LEARNEMC, LLC

EMC Question of the Week
2017 - 2020

Copyright  2021 LearnEMC, LLC
P.O. Box 729
Stoughton, WI 53589
LearnEMC.com

Forward
On March 13, 2017, LearnEMC began posting an EMC Question of the Week on their website at
LearnEMC.com. Each week a new multiple-choice question is posted along with a link to the correct
answer and a brief explanation. As of the end of 2020, 200 multiple choice questions had been asked
and answered.
EMC Question of the Week answers are viewed by hundreds of people each week. Sometimes thesolution is fairly obvious to anyone working in the field of electromagnetic compatibility. Sometimes
the questions are a bit more difficult. This book is a compilation of questions and answers thatappeared from March 2017 to the end of 2020. Some of the questions have been updated or edited for
publishing in this format, but the technical content is essentially the same.
Each question has been assigned a topic and a level (basic, intermediate, or advanced). You canlocate questions by topic or by level using the index at the end of this book. Levels are intended toindicate how deep one’s understanding of a particular topic would typically need to be in order toanswer a question correctly. For example, someone with a basic understanding of the transient
protection topic should be able to answer a basic level transient protection question. However, morethan a basic understanding of the topic would typically be required to answer a question at anintermediate or advanced level.
If you’re an EMC engineer, or someone who wants to improve your general knowledge regarding“all things EMC,” we hope you will enjoy this book. If you see an error in a question or solution, wewould appreciate hearing from you. Corrections or comments can be emailed toquestionoftheweek@LearnEMC.com.
I would like to thank Nancy Hubing for serving as the technical editor of this book and Mark Fossen
for designing the cover.
One Final Note: April 1 is April Fools’ Day in the United States. Any question dated April 1
shouldn’t be taken too seriously.

Todd Hubing

March 13, 2017
Topic: EMC Regulations and Testing
Level: Fundamental

Question 1
Quasi-peak values are alwaysa.
b.
c.
d.

equal-to or less-than peak values
equal-to or less-than average values
both "a" and "b"none of the above

AnswerThe correct answer is “a.” Quasi-peak values are always less than or equal to peak values. For
continuous signals (i.e., signals that exhibit constant power at the frequency being evaluated), the quasipeak value is equal to the peak value. For intermittent signals, the quasi-peak detector records a valuethat rises when the signal is present and falls when the signal is absent. The rise time is defined by atime constant known as the "attack time constant." The fall time is defined by a "decay time constant."
The quasi-peak value is the average value of this rising and falling power level. The CISPR quasi-peakdetector (0.15 - 30 MHz) has an attack time constant of 1 ms and a decay time constant of 160 ms.

EMC Question of the Week

1

March 20, 2017
Topic: EMC Measurements
Level: Fundamental

Question 2
If a 100 MHz sine wave with a peak amplitude of 1 volt is input to a spectrum analyzer, theamplitude of the harmonic at 100 MHz recorded by the analyzer should bea.
b.
c.
d.

120 dB(μV)
117 dB(μV)
60 dB(μV)
depends on the RBW of the analyzerAnswer
The correct answer is “b.” The peak amplitude of the sine wave is 1 volt, which is 120 dB(μV), but
spectrum analyzers record the average power detected within the resolution bandwidth (RBW) of theanalyzer. All of the power in the sine wave will fall within one resolution bandwidth regardless of theRBW setting. When that average power is converted to a voltage, it will be the RMS voltage of theinput signal. The RMS voltage of a 1-volt sine wave will be 0.707 volts or 117 dB(μV).

EMC Question of the Week2March 27, 2017
Topic: EM Shielding
Level: Fundamental

Question 3
Of the following choices, the best material to use to shield a 60 Hz magnetic field isa.
b.
c.
d.

coppersteel
leadeither "b" or "c"AnswerThe correct answer is “b.” None of these materials has a high enough conductivity to block a magnetic
field using eddy currents at 60 Hz. The best remaining option is to redirect the field using a material
with a high relative permeability. The relative permeability of copper and lead is close to 1 (about thesame as plastic or air). The only material in this list that potentially has a high relative permeability issteel.
The figure below illustrates how low-frequency magnetic fields from a motor might be redirectedaround a circuit board with a permeable material such as steel.

EMC Question of the Week

3

December 21, 2020
Topic: Circuit Board Design and Layout
Level: Intermediate

Question 199
On a circuit board where R1 is a resistor, C1 is a capacitor, and L1 is an inductor, B1 is mostlikely aa.
b.
c.
d.

ferrite beadvaristor
diodedecoupling capacitor

AnswerThe correct answer is “d.” The B indicates a bypass capacitor, which is another name for a power bus
decoupling capacitor. Some boards use BC to indicate bypass capacitors, while many boards simply
use C. Bypass capacitors are typically no different than capacitors used for other purposes in a board
design. However, labeling them differently makes it easier to locate them during a design review.
The board in the figure below has decoupling capacitors labeled this way. We can quickly see that allof the visible decoupling capacitors are poorly connected. Note that all of them connect to a ground fillrather than connecting to the ground plane. This results in a much higher connection inductance thannecessary. (To convince yourself of this, trace the path from the power pin through the decoupling
capacitor to the ground pin.)
Connecting high-frequency
components to a ground fillon an outer layer instead of
directly to the ground plane
is a common circuit board
design mistake.

EMC Question of the Week

199

December 28, 2020
Topic: Time / Frequency Domain Representation of Signals
Level: Intermediate

Question 200
The amplitude of the 5th harmonic of a 3.3-volt, 10-MHz clock signal with a 1-ns transition time
is approximatelya.
b.
c.
d.

600 mV
300 mV
30 mV
60 V

AnswerThe correct answer is “b.” There are several ways to quickly arrive at this solution. Recognizing thatthe transition time is 1/100 of the period, we know that the envelope of the first 30 – 50 harmonics fallsoff proportional to the frequency. The rms amplitude of the first harmonic is always about half the peak
amplitude of the clock signal, so that’s about 1.6 volts. The fifth harmonic is one fifth of that or about
0.3 volts.
Another slightly more accurate option is to plug the parameters of the clock signal into an equation for
the amplitude of the nth harmonic, as indicated in the figure. A safer option is to use the equation for
the envelope of the harmonic amplitudes (also indicated in the figure).
Finally, if you want to reduce the chances of making a miscalculation, you can use an online harmonic
calculator like the one on the Harmonics of a Trapezoidal Waveform calculator on the LearnEMC
website.

EMC Question of the Week200

Index of Questions by Topic
Topic

Question Numbers

Capacitance ------------------------------------------------------ 13
Circuit Theory ---------------------------------------------------- 89
Circuit Board Design and Layout ---------------------------- 15, 23, 30, 31, 40, 42, 51, 84, 88, 104,
106, 128, 163, 165, 166, 168, 172, 174,176, 190, 196, 199
Components ------------------------------------------------------ 32, 33, 34, 36, 64, 65, 73
Crosstalk ---------------------------------------------------------- 105
Current Return ---------------------------------------------------- 17, 164
Electrical Balance ------------------------------------------------ 50, 70, 86, 107, 129, 155, 184, 194
Electrostatic Discharge ---------------------------------------- 5, 47, 63
EM Coupling Mechanisms ------------------------------------ 6, 18, 35, 37, 44, 45, 52, 68, 77, 94,102, 123, 135, 145, 159, 179, 189
EMC Modeling --------------------------------------------------- 14, 46, 111, 169, 175
EM Propagation ------------------------------------------------- 49, 138, 182, 183, 197
EM Shielding ----------------------------------------------------- 3, 4, 16, 55, 83, 92, 110, 112, 139, 150,152
EM Theory -------------------------------------------------------- 19, 25, 27, 28, 29, 38, 39, 71, 72, 118,
125, 126, 127, 130, 133, 134, 154, 178,
185
EMC Management ---------------------------------------------- 101
EMC Measurements -------------------------------------------- 2, 24, 53, 93, 121, 142, 156
EMC Regulations and Testing ------------------------------- 1, 48, 58, 61, 79, 85, 91, 109, 119, 136,143, 158, 195, 198
Filtering ------------------------------------------------------------ 80, 81, 98, 100, 157
Grounding --------------------------------------------------------- 57, 75, 76, 95, 122, 141, 162
Inductance -------------------------------------------------------- 8, 90, 124, 140, 188
Lightning ----------------------------------------------------------- 56, 62, 120
Linear Systems -------------------------------------------------- 117, 144, 186, 187
Power Circuits ---------------------------------------------------- 54, 59, 96, 151, 170, 171
Power Bus Decoupling ----------------------------------------- 22, 116, 137, 149, 192
Radiated Emissions -------------------------------------------- 12
Resistance -------------------------------------------------------- 7, 78
System EMC Design ------------------------------------------- 153, 173, 181
Time/Frequency Domain Representation of Signals --- 82, 87, 97, 114, 177, 200
Transmission Lines --------------------------------------------- 9, 11, 21, 26, 41, 43, 60, 66, 67, 69,74, 103, 113, 160, 167, 180, 193
Transient Protection -------------------------------------------- 20, 99, 148, 191
Working with Decibels ----------------------------------------- 10, 115, 131, 132, 146, 147
EMC Question of the Week

201

Index of Questions by Level
Level

Question Numbers

Fundamental ----------------------------------------------------- 1, 2, 3, 5, 6, 7, 8, 9, 10, 11, 13, 14,15, 16, 17, 18, 19, 20, 21, 22, 23, 24,25, 26, 27, 28, 30, 31, 35, 36, 37, 38,39, 42, 44, 45, 46, 47, 48, 49, 50, 51,52, 53, 56, 57, 59, 60, 62, 63, 64, 65,66, 67, 70, 71, 72, 73, 76, 77, 78, 80,82, 83, 84, 85, 87, 89, 91, 92, 93, 94,98, 99, 101, 102, 105, 106, 109, 111,112, 114, 115, 117, 118, 119, 120, 121,122, 123, 125, 126, 127, 129, 130, 131,132, 135, 137, 138, 140, 141, 142, 143,145, 146, 147, 148, 149, 156, 158, 159,163, 164, 165, 167, 168, 170, 171, 174,176, 177, 184, 185, 186, 187, 188, 189,191, 192
Intermediate ------------------------------------------------------ 4, 12, 29, 32, 33, 34, 40, 43, 54, 55,58, 61, 68, 69, 74, 75, 79, 81, 86, 88,90, 95, 96, 97, 100, 103, 104, 107, 110,113, 116, 124, 128, 133, 134, 136, 139,144, 150, 151, 152, 153, 154, 155, 157,160, 162, 166, 172, 173, 179, 180, 181,182, 183, 190, 193, 194, 195, 196, 197,198, 199, 200

Advanced ---------------------------------------------------------- 41, 169, 175

EMC Question of the Week

202