emc basics

EMC Basics #1: Welcome!; and Clocks: critical circuits for EMCDaryl Gerke, Kimmel Gerke Associates

2/24/2011 1:09 PM EST

(Editor's note: we are pleased to begin a new series on the vital and sometimes unappreciated

topic of electromagnetic compatibility (EMC), presented by well-known expert Daryl Gerke of

Kimmel Gerke Associates. Here is his introduction to you, followed immediately below it by first

entry in the series, which looks at printed circuit board EMC, starting with the clock circuit.)

Hi! I'm Daryl Gerke, and I am delighted to again be working with the good folks at EDN, now part

of UBM Electronics. Way back in 1994, my business partner (Bill Kimmel) and I wrote the

original EDN Designer's Guide to EMC. At that time, we wanted to share our collective half

century of industry experience with our design colleagues.

As full-time EMI/EMC consultants since 1987, we had seen a multitude of EMI problems.

Although spread across different industries, the underlying causes were often similar. So when

former EDN Editor Steve Leibson (still blogging for EDN) put out a call for design tutorials, we

responded with what eventually became the Designer's Guide.

Fast-forward 17 years and the EMI problems are still with us. At this stage in our careers, we've

become "old warriors" with over 80 years of collective experience. Our goal now will be to help

you sharpen your "EMI spears". I hope you enjoy our efforts.

We will focus on design and troubleshooting, not test and regulations. As we are fond of saying,

"An ounce of EMI prevention is often worth a pound of EMI shielding." This is best accomplished

at the design stage, when most EMI fixes are cheap or even free. Thus, we felt EDN was the

perfect place to share our insights.

For personal information, please visit http://www.emiguru.com. You'll find lots of additional

resources there, too. Most are free, and a few are available for a nominal charge. This blog also

originates there. Finally, if you are curious about consulting, check out my other blog at

http://www.jumptoconsulting.com. (Be sure to visit the special welcome for geeks.)

And now, let's get started on our first entry in the EMC series:

Welcome to the first post on EMC issues here at Planet Analog and EE Times. Since this is a

design-oriented site, we'll begin with some EMC design issues. Specifically, we'll address what

should you look for when doing an EMC design review on your printed circuit boards (PCBs).

Not doing these kinds of reviews? Well, you should. An hour or two at the beginning the project

can save thousands of dollars and a lot of grief at the end of the project. One extra trip to the

EMC lab can easily cost $10K or more when you include your engineering time. And who know

how much it costs by being late to market?

Convinced yet? I hope so. A quick EMC design review is pretty simple. We do these reviews for

clients all the time, and you can do them for yourself. For the next half dozen posts, we'll give

you a quick overview.

http://www.jumptoconsulting.com/

http://www.emiguru.com/

To start, we look at the following five critical circuits: clocks, resets, power regulators, analog, and

I/O. These five circuits probably account for 90% of the EMI problems at the PCB level. Today,

we'll look at clocks.

As the most periodic of signals, clocks are the richest in harmonics that often result in radiated

emissions problems. As a minimum, we worry about the first 20 harmonics. Thus, for a 50 MHz

clock, we are concerned all the way up to 1 GHz. But that is just a starting point, as we've seen

higher harmonics cause problems, particularly if they excite a resonance.

We also check for clock-like or clock-derived circuits, such as memory enables or busses. These

may operated at a sub-harmonic of system clocks, but can still cause emissions problems. Of

course, many systems have multiple clocks, so all the clocks be addressed.

Prior to testing, we recommend making a chart showing clock harmonics all the way up to the

maximum frequency of concern for radiated emissions. Then repeat the chart for all clocks

divided by two, and again for all clocks divided by four. (Other division ratios may be needed if the

system uses other clock derivations.) All this can be done easily on a spread sheet.

This data is very helpful during testing. If a radiated emission failure occurs, you can quickly

check your spread sheet to determine which clock is a culprit. If a subdivision, this may also point

to additional clock-like circuits.

Incidentally, if your failure frequency is NOT on your charts, the test data may be pointing you to a

parasitic oscillation. We'll talk about these in a future posting.

Finally, typical solutions for clock problems include power decoupling, series termination (or even

filtering) on outputs, and attention to clock trace routing. Also, keep clock circuits away from I/O

ports to prevent unwanted radiated coupling to the I/O. In extreme cases, selective shielding may

also be needed on the PCB.

We'll revisit these issues in more detail in future entries. Next up: resets.

About the author

Daryl Gerke, an EMI/EMC consultant since 1987, along with business

partner Bill Kimmel, focuses on design and troubleshooting (not test and regulations). He and Kimmel

have been chasing EMI problems for over 80 years (combined, of course.)He is a published author and columnist,

and theirEDN Designer's Guide to EMC (1994) is still in relevant and in demand. He can be

reached viahttp://www.emiguru.com or his other blog athttp://www.jumptoconsulting.com

http://www.jumptoconsulting.com/

http://www.emiguru.com/

EMC Basics #2: Resets as critical circuitsDaryl Gerke, Kimmel Gerke Associates

3/7/2011 6:12 PM EST

[Editor's note: we are pleased to continue our new series on the vital and sometimes

unappreciated topic of electromagnetic compatibility (EMC), presented by well-known expert

Daryl Gerke of Kimmel Gerke Associates. There is a link to Entry #1 at the end of this item.]

After clocks, we like to focus on reset circuits when doing an EMC board

review. The reset circuits are often upset by transients such as ESD (electrostatic discharge) or

EFT (electrical fast transient). A secondary threat is RFI (radio frequency interference.) The latter

is not very common, but we have seen it happen at high RF levels. Fortunately, these problems

are easy to prevent.

False reset effects can range from simple nuisances to a complete system lockup. The actual

response is often dictated by the system software. As such, it can be easy to fix a reset problem

at the software level. Alternately, one can often push the "reset" button or even power down to

restart the system. In critical systems, however, this may not be an option so hardware fixes may

still be needed.

The first thing we check is adequate power decoupling. This is particularly important when using

a "voltage monitor/power-on reset" IC. Since these often use sensitive internal comparators, even

a short disturbance on the Vcc can initiate an unwanted reset.

Incidentally, this was a serious problem with early reset ICs. Since then, the IC vendors have

incorporated small internal delays (such as Schmidt triggering) with good success. Nevertheless,

we still pay attention to decoupling to assure an extra margin of safety.

Next, we check the inputs. On simple devices, there may be none, as the IC relies solely on the

Vcc rail. More sophisticated devices, however, may include a separate sense input or an external

reset input. The latter is often connected to a button, with the input pulled high or low to initiate

the reset. Both input types may need light filtering—a 1000 pF capacitor can work wonders.

If the external reset goes off the board, additional filtering may be needed. Consider a ferrite in

series with the button, followed by a 1000 pF capacitor at the IC input. Yes, this will slow down

the system response to a reset, but if that is a problem, just push the button faster! After all, the

typical delays are less than a microsecond.

The last thing to check are the outputs. If they run more than an inch or two, consider a RC filter

at the IC output, plus 1000 pF capacitors at the loads. A better choice, if available, is to place the

reset controller IC close to the device(s) it is controlling.

To recap, the typical hardware solutions for reset problems include decoupling of the Vcc, filtering

of inputs (particularly if an external reset control is used), and filtering of outputs if the trace

lengths are over an inch or two. Finally, do not overlook software fixes, a very cheap and very

effective solution to reset problems.

Previous entries in the series

EMC Basics #1: Welcome!; and Clocks: critical circuits for EMC

EMC Basics #3: Voltage regulators as critical circuitsDaryl Gerke, Kimmel Gerke Associates

3/24/2011 2:16 PM EDT

[Editor's note: we are pleased to continue our new series on the vital and sometimes

unappreciated topic of electromagnetic compatibility (EMC), presented by well-known expert

Daryl Gerke of Kimmel Gerke Associates. There is a link to previous entries at the end of this

item.]

After clocks and resets, we like to focus on voltage regulator circuits when doing an EMC board

review. Voltage regulators can cause both radiated emissions and susceptibility problems.

Emissions are the result of parasitic oscillations, and susceptibility is the result of RFI (radio

frequency interference.)

The first problem, parasitic oscillations, are due to high-frequency

feedback from the output to the input. The criteria for an oscillator are (a) 180°phase shift from

output to input, and (b) gain greater than one at the feedback frequency. Or, as the old saying

goes, "Oscillators won't, but amplifiers will."

With today's devices, oscillations typically occur in the 100 to 500-MHz range. We've seen these

problems occur with both linear and switching regulators. Both types have feedback, and both

have gain. The resulting levels are often high enough to cause radiated emissions failures during

EMI testing.

Note that these are free-running oscillations. As such, they will NOT be exact harmonics of any

oscillators. Also, the frequencies may vary from system to system. For example, it is not unusual

to see an oscillation at 222 MHz in one system, and to see an oscillation at 231 MHz in another.

Same circuits, same layout, different parasitics. These are useful clues when troubleshooting

radiated emissions.

The second problem, RFI, is due to rectification. Even a small amount of demodulated AC or DC

voltage at a critical feedback node can drive the regulator out of range. In digital systems, this can

http://www.eetimes.com/design/analog-design/4213508/EMC-Basics--1--Welcome--and-Clocks--critical-circuits-for-EMC?Ecosystem=analog-design

result in unpredictable and unrepeatable behavior, such as system lockups. In extreme cases, the

out of specification voltages can even change the state of programmable components.

Like parasitic oscillations, the RF susceptibility problems typically occur at 100 MHz and above.

At those frequencies, the dimensions of the circuit boards and traces become efficient antennas.

This is particularly problematic for circuit boards that are not in a shielded enclosure.

Fortunately, both problems are easy to prevent. Small high-frequency capacitors placed directly

across the component inputs and outputs will "short out" both adverse effects. We typically

recommend 1000 pF capacitors at these locations. Keep the leads short!

Note that many regulator circuits have electrolytic capacitors across their inputs or outputs. These

capacitors alone are NOT adequate at high frequencies. Most electrolytics are not a good "short"

at frequencies above 10 MHz. In those cases, you must add the small capacitors in parallel,

located at the device. Just like stereo speakers, think "woofer-tweeter" capacitors.

To recap, add small high frequency capacitors at regulator device inputs and outputs. These

provide cheap insurance against unwanted parasitic oscillations, and at the same time protect

against RF threats. You will never know when they are working, but if you need them and they

are not there, you WILL experience EMI problems.



EMC Basics #2: Resets as Critical Circuits

EMC Basics #4: Analog devices as critical circuitsDaryl Gerke, Kimmel Gerke Associates

4/18/2011 8:29 AM EDT

[Editor's note: we are pleased to continue our series on the vital and sometimes unappreciated


Kimmel Gerke Associates. There is a link to previous entries at the end of this item.]

If a circuit board includes analog circuits, we also like to focus on those for an EMC board review.

Similar to voltage regulators (discussed in the previous posting), analog circuits can also cause

radiated emissions and susceptibility problems. Once again, the emissions are the result of

parasitic oscillations, and the susceptibility the result of RFI (radio frequency interference.)

Analog circuits differ, however, from voltage regulators. In

analog circuits, the RF susceptibility problems predominate. Parasitic oscillations (resulting in

http://www.eetimes.com/design/analog-design/4213840/EMC-Basics--2--Resets-as-critical-circuits


unwanted emissions) can occur, but are less likely. Since voltage regulators are already

feedback-based devices, unwanted oscillations are more likely than with non-feedback based

devices.

The RF susceptibility problems are typically due to rectification. A CW (continuous wave) RF

threat can result in a DC offset, which can often be blocked with a capacitor. With a modulated

RF threat, however, a demodulated signal is the result. If the demodulation is within the expected

signal passband, there is no way to filter it.

That is why modulation is used during most RFI tests. A 1000-Hz modulation is typical, but that

may be further constrained depending on the equipment under test. For example, most medical

devices require modulation within the passband of the physiological function to be measured (a

few Hertz or less is typical.) The goal is to uncover unwanted effects in the presence of RFI.

Fortunately, the problems are easy to prevent at the circuit level. Start with high-frequency

filtering on the analog circuit inputs. As a minimum, place small capacitors (100 to 1000 pF

typical) placed directly across the component inputs to "short out" the RF energy. For additional

protection, these capacitors can be augmented with series resistors or ferrites. The goal is to

prevent the RF from reaching critical input circuits in the first place.

Due to circuit bandwidth, the allowable input filtering may be limited. Fortunately, most analog

circuits operate at audio frequencies (or below), so the small amount of capacitance needed for

RF protection usually does not affect normal operation. The key is to use enough filtering, but not

too much. In some cases, even 10 pF across inputs is enough to protect against RF threats.

Added protection may be needed. Power decoupling should include high-frequency ceramic

capacitors (1000 pF typical) in addition to larger low-frequency capacitors. These should be

installed adjacent to the analog devices. In extreme cases, output filtering and even local

shielding may be necessary for very sensitive devices (or very large threats.)

To recap, consider high-frequency protection for your analog circuits. Don't assume that just

because they operate at low frequencies they won't be affected by high frequency (RF) threats.

With today's modern devices, sooner or later RFI problems will occur unless protection is

included. Previous entries in the series



EMC Basics #3: Voltage regulators as critical circuits

EMC Basics #5: I/O as critical circuitsDaryl Gerke, Kimmel Gerke Associates

5/2/2011 10:36 AM EDT



Kimmel Gerke Associates. There is a link to previous entries at the end of this item.]

http://www.eetimes.com/design/analog-design/4214472/EMC-Basics--3--Voltage-regulators-as-critical-circuits



Last, but not least, we like to look at the I/O (Input/Output) circuits during

EMI circuit board reviews. As a port of entry for external currents, I/O circuits are particularly

vulnerable to threats like ESD (electrostatic discharge) and RFI (radio frequency interference).

Radiated emissions are also a concern, as the I/O circuits are often the last chance to keep

unwanted currents from leaving the board.

There are several parameters of concern -- internal/external, digital, analog, relays, and contact

closures. They each present different problems, and may require different solutions. One size

does not fit all.

Internal/external -- I/O circuits connected to external traces/cables are the primary concern. I/O

circuits are connected to wiring or traces that leave the board, and that can act as "hidden

antennas." If the system is shielded, internal I/O is less of a concern. If not shielded, however,

both internal and external I/O deserve EMI attention. Regardless, we like to check out both types

of I/O.

Digital inputs/outputs -- The key concern for digital interfaces is ESD. A high level discharge

may cause damage, while lower levels may simply cause upsets. The solutions include transient

protection (must be fast enough), filtering, or even software (ACK/NACK protocols, etc.)

A secondary concern is radiated emissions, with small currents sneaking out the I/O port. The

resulting voltages are usually so small you can't see them with an oscilloscope. Radiated

susceptibility is rare with digital I/O, although we have seen problems at very high RF levels. The

solutions for both radiated problems include filtering at the interface and/or or shielding of external

cables.

Analog inputs/outputs -- The key concern for analog interfaces is RF. High RF levels can cause

rectification in the I/O circuits, resulting in either a DC offset (no modulation) or a low frequency

AC signal (with modulation.) If the modulation is in the signal passband, you can no longer filter it.

Thus, the best strategy is to prevent rectification from occurring in the first place.

Typical solutions include high frequency filters and/or shielding of the external cables. If diodes

are included for ESD protection, the filtering must precede the diodes to prevent rectification at

the protection diodes.

Relay outputs -- Since relay drivers are usually digital, the regular digital concerns apply. In

addition, inductive transients from the relay coils may pose a self-compatibility problem. Snubber

circuits may be needed at either the relay (best) or at the driving circuit on the boards.

Contact inputs -- Since the receiving circuits are usually digital, the regular digital concerns

apply. Do NOT assume that because contacts are relatively slow that they are immune to EMI

problems. Remember, high frequency EMI currents love to exploit unprotected low frequency

ports.

To recap, when designing or reviewing circuit boards for EMI, ALL of the I/O circuits deserve

EMI attention! Even one unprotected I/O port can wreak EMI havoc.





EMC Basics #4: Analog devices as critical circuits

EMC Basics #6: Looking at circuit board "stackup"Daryl Gerke, Kimmel Gerke Associates

6/5/2011 5:09 PM EDT



Kimmel Gerke Associates. Note that there are links to all previous entries at the end of this item.]

After the critical circuits, we like to examine the printed-

circuit board (PCB) stackup. Like the schematic, board stackup decisions are often made prior to

routing and placement, so this is a good time to look at the proposed stackup. When routing and

placement are complete, a second EMC review of the board is prudent. Stackup matters for

EMC!

The first stackup question is the number of layers. If it is one or two layers (no power or ground

planes), this raises serious EMI serious concerns about both the layout and the technology used.

With today's devices, one- or two-layer designs are usually OK for embedded controllers with

clocks under 10 MHz. But even here, EMI precautions must be taken.

We'll address EMC design recommendations for one/two layer guidelines in a future post. For

now, we'll assume multi-layers. As a minimum, you can start with four layers: a ground plane, a

power plane, and two signal layers. The multi-layer concepts can be extended to as many layers

as you want or need (or can build.)

Multi-layer boards are preferred for high-frequency designs. This generally means RF (radio

frequency) circuits or digital circuits with clocks over 10 MHz. But even low-frequency analog

http://www.eetimes.com/design/analog-design/4215198/EMC-Basics--4--Analog-devices-as-critical-circuits




circuits (audio or instrumentation) can benefit from multi-layer designs when subjected to RF

susceptibility requirements. Remember, low-frequency circuits and be affected by high-frequency

threats.

Our experience has shown that multi-layer boards provide at least 10× reduction in radiated

emissions, and 10× improvement in immunity (both RF and ESD.) We have repeatedly seen that

when replacing simple two-layer boards with four-layer boards. The power and ground traces are

now solid planes -- everything else is the same. But as the old saying goes, your mileage may

vary.

The first EMC miracle occurs due to proximity of the planes to signal traces. The image-plane

effect provides a return path for high-frequency currents that greatly reduces loop size. Every

trace is now a transmission line, instead of an unwanted loop antenna. This works as long as the

adjacent plane is continuous all along the trace.

The second EMC miracle occurs due to reducing power/ground loops and impedances. These

loops form additional hidden antennas for emissions and immunity. We refer to these loops as the

"back door" for EMC. No, the clock Vcc CURRENT is NOT CONSTANT; rather, the clock Vcc

current pulses at the clock frequency as the internal loads change.

So what do we look for in the board stackup? Here are four simple features to examine:

(1) Are all trace layers adjacent to a solid plane? Either power or ground planes are fine, since a

well-decoupled power plane is just another high-frequency ground (return) plane.

(2) Are associated power and ground plane adjacent? For example, are the analog-voltage

planes next to analog ground, and digital-voltage planes next to digital ground? Are there

overlaps?

(3) If a plane is split (common for voltage planes), will traces run across these cuts? If so, you are

just begging for EMI problems. It is best to define design rules prior to routing, and then examine

the actual routing results. Additional split-plane issues will be addressed in a future post.

(4) Are the trace and solid planes symmetrical about the center of the board? While not an EMC

issue, this can affect board construction and is worth a quick look.

For simple circuit boards, the entire process of examining critical circuits and the board stackup

for EMC should not take more than a couple of hours. This time is well spent early in the design,

to prevent EMI disasters later!

[Additional Editor's Note: for a somewhat "whimsical" look at printed circuit boards, click here.]






EMC Basics #5: I/O as critical circuits

http://www.eetimes.com/design/analog-design/4215640/EMC-Basics--5--I-O-as-critical-circuits





http://www.eetimes.com/electronics-blogs/other/4216510/What-else-can-you-do-with-our-ubiquitous-PC-board-material-

EMC Basics #7: An introduction to troubleshooting EMI problemsDaryl Gerke, Kimmel Gerke Associates

6/27/2011 9:36 PM EDT




Time to switch gears for a while, and look at troubleshooting

EMI problems. We'll examine four key EMI problems -- emissions, ESD (electrostatic discharge),

RFI (radio frequency interference), and power disturbances. We'll look at these problems in two

contexts -- the EMI test lab, and the engineering lab. We'll also discuss specific troubleshooting

techniques.

Troubleshooting consists of trying to isolate a problem and the underlying causes, and then

applying appropriate fixes. Often times, we are acting like a medical doctor to diagnose an EMI

illness.

Diagnosis is important -- don't just start throwing solutions at the problem.The medical profession

has a saying for this --"Prescription without diagnosis is malpractice." I think this applies to EMI

problems, too.

To continue with the medical analogy, doctors use a methodology known as differential diagnosis.

That means ruling things in, and ruling things out. In simple terms, diagnosis is often a process of

elimination. Or at least, a process of playing the odds.

Diagnosis involves several stages: looking at clues, examining the equipment, and perhaps

gathering additional information (usually through tests.)

The first step is to look at the clues. For example:

• What are the symptoms? Resets? Lockup? Bizarre readings?

• How bad is the problem? Small outage? Damage? Catching on fire?

• Is there an obvious cause and effect? In the test lab, this may be very obvious. In the field,

this may be unknown, so you may have to speculate.

• What are some key parameters? Frequencies? Amplitudes? Dimensions? Impedances?

The next step is to examine the equipment. For example:

• How does the electrical design look? Multilayer or two layer boards? Layout? Etc.?

• How does the mechanical design look? Metal enclosure or all plastic? Seams? Penetrations?

• What about cables and connectors? Shielded? Filters?

• And what about the power interface? Filters? Transient protection?

At this point, one should make a preliminary diagnosis. If the data is still fuzzy, you may need

additional testing. The tests can either be monitors,or failure forcers. Both can provide critical

information.

By the way, it is OK to change your diagnosis as you proceed -- doctors do this all the time. More

important, don't fall in love with your initial diagnosis, but keep an open mind as new data

becomes available.

Once comfortable with a diagnosis, you are finally ready to try fixes (prescriptions.) Install, test,

and observe. If nothing happens, try another fix. And so on. Keep notes as you go along so you

can backtrack.

By the way -- don't try only one fix at a time, but rather stack them up. To change analogies, EMI

problems are often like a leaky boat. If you have five holes in the boat, but you only apply one

patch one at a time, you'll never get dry.

A final admonition -- at this stage, don't worry about the practicality of your fixes. The initial goal is

to find a fix - any fix. Once you find that first fix, you can always try for a better one.

Over this next series, we'll examine various EMI problems. We'll look at the symptoms, and we'll

discuss various troubleshooting tests. We'll also include recommended fixes. Stay tuned...







EMC Basics #6: Looking at circuit board "stackup"

Also relevant to this topic:

Debugging: The 9 Indispensible Rules for Finding Even the Most Elusive Software and

Hardware Problems (Chapter 5, Part 3 of 3)(and see its preceding sections, which are linked

within)

EMC Basics #8: An introduction to troubleshooting EMI problems (con't)Daryl Gerke, Kimmel Gerke Associates

7/11/2011 2:40 PM EDT




http://www.eetimes.com/design/automotive-design/4010329/Debugging-The-9-Indispensible-Rules-for-Finding-Even-the-Most-Elusive-Software-and-Hardware-Problems-Chapter-5-Part-3-of-3-

http://www.eetimes.com/design/automotive-design/4010329/Debugging-The-9-Indispensible-Rules-for-Finding-Even-the-Most-Elusive-Software-and-Hardware-Problems-Chapter-5-Part-3-of-3-

http://www.eetimes.com/design/analog-design/4216623/EMC-Basics--6--Looking-at-circuit-board--stackup-






Unintended electromagnetic emissions can cause interference to nearby communications

receivers—radio, TV, GPS, WiFi, and more. Many years ago, EMI problems were called RFI,

or radio frequency interference.Marconi suffered EMI (RFI) problems over 100 years ago.

Mandatory emissions tests are required for most electronics devices.These include both radiated

emissions (RE) and conducted emissions(CE). The RE tests look at electromagnetic fields from

the entire system, while the CE tests look at voltages/currents on the input-power mains.

As an aside, although the CE tests are not radiated tests, they still aim to prevent direct radiation

from the power lines. The CE tests also aim to prevent interference from directly coupling through

the power system to other equipment.

There are two broad categories for emissions tests, with different goals:

•Commercial limits aim to protect a nearby TV receiver

•Military/avionic/vehicular limits aim to protect a nearby radio receiver

Since radio receivers are much more sensitive than television receivers, the latter limits are much

lower. For example, at 100 MHz the military/avionic limits are often 40 dB or more lower than

corresponding commercial limits. The exact differences vary depending on the actual

environments.

Even if you pass all the emissions test at the EMC lab, you can still have problems in the field. If

that happens, don't assume you can hide behind your FCC/CE certifications. In the US, the FCC

can invoke the "noninterference clause". You cause a problem, you clean it up.

There are two major components to emissions problems: hidden transmitters and hidden

antennas. Even though they are not really hidden, most designers don't see them as transmitters

and antennas (RF designers being an exception, of course.) But the electrons don't care—if it

looks like a transmitter and an antenna, it's time to party.

The primary hidden transmitters in most systems are digital circuits. Highly repetitive signals such

as clocks and clock-like signals (busses, repetitive control lines, etc.) generate strong harmonics.

But even switching power electronics (power supplies and motor drives) can get into the act. As a

helpful hint, we usually assume the first twenty harmonics of any repetitive signal are potential

transmitters.

But even lower frequency circuits can be hidden transmitters, thanks to parasitic oscillations.

Long a problem with vacuum tubes, we've seen a significant increase these unwanted oscillations

in solid state voltage regulators, op amps, and other more. As a helpful hint, these usually occur

above 100 MHz.

The hidden antennas are highly dependent on physical dimensions and frequency. The higher

the frequency and the longer the antenna, the more the radiation. We usually assume anything

over 1/20 wavelength is an efficient antenna. That means six inches at 100 MHz, and about 3/4

inch at 1 GHz.

As such, cables are very likely hidden antennas, followed by traces on circuit boards and even

the components themselves in the GHz range. Openings in shielded enclosures can act as "slot

antennas", and follow the same guidelines. A two-inch slot at 300 MHz leaks like a sieve.

So how do we troubleshoot these emissions problems? Here are five quick suggestions:

•Turn off clocks or change their frequencies to see if the emissions more or disappear. This can

isolate the hidden transmitters.

•Use sniffer probes to identify hidden transmitter circuits. These are small hand held magnetic

probes you connect to a spectrum analyzer. Since they are quite localized, you can quickly sniff

around a circuit board for hot spots for emissions.

•Remove cables to see if the emissions change. This can isolate the hidden antennas.

•Use current probes on the cables. These are high frequency probes you clamp on a cable and

connect to a spectrum analyzer. Currents in excess of a few microamps are suspect.

•Shield the entire enclosure with aluminum foil. This is very useful in identifying any hidden

antennas in the mechanical enclosure.

Next time, we'll share some troubleshooting techniques for ESD (electrostatic discharge.)








EMC Basics #7: An introduction to troubleshooting EMI problems

EMC Basics #9: Troubleshooting RFI EMC problemsDaryl Gerke, Kimmel Gerke Associates

8/7/2011 3:51 PM EDT



Kimmel Gerke Associates. Note that there are links to all previous entries at the end of this

article.]

http://www.eetimes.com/design/analog-design/4217287/EMC-Basics--7--An-introduction-to-troubleshooting-EMI-problems







RFI (radio frequency interference) is a rapidly increasing EMC problem, thanks

to the proliferation of wireless devices. These range from low power Wi-Fi and cell phone

transmitters to high power radio/television broadcast transmitter to extremely high powered radar

systems.

[As an aside, commercial testing has resulted in more robust products. Prior to 1996, when CE

testing became mandatory, we often saw RFI problems in the field. These were due to both

broadcast transmitters (radio and television) as well as mobile transmitters (cell phones, hand

held VHF/UHF transceivers, vehicular transmitters, and similar. Today, however, most RFI

problems get caught prior to market release. Some, however, still make it into the field.]

RFI as an EMI source

Transmitter power is not the sole issue, but rather a combination of power and proximity. The

lowly cell phone a few inches away may cause more problems than the broadcast transmitter a

mile away. The key parameter is the magnitude of the electromagnetic field at the victim.

In the EMI world, we focus on the electric field magnitude. The field levels can be easily

measured with suitable equipment. But here is a simple approximation that will get you in the right

ball park for an initial assessment:

E = [5.5 × √(PA)] / d,

where

E = Electric field in Volts/meter

P = Transmitter power in Watts

A = Antenna gain relative to isotropic

d = Distance from transmitter antenna to victim in meters

This formula assumes a point source and a "far field", both valid for most RFI situations.

For example, a 1W radio at 1 meter with a relative gain of 1 (good assumptions for a hand-held

radio or cell phone) produces a field level of 5.5 V/m. In fact, it is this hand-held transmitter model

that results in the 3 V/m and 10 V/m limits for commercial equipment. Higher field levels reflect

higher transmitter levels, such as the 200 V/m limit common for many military/automotive

environments.

RFI coupling paths

Since radio transmitters work by electromagnetic radiation, the primary path is radiated. As

"hidden antennas" are involved, physical dimensions are critical. For frequencies <300 MHz,

cables are the most likely factor. For frequencies >300 MHz, everybody gets in the act: circuit

board traces, enclosure openings (slot antennas), and even components themselves.

A good rule of thumb is to assume any conductor greater than 1/20 wavelength long is a potential

antenna. We've seen smaller antennas, but this criterion is widely used in the EMI community.

This means six inches (15 cm) at 100 MHz, two inches (5 cm) at 300 MHz, and ¾ inch (2 cm) at 1

GHz.

Direct conduction is also possible, but less likely. Nevertheless, when troubleshooting an RFI

problem, don't overlook this possibility. We've seen it happen.

RFI victims

The primary RFI failure mode is rectification, but failure levels vary with circuit types. The more

sensitive the circuit, the lower the thresholds will be. Here are some typical levels:

Analog circuits: 0.1 to 1 V/m

Power circuits: 1-10 V/m

Digital circuits: 10-100 V/m

These are simple guides. As the saying goes, "Your mileage may vary..."

Different circuits exhibit different symptoms, which may be helpful in troubleshooting:

When analog circuits are upset, you may get errors in sensor information, but everything else

works fine.

When power circuits are upset, the system often locks up or exhibits other strange behavior.

When digital circuits are upset, repeatable upsets are typical: resets, unwanted interrupts, or

memory upsets. The digital upsets are similar to those seen with ESD and other transients that

“flip” critical bits. .

Troubleshooting RFI problems

If you fail an RFI test at the lab, you already have details of frequency, amplitude, and failure

mode. In you fail in the field, the picture is less clear. You may need to make some

measurements, or to use approximations as described above.

Here are five quick RFI troubleshooting suggestions:

1. Remove cables to see if RF susceptibility changes. As an alternate, apply clamp on

ferrites for frequencies above 100 MHz.

2. Wrap the unit under test in aluminum foil. This will show if shielding is adequate (or will

help if Unit Under Test is unshielded.)

3. Harden critical circuits -- ferrites and 1000 pF capacitors are very helpful above 100 MHz.

Apply to inputs, power, and even outputs.

4. At the systems/box level, troubleshooting is best done in a shielded enclosure with

suitable equipment. In a pinch, a hand-held VHF/UHF radio can be useful. If testing in the

field, keep transmissions short (1-2 seconds) on unused frequencies.

5. At the PCB level, a signal generator connected to a sniffer probe can also be helpful in

"injecting" a signal at the component/trace level. Not new, this technique was used 50

years or more ago by those who repaired radios and televisions. It still works today.

Next time, we'll look at troubleshooting power disturbances.








EMC Basics #7: An introduction to troubleshooting EMI problems

EMC Basics #8: An introduction to troubleshooting EMI problems (con't)

EMC Basics #10: EMC troubleshooting and power disturbancesDaryl Gerke, Kimmel Gerke Associates

9/8/2011 1:16 PM EDT



Kimmel Gerke Associates. Note that there are links to all previous entries here.]

This is the final installment in the mini-series on

EMI/EMC troubleshooting. Often seen as an EMC stepchild, power disturbances are becoming

increasingly important.

At the design level, we’ve seen a significant increase in recent years with power disturbances.

These problems are driven by improved power components, which allow faster switching rates,

higher power levels, and unfortunately, increased EMI problems.

http://www.eetimes.com/ContentEETimes/Documents/Schweber/EMCBasicsseries.pdf

http://www.eetimes.com/design/analog-design/4217742/EMC-Basics--8--An-introduction-to-troubleshooting-EMI-problems--con-t-

http://www.eetimes.com/design/analog-design/4217287/EMC-Basics--7--An-introduction-to-troubleshooting-EMI-problems







We’ve seen an increase in power disturbance problems at the systems level, too. The recent

2011 IEEE EMC symposium even held a full-day special session on EMC issues and the “Smart

Grid”. The session was well attended and promoted a lot of discussion. As one wag observed,

“megawatts are finally meeting gigahertz.”

But back to the design issues. Due to these problems, mandatory power-disturbance testing is

now required for EMC qualification on a wide range of products. The specific tests vary with

industry, platform, and even location. One size does not fit all when it comes to power

disturbances.

Power disturbance specifications

There are power-disturbance specifications for electronics used on AC mains (often varies with

country), military platforms, vehicles, telecommunications facilities, commercial aircraft, and more.

Most are unique to the environment, and are typically based on empirical data.

Some specifications are part of the general EMC requirements, while others may reside in

separate documents. In the latter case, the popular term is “Power Quality,” or PQ. While the

EMC requirements focus on transients, the PQ requirements usually address longer term power

perturbations like sags, over/under voltages, outages, and others.

EFT and surge

Two very popular commercial EMC requirements are the EFT (Electrical Fast Transient) and the

lightning surge. These are applied to the AC inputs. In the real world, these are two of the more

common causes of equipment malfunctions and damage. Other industries, such as military,

vehicular, and telecommunications, have similar requirements for both AC and DC inputs.

The EFT tests simulate arcing at contacts, which results in short bursts of very fast transients.

The EFT is described in ANSI/IEEE C62.41; the corresponding CE test requirement is EN61000-

4-4. The individual transients uses a 5 nsec rise time, which is pretty close to the nominal 1 nsec

for ESD. As such, upsets such as resets or other “bit-flipping” is common.

The surge tests simulate a lightning hit on the power mains. The surge is also described in

ANSI/IEEE C62.41; the corresponding CE test requirement is EN61000-4-5. These transients

(both voltage and current) are much slower but with much more energy than the EFT. As a result,

both upsets and damage are common.

Power quality

An excellent resource for PQ on the AC mains in North America is IEEE Std-1000, also known as

the “Emerald Book.” This guide is put out by the IEEE Power Engineering Society, and focuses

on wiring practices for computer equipment. As such, it is an excellent place to start. We have

even used this as the basis for developing internal power specifications.

There are unique PQ specifications for other industries, too. Sometimes these are separate

documents, and sometimes they are separate chapters in detailed equipment specifications.

Several of the European Norms address PQ concerns for European power mains as well.

Troubleshooting power disturbances

There are two methods when troubleshooting power disturbances—failure forcers and monitors.

You may need a combination of both to isolate and fix a problem.

To force failures, both the EFT and Surge tests are good starting points. Start at low levels and

work your way up. And remember, the surge can cause damage, so don’t do surge testing on a

valuable one-of-a-kind prototype.

For monitoring power at the equipment, power disturbance analyzers (PDAs) are very helpful. If

you don’t own one, these can be rented and left in place for a period of time. These devices will

check numerous various parameters, such as over/under voltages, outages, transients, and

more. They date/time stamp the events, and even capture the waveforms for later evaluation.

One caveat with a PDA: due to the bandwidth, they may miss EFT events, so you may want to

augment one with a storage oscilloscope. The bandwidth should be 100 MHz or higher.

Here are five quick power disturbance suggestions:1. Add a modular power filter at the input. If in a metal enclosure, be sure to locate directly at the power entrance, and

provide a low-impedance ground connection between filter and enclosure.2. Add transient protection at the input. Install both differential mode and common mode devices. For the surge, MOVs

are adequate. For the EFT, you will usually need faster silicon devices, with short connections.3. If EFT upsets occur, try adding a multi-turn common-mode ferrite (3-4 turns through the core) to the input power line.

Note that single-turn ferrites may not be adequate.4. If resets occur due to EFT, try adding 0.01 μF capacitors and multi-turn ferrites right at the reset circuit. Inputs are

particularly vulnerable, but be sure any reset/voltage monitor devices are also well decoupled at the chip.5. If lightning damage occurs, try to assess the failure path. Consider isolation transformers and transient protectors

designed to for the full lightning-surge levels.

At various times, we’ve done all of the above. Next time, we’ll start looking at some EMC

shielding problems and solutions.

EMC Basics #11: An introduction to EMC shieldingDaryl Gerke, Kimmel Gerke Associates

10/7/2011 12:33 PM EDT




Time to shift gears, and look at the mechanical side of EMC — or more correctly, the

electromechanical side of EMC. In this next mini-series, we'll look at various aspects of shielding,

and how it works. It is not enough to just hang metal — you need to understand what you are

doing, and why.

http://eetimes.com/ContentEETimes/Documents/Schweber/EMCBasicsseries.pdf

The primary purpose of a shield is to block electromagnetic radiation. This includes both

radiated emissions and radiated susceptibility. In fact, most of the time a shield behaves in a

reciprocal manner, and what works for one direction works equally well for the other. The

exceptions are subtle, and will be ignored for now.

Shields can be applied at different levels. The most common for electronic equipment is at the

"box" level, but shielding can also be employed at the component, board, or even systems

level. (In the latter case, think of shielded rooms.) In fact, multiple levels of shielding are quite

common. You don't need to depend on just one shield for all your EMI protection.

Shielding performance is traditionally defined as "shielding effectiveness" (SE). This is the ratio

of the field level before the shield is in place, divided by the field level after the shield is in place.

It is customary to express this parameter in deciBels. The number should always be zero (no

shielding) or positive. (If negative, you must be creating energy. Quick — patent it!)

As with many EMC issues, there is a lot of duality with shielding:

•Two modes - reflection and absorption

•Two design issues - materials and mechanical

•Two field concerns - near field and far field

•Two frequency concerns - low frequency and high frequency

•Two impedance concerns - low (magnetic) and high (electric)

With all these variables, it is no wonder one shield design does not work for all cases.

All of the above leads to thinking of shielding in three regimes: magnetic, electric, and

electromagnetic. Figure 1 is a curve from a military design handbook showing the SE of copper.

Notice the two modes and three regimes. Don't panic: we'll look at this in more detail to help

you decode the mysteries of shielding.

Figure 1: Typical shielding curves for copper

As you can see, there are a number of things to consider when designing an EMC shield, both

electrical and mechanical. If you need shielding, it is not enough to just throw it over the wall to

the mechanical engineers. You need to be involved in the design decisions, too.

We'll explore all of these topics in more detail in future posts. We'll also augment these with

practical design guidelines along with our favorite shielding "rules of thumb."

EMC Basics #12: Shielding materials solve electromagnetic-compatibility issuesDaryl Gerke, Kimmel Gerke Associates

11/20/2011 8:29 AM EST




The first shielding decision is usually about materials. Do we need thick steel or mu-metal, or

will a thin shield or even conductive paint suffice? Well, it depends...

http://www.eetimes.com/ContentEETimes/Documents/Schweber/EMCBasicsseries.pdf

To better understand this issue, we'll delve into basic shielding theory. Don't panic — we

won't be deriving Maxwell's famous equations or bogging you down in electromagnetic field

theory. Rather, we'll use a simple theory developed in the 1930's by Dr. Sergei Shelkunoff that

still serves us very well almost a century later.

If you look at the shielding diagram in the previous post, you can see that prior to

Shelkunoff, it must have been very difficult to figure out what was going on. Did shielding

increase or decrease with frequency? Was it linear or exponential? What was the rate of

change? The answer, common in the EMI world, was "It depends..."

Shelkunoff proposed a simple transmission line model for shielding — specifically a lossy

transmission line. This resulted in two major mechanisms, reflection (R) and absorption (A). He

also added a fudge factor for reflections through a thin shield that he dubbed B.

The resulting top-level equation was rather elegant:

SE (dB) = R(dB) + A (dB) + B

The mechanisms are illustrated in Figure 1.

Figure 1: Basic shielding mechanisms modeled as transmission-line effects.

Incidentally, since B is relatively small for most EMI issues, most of us just ignore it. (It can

be important, however, for very thin shields or at optical frequencies.) As such, we'll focus on

the two remaining shielding mechanisms, R and A.

Reflection: This is a surface mechanism, and is the result of the mismatch (transmission line

theory) of the "barrier impedance" and the "wave impedance." The former is simply the surface

impedance, given in "ohms/square", while the latter is the ratio of the magnitudes of the

electric and magnetic fields.

In free space, the wave impedance is 377 Ω, but at very low frequencies (such as 50/60 Hz)

the wave impedance may be drastically altered by the circuit impedance.

Since the barrier impedance for metals and metallic coating is often measured in milliohms,

you can see we have a huge mismatch at higher frequencies. As a result, reflection is the

primary mechanism for shielding at radio frequencies (RF) above 10 kHz. Even thin coatings like

conductive paints can provide 60-80 dB or more of shielding.

Absorption: This is a volume mechanism, and is the result of loss through the shield (lossy

transmission line theory), and is the result of "skin depths." The loss is exponential. One skin

depth results in 8.68 dB (one neper) of loss, two skin depths in 17.4 dB of loss, etc. Since we

usually don't need absorption for RF frequencies, any added absorption is a bonus.

At power frequencies with high currents (low-impedance fields), the reflection is minimal so

you need absorption. Furthermore, skin depths are hard to come by at low frequency. At 60 Hz,

you need at least 3-4 inches of aluminum to start to have even a small effect. So what to do?

Well, you can boost the skin depth by permeability. The improvement is proportional to the

square of the relative permeability. Thus, 0.1 inch/2.5mm of steel ( μr= 1000) gives the same

absorption as about 3 inches/75mm of aluminum. This is why we use steel (or other permeable

materials) for shields around power supplies or around devices that are sensitive to power-line

magnetic fields.

The bottom line: the two mechanisms often drive our choice of shielding materials. At RF

frequencies (above 10 kHz) thin conductive coatings are usually fine. For power frequencies

(50/60/400 Hz) with high currents, thick permeable materials are often required.

In the next post, we'll start to look at the mechanical issues of shielding, such as how seams

and other discontinuities affect RF shielding. These are the common limitations of RF shielding.

We'll also share some of our favorite "rules of thumb."

EMC Basics #13: EMC shielding--destroying a shield

Daryl Gerke, Kimmel Gerke Associates

1/22/2012 5:43 PM EST




There are two ways to destroy a high-frequency shield: seams and penetrations. As discussed in

the previous article (EMC Basics #12), even thin conductive materials work well for frequencies

above about 10 kHz. Thus, the weak points are mechanical rather than materials.

Building a high-frequency shield is like building a wooden water tank. Once the planks are thick

enough, the leaks occur at seams, joints, penetrations, and even knotholes. And even a small

hole can be a problem : drill a ¼ inch hole in the bottom of the tank, and eventually all the water

leaks out.

Incidentally, the “planks” don’t need to be thick for high-frequency EMC shielding, as long as

they are conductive. Obviously, wood doesn't work for EMC, but aluminum foil is very effective,

along with surface treatments like conductive paint or plating. Remember, however, you still

may need the “thick conductive planks” for low-frequency magnetic field shielding (60 Hz and

harmonics).

Before we go further, we need to look at some simple physics.Although the water-tank analogy

is useful, it falls apart in several ways. Here are a couple of key points:

•For seams, the longest dimension is critical -- NOT the area. Unlike water, a six-inch seam will

leak the same as a six-inch hole under worst-case conditions. The difference is that the seam will

be highly polarized, while the hole will not. When designing an EMC enclosure, we need to be

pessimistic. After all, Murphy and his law will make sure that the worst case will occur.

•For penetrations, the depth of penetration is critical, NOT the hole size. If a wire, cable, or even

a pipe extends beyond the EMC shield and is NOT shorted to the shield, the extensions act like

antennas connected by a coaxial cable. For example, carrying wires or cables through a hole to a

connector on the circuit board can completely destroy a shield at high frequencies.

A good rule of thumb for seams and penetrations is the “1/20 wavelength rule.” Antenna

designers often use this guideline as a practical limit when making small antennas. You don’t

need a half wavelength (or even a quarter wavelength) to support electromagnetic radiation –

1/20 of a wavelength will still do a credible job.

http://www.eetimes.com/design/ContentEETimes/Documents/Schweber/EMCBasicsseries.pdf

As EMC designers, we’re trying to do just the opposite – that is, NOT design antennas into our

shields. Yet, that is what happens. The seams look like slot antennas, and the penetrations look

like monopole antennas. Both can radiate (leak) a surprising amount of energy at high

frequencies.

Most EMC engineers use a 1/20 of a wavelength as a starting point, but even that only provides

about 20 dB (10×) reduction. If you need 40 dB (100×) this reduces to 1/200 wavelength, and 60

dB (1000×) reduces to 1/2000 wavelength.

You can quickly calculate physical dimensions using this formula,based on the speed of light in

free space:

Frequency (MHz) × Wavelength (meters) = 300

•For example, at 100 MHz, a wavelength is 3 meters, so a 15-cm seam or penetration (about six

inches) provides 20 dB of shielding. If you need 40 dB, that reduces to 1.5 cm, and if you need 60

dB, that further reduces to 1.5 mm.

•It’s even worse at 1 GHz, where a wavelength is 30 cm. A seam or penetration of 1.5 cm (less

than an inch) only provides 20 dB of shielding. If you need 40 dB, that reduces to 1.5 mm, and 60

dB it is only 0.15 mm. No wonder we need EMI gaskets (or even welded seams) and bulkhead

connectors at the higher frequencies!

We’ll look at how to plug those leaks in a future post of this series. In the meantime, the

1/20th wavelength rule is a good place to start. After all, if you can’t get a 10× reduction, why

even bother with shielding in the first place?

EMC Basics #14: Making plastic coatings work for EMIBill Kimmel, PE, Kimmel Gerke Associates

3/1/2012 9:56 PM EST


topic of electromagnetic compatibility (EMC), presented by well-known experts Daryl Gerke and

Bill Kimmel of Kimmel Gerke Associates. There are links to all previous entries in the EMC

Basics series here.]

Two factors are combining to create EMI shielding problems: increasing clock frequencies and

the shift to plastic housings. The trend to higher clock frequencies continues. Modern laptop and

desktop computers run in the GHz range, a thousand times faster than the early personal




computers, but even modest applications are running clocks in the 20- to 100-MHz range. CISPR

22 calls for emission testing up to six GHz, depending on maximum clock frequency.

For shielding effectiveness, you need to keep openings in your enclosure less than 1/20

wavelength of the highest-applicable frequency, typically to the tenth harmonic of the fastest

clock. If you have a 100-MHz clock, you will be testing to 1 GHz, so expect to limit openings to

about 2 cm, and even less if you have higher clock frequencies. It has now reached the point

where continuous closure is necessary; occasional contact is not sufficient.

The second aspect is the increasing use of plastic enclosures. As we know, plastic provides no

shielding unless provided with a conductive coating. The conductivity of the coating is not the

driving factor in high-frequency shielding effectiveness; it's the longest dimension of the

opening, which almost always occurs at the mating seams. So select your coating for criteria

other than EMI, such as availability, cost, durability, and ease of application.

Herein lies the problem: it's difficult to get conductive closure at the seams, and the problem is

with the design of the mold. Done right, the shield works very well. Unfortunately, most of the

molded plastics are poorly designed to contain EMI. Radiated-emissions failure is almost a

foregone conclusion, if the plastic enclosure is not properly designed.

How to make the shield work

You need to bring the conductive coating right up to the mating seams, then ensure the surfaces

conductively mate pretty-much continuously along the entire seam. This requires the plastic

enclosure be designed so as to facilitate proper coating.

A reliable method is to use tongue and groove, making sure the mating surfaces are stiff enough

to ensure continuous contact. Better, yet, the groove can provide a "nesting" place for

conductive EMI gasketing, Figure 1.

http://en.wikipedia.org/wiki/List_of_EMC_directives

http://en.wikipedia.org/wiki/List_of_EMC_directives

Figure 1: a) Conductive coating brought up to mating surfaces;

b) EMI gasketing placed in groove.

However, we encounter strong resistance when discussing this with the mechanical designers.

They want to mask off the coating back from the seam to ensure the coating doesn't show on

the outside. The fact is, if the coating doesn't get into the seams and close the gaps, the

shield will not work.

In fact, if you don't close the gaps, the measured emissions may well increase! Why? The shield

may well collimate the available RF energy, resulting in emission hot spots.

You also need to make provision to terminate cable shields and filters. In most cases, these need

to be grounded firmly to the shield, as casual contact will not do. The worst case is to run the

cable shield without terminating it to the enclosure shield: that's a guaranteed test failure.

Summary

Don't fool yourself. If you want the shield to work, design the enclosure to close the seams.

Tongue and groove works well, preferably with EMI gasketing. Also make sure your cable shields

and cable filters are well terminated to the coating.

EMC Basics #15: Use gaskets to seal and solve leaky RF seam problemsDaryl Gerke, Kimmel Gerke Associates

5/9/2012 5:41 PM EDT


topic of electromagnetic compatibility (EMC), presented by well-known experts Daryl Gerke and

Bill Kimmel of Kimmel Gerke Associates. There are links to all previous entries in the EMC

Basics series here.]

Continuing with our shielding theme, we'll look at EMI gaskets. As discussed previously, seams

and other openings can be a "weak link" in EMI shielding. Two key concerns are gasket choice

and gasket mounting. We'll look at the former now, and save gasket mounting for a future

article.

Years ago, EMI gaskets were widely used in military systems and radio communications

equipment, but rarely seen in commercial equipment. Thanks to increasing processor speeds

and increasing EMI threats, gaskets are now common in a wide range of electronic equipment.

As a rule of thumb, we recommend gaskets whenever shielding needs exceed 60 dB, although

gaskets can still help at lower levels.

http://cms.eetimes.com/ContentEETimes/Documents/Schweber/EMCBasicsseries.pdf



We regularly encounter EMI problems due to poor shielding and gasketing. While you might

be tempted to leave EMI gasket choices solely to the mechanical engineers, don't do that. You

need to work with your mechanical colleagues. Like many EMI problems, both disciplines need

to be involved. Fortunately, the solutions are usually simple once you understand some basic

principles.

How gaskets work

EMI gaskets perform their magic by providing a conductive path across seams and other

discontinuities in an electronic enclosure. This “shorts out” any potential difference across the

shield surface while maintaining smooth current flow. As a mechanical analogy, gaskets simply

plug the leaks.

In a perfect EMI shield (a “Faraday cage”), the EMI currents induced on the shield remain

inside (or outside) the shield. In the real world, however, seams or other joints present a

discontinuity. Shield currents are diverted, and a voltage appears across the seam or joint. As a

result, time-varying voltages and currents can launch an electromagnetic wave, just like a wire

antenna.

In fact, seams in shields are often modeled as “slot antennas.” The only difference between a

wire antenna and a slot antenna is that a wire antenna is metal surrounded by space, and a slot

antenna is space surrounded by metal. That means even a thin slot (such as two metal surfaces

separated by paint) can radiate if it is long enough.

A critical parameter is length, not thickness. Most of us in the EMI business worry when slots

are longer than 1/20 wavelength (e.g. 5 cm at 300 MHz, 1.5 cm at 1 GHz.) The secret to success

is to minimize impedance across the joint with clean, continuous metal-to-metal contact.

An alternate would be to reduce the current flowing in the shield, but this usually isn’t

practical. However, don’t overlook this. We once had a case where hundreds of amps of high-

frequency power-return currents were flowing in the cabinet. That case gave us two options to

explore -- either improve the gaskets, or reduce the currents -- we ended up choosing the latter

with good success.

Types of Gaskets

There are several types of popular EMI gaskets. All will work well when properly installed, so

the choice is often usually based on overall mechanical issues. Here are some pros and cons on

different EMI gaskets:

•Beryllium-Copper: These gaskets provide very-high EMI performance. The material has high

conductivity and is very springy, which makes it ideal for doors and panels. The material can be

formed into many shapes, such as fingerstock, serrations, and spirals, and can be plated for

corrosion protection. The drawbacks are cost, mechanical vulnerability (such as snagging of

fingerstock), and the lack of an environmental seal.

•Conductive elastomers: These gaskets provide good performance. They often have metallic

particles or wires embedded in them, so they do require pressure to assure an EMI seal. A big

advantage is that they can also provide an environmental seal as well as an RF seal. The main

drawback is the compression force, but hollow elastomer gaskets often overcome this issue.

•Wire mesh: Like fingerstock, these gaskets can provide very high levels of EMI performance. A

drawback is that many mesh gaskets take a set, and thus can not be reused. Those are fine for

permanent seals, but are not suitable for doors or access panels. They also lack an

environmental seal.

•Conductive cloth over foam: These gaskets are very popular in commercial applications, and are

quite cost effective. Most use a silver-plated cloth over foam to create a soft gasket that can

take up a lot of mechanical slack. The major drawback is a lack of an environmental seal.

•Conductive epoxies: these are permanent gaskets, formed from a metal impregnated caulk.

Silver loading is very common, and the seal is usually also watertight. The major drawback is that

any repaired joint must be cleaned and recaulked to maintain a seal.

•Form-in-place gaskets: These gaskets often make sense for high volume applications that can

automate the creation of a gasket right on an enclosure. These are similar to conductive

elastomers. When cured, these gaskets are resilient and thus may be reusable. The major

drawback is that they are not practical for low volumes.

Corrosion

No discussion of gaskets would be complete without a few comments on corrosion. Even a

small amount of corrosion can render a good gasket ineffective. Equipment used in harsh

environments, such as medical, military, vehicular, or industrial are often subject to corrosion.

Fortunately, corrosion is not usually a concern for commercial products.)

To fight corrosion, plated gaskets can be used to minimize the effects of dissimilar metals.

Another option is to seal out moisture at the gasket interface. Hybrid gaskets are available that

incorporate both an environmental seal and an EMI seal. In those cases, be sure to install the

environmental seal to the outside to protect the EMI gasket from external moisture.

In conclusion, consider gaskets in your shield designs, particularly if you need high levels of

shielding. And work with your mechanical colleagues to assure the best choice.