EMI receiver or
spectrum analyzer a fixed distance away (1 to 3m) from
where you're troubleshooting you can monitor your results
real-time.
IDENTIFY THE SOURCES—The first step should always
be toidentify the likely sources. If you're failing at 300 MHz
or 500 MHz, for example, are these the third or fifth
harmonics of a 100 MHz clock oscillator? How about the
memory clocking? Generally memory address and data
busses are fairly random. The exception would be the AO
or DO line, which 1s clocking at a relatively non-random
rate. What about clock lines to ASICs or FPGAs? If you
have multiple crystal oscillators, which could be the cause
of a particular harmonic, spraying "freeze spray" on one
then the other—can often identity the oftending oscillator.
FREQUENCY —The frequency is key to any radiated
emission problem. As a quick rule of thumb, the higher
the frequency, the more likely the coupling path 1s
radiated. The lower the frequency, the more likely the
path is conducted. In fact the common break frequency is
30 MHz. Below that, we measure conducted emissions
(CE)—above that we measure RE. If your product uses a
high-frequency crystal oscillator with fast edge speeds, the
harmonic content can be estimated with the formula in
Equation 3.
1
xr
Equation 3: Maximal RE frequency estimate, where £ = EMI frequency (Hz)
and t, =risetime (5).
For example, with 1ns logic, the harmonic content may
be centered around 300 MHz. Another rule of thumb is
that for frequencies below about 300 MHz, the problem is
most likely due to common-mode emissions from cables
and above that; the problem is most likely radiation from
slots or seams in the metal chassis or circuit board
radiation.
DIMENSIONS —The dimensions of physical structures are
also an important factor in troubleshooting an emissions
problem. Recall that the wavelength (m) of a resonant wire
at frequency £ in free space 1s:
= © _3х10°
f
Equation 4: Wavelength of a wire, where c =speed of light in m4 and f =
frequency in Hz.
The dimensions in physical structures, like circuit
boards must be reduced by the velocity factor of the board
material (example, 4.7 for FR4 circuit boards). However,
typical cables, such as USB or video) are approximately
1m long and can be considered as being in free space.
Wires or slots may resonate strongly at multiples of a
quarter wavelength. For example, a 1m long cable has a
full-wave resonance of 300MHz, but may also radiate
strongly at 150MHz and 75MHz. Slots or seams of 8 to
5
15cm may resonate in the area of 500 MHz to 800 MHz.
As a general rule of thumb, radiating cables or chassis
slots of 1/20™ wavelength or greater, start to become
significant coupling paths for RE.
PROBES—There are a variety of useful probes that may be
used to troubleshoot RE problems: E-field, H-field and
current probes. The E-field and H-field probes are easily
made in the lab. All are available from several
manufacturers’. An E-field probe may be made by
extending the center conductor about 0.5 cm from a
section of semi-rigid coax or high-quality flexible coax;
then attaching a coax connector to the other end. Shorting
of the probe to circuit traces may be avoided by wrapping
insulating tape around the end. A useful H-field probe
may be fashioned by looping the center conductor around
and soldering it to the shield to form a small loop of 0.5cm
to 5 cmin diameter (the larger the loop), the more
sensitivity. A better H-field probe design uses sem-rigid
coax to form the loop (see examples in Figure 7). These
probes are then connected to the input of an EMI receiver
or spectrum analyzer to display the harmonics as the probe
is brought into close contact with the circuit traces or
chassis slots. Depending on the diameter of your H-field
probe, you may need to use a broadband preamplifier
between the probe and analyzer”.
Figure 7: Examples of commercial E-field and Hfield probes from Beehive
Electronics.
Figure 8: Examples of home made Hfield probes.
1 Probe manufacturers include Fischer Custom Communications (www. fischercc.com), Beehive Electronics (www.beehive—electronics.com) or
Teseg (Wwww.teseg.com).
2 I made my own broadband preamp using a MiniCircuits model ZX60—3018G—S, which covers 20 MHz to 3000 MHz at 18—23 dB gain and
2.7 dB noise figure. It sells for USD 50.
SAFETY & EMC 2008 35
Figure 9a: Uke of simple FFfield probes to locate emission sources.
Meerkat Main PC Board
M. E ' J
Cable
100MHz 100MHz XTAL
Harmonics feeds PLL
on cable J Creating 133MHz
clock and maybe
us 933MHz ham
Heatsink is
Ok, but "hot ” ”Hot” 100MHz
100 Mhz ham harmonics
undemeath
50MHZ ham
24MHz XTAL ;
mayalss be Cabins Ok
creating the 933MHz
harmonic at harmomic
933MHz
933MHz harmonic coupling
to I/O pins due to proudmity
to 24 MHz xtal oscillator
Figure 9b: I map out all the potential sources; documenting them on a
photograph of the circuit board.
"all |
Figure 10: I made my own broadband prceamp using a Mint Circuits model
ZX60-3018G58. It is powered it with two 6V Duracell #28A batteries, which
fit in a standard 'AA' battery holder, The amplificr covers 20 to 3000 MHz at
18-23 dB gain and is used to boost the probe signals.
TROUBLESHOOTING STEPS—Generally, once you are
finished mapping out your sources, you should start with
the lower harmonics and work upwards. Often,
lower-frequency sources will cause significant
high-frequency harmonics, depending upon the rise time.
By fixing the low-frequency source, you'll often resolve
high-frequency harmonics, as well. Next, check cables
and then the enclosure.
CABLES—Check your cables first, as they are often the
worst offenders. Moving a 'hot' cable will alter the RE
levels. I usually unplug all cables; then try plugging each
one in individually to find all that are radiating. Remember
that there may be more than one bad cable! Snapping a
ferrite choke around the base of the cable will probably
help as an interim fix. I’ ve found that most cable
emissions are very likely due to poor grounding at the I/O
connector.
(Cable currents may also be measured directly versus
frequency with a current probe’. Examples are shown in
Figure 11.
LAN conn
needs gnd
shell.
7 ps в $ :
N t: poor/failing margins. ° did
Note a lack of good “ OU."
connection between . | |
chassis enclosure and |= Pili wd Hl и
ZA, WE
connector ground. FA | MY
20 КАИ Мне hE я essen dea ess
"ЧЕЛ | TT
AQ Feeriendrcenah rte NADO To pea a... Sg afin oy =";
"a 40 ca an и FT = on
CHAMBER DATA FREQUENCY Emma)
Figure 12: Poor I/O connector grounding to the chassis allows the cable to
radiate and usually fail the RE test. A lack of solid ground can allow CM
currents generated inside the product to flow out the 1/0 cable and radiate
sually causing RE failures. The included graph shows poor margins to the
CISPR {1 (ass A 3m RE limit (for ISM products, in this case). I'TE products,
such as PGs and printers have a limit 10 dB lower.
Current probe on USB cable.
Connection between connector
Bef After
ground shell and chassis ore
enclosure made with Some harmonics dropped by 10-15 dB!
screwdriver blade.
Figure 13: Cables should be tested individually. Here, I have a current probe
clamped around the cable under test and am monitoring the harmonics with a
simple hand—held spectrum analyzer’. As I ground the connector shell to the
chassis with the screwdriver blade, the harmonics are reduced 10 to 15 dB!
3 Commercial current probes are available from Fischer Custom Communications (www.fischercc.com), Teseq (www.teseq.com) or Solar
Electronics (www.solar—emc.com).
4 The handheld spectrum analyzer being used for the cable test is made by Thurlby Thander Instruments (www.tti—test.com). It sells for
approximately USD 1500 and covers 150 kHz to 1 GHz.
36 SAFETY € EMC 2008
It is possible to actually predict whether a particular
cable will pass or fail by measuring the CM current at the
offending frequency, solving for IC (Figure 14 and
Equation 5 below) and plugging this into Equation 2 to
solve for the field level in V/m. The length of the cable is
L and the offending harmonic frequency is f. Use a test
distance of either 3 or 10m to predict the outcome at those
test distances.
(GP ZF)WHO L 3AO8Ÿ GP FONVO3dWNI H3ASNVEL
FREQUENCY
Figure 14: Transfer impedance (Z;) graph of a typical current probe (courtesy
of Fischer Custom Communications). The x—axis is frequency, while the
y-axis is dB{2. Uke this to calculate the value of IC, given the measured
voltage at the probe terminals (Vg) and Zr
de Pa. Ze
ClaBua dBuV TlaBQ
Equation 5: Calculation of I: given the measured Vand Zr (from Figure 14).
Next, plug I- into Equation 2 to calculate the predicted E—field emission level
tn Écrire die rd pas ell die me le
being meme.
SLOTS & SEAMS—Once the cables and associated 1/0
connectors are addressed, it's time to probe for radiation
leakage through slots or seams in the chassis. Remember,
that the length of the slot or seam is important. Any seam
with leakage whose effective length is longer than 1/20* of
a wavelength at the harmonic of concern has the potential
to be an effective radiator. For example, a slot of 2.5cm
can just start radiating harmonics at 1000 MHz. I use a
permanent marking pen to record the areas of leakage and
frequencies of concern from every seam£lot on the
chassis.
Once these are marked, I'll carefully cover over all the
openings with copper tape and re-measure the RE levels.
Keeping an eye on the RE levels, I'll start removing the
tape piece-by-piece to determine which slots or seams are
actually causing problems. Often, just a fewslots or seams
are causing the most problems. Once the leakages are
identified, you can determine the appropriate fixes with
your mechanical engineer.
It's also possible to use a differential probe and
high-bandwidth oscilloscope to measure any voltage
5
differences between pieces of sheet metal on the chassis
enclosure. If any voltage is measured, it indicates a poor
connection and potential leakage. Figure 13 shows how to
place one tip on each side of the joint. Ideally, the voltage
should measure zero.
—
Figure 15: Ue of a differential probe and higlr frequency oscilloscope or
spectrum analyzer to measure the potential difference across suspected gaps
in the seams of a product enclosure. Ideally, this voltage should measure zero.
TROUBLESHOOTING KIT—For speedy troubleshooting
and analysis, I've assembled an EMC troubleshooting kit
into a portable case, which can be wheeled right to an
engineer's workbench. Major contents include a small
spectrum analyzer, a broadband preamplifier, small
antennas, various probes and other accessories. Other
useful items to include into your troubleshooting kit
include ferrite chokes, aluminum foil, copper tape, power
line filters, signal filters and various values of resistors
and capacitors. Figure 15 shows an overall view of the
contents.
Figure16: Contents of the special EMC troubleshooting kit I've assembled. 1
can probe for various RE problems, as well as test for ESD and radiated
immunity. Performing these tests early in the design cycle, results in a
greater chance of passing the required EMC product qualification tests.
Summary
In order to pass required EMC tests for radiated
emissions, it is necessary to understand the basic
concepts of current flow through loops, as well as
differential- and common-mode currents and how they're
generated. Troubleshooting an existing design is simply
the process of identifying the likely sources, determining
the coupling mechanisms through probing, and applying
temporary fixes. Once these fixes have been applied and
the product passes, then the electronic and mechanical
engineers may determine the most cost-effective
solutions. Obviously, troubleshooting or characterizing
products early in the design cycle Continued on Page50
SAFETY & EMC 2008 37
5
If there is only one type in an application unit, the type of
sample should be delivered. Representative samples
should be chosen and delivered if certification application
of a series of products is within the same application unit.
d) Relativestandards (Referto Table frompage 44topage 47)
e) Factory inspection
On-site examination on factories is an important
precondition for enterprises to obtain CESI certification
for the first time, as well as mantaining effectiveness of
certificate. Usually, the examination includes initial
examination and at least one time reinspection per year, as
to the latter, the enterprises would be informed or not
informed beforehand. Content of factory examination
includes examination on certified products and
requirements on quality system of the factory, which
involve factory's: responsibility and resource, documents
and records, procurement and incoming goods
examination, manufacture process control and process
examination, routine examination and confirmation
examination, examination on instruments and equipments,
control on unqualified products, internal quality
examination, coincidence of products and packaging,
removal and deposition of products, totally in ten aspects.
Those requirements would ensure that batch manufactured
certificated products can comply with requirement of CESI
certification standard, and consist with certificated type of
sample.
I believe CESI electronic component certification will
be an effective assistant means for your products to enter
China market.
Please browse relative information at: www.cesi.ac.cn
Contact: +#86—10—6400 8580/34029206
Continued from page 37
are preferred in order to reduce overall implementation
costs.
Kenneth Wyatt—Sr. EMC Engineer, Wyatt Technical
Services, LLC, holds degrees in biology and electronic
engineering and has worked as a senior EMC engineer for
Hewlett—Packard and Agilent Technologies for 21 years. He also
worked as a product development engineer for 10 years at various
aerospace firms on projects ranging from DC=DC power converters
to RF and microwave systems for shipboard and space systems. A
prolific author and presenter, he has written or presented topics
including RF amplifier design, RF network analysis software,
EMC design of products and use of harmonic comb generators for
predicting shielding effectiveness. He has been published in
magazines such as, RF Design, EMC Design & Test, Electronic
Design, Microwave Journal, HP Journal and several others.
Kenneth is a senior member of the IEEE and a long time
member of the EMC Society where he serves as their official
photographer. He is also a member of the dB Society and is a
licensed amateur radio operator.
His comprehensive yet practical EMC design, measurement and
troubleshooting seminars have been presented across the U.S.,
Europe and Asia. He currently resides in Colorado and may be
contacted at [email protected]—~seminars. com. His Web site is: www.
emc-seninars.com.
Continued from page 47
Safety and EMC Testing Center of Electronics Industry
(SEC) is legally owned by China Electronics
Standardization Institute (shortened as CESI). SEC1s a
government approved Testing Lab (approved by the former
Ministry of Electronic Industry of the P. R. C)) specialized
in safety and EMCtesting of electronic products, aimingat
supporting the government on quality supervision. SEC is
an independent and non-profit testing body.
SEC was authorized by Certification and Accreditation
Administration of the People's Republic of China (CNCA)
to evaluate safety and EMC performance of products.
CESI mark is one of several marks designated by CNCA,
50 SAFETY € EMC 2008
which can be accepted by 3C certification. The
components having acquired CESI certification don't need
to be tested again during 3C certification process of
equipments. That also means the manufacturers having
obtained CESI marks on their products have passed the
certification of national-level certificate authority. As a
result, value-added of products will undoubtedly increase.
As a matter of fact, certificated components can be used
by electronic equipment manufacturers more preferably;
consequently, it will lead to the increase of product sales
volume and bring more benefits to the manufacturers.
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