KickStart
b y M ike Tooley
Part 5: Getting to grips with EMC
Our occasional KickStart series aims to
show readers how to use readily available
low-cost components and devices to
solve a wide range of common problems
in the shortest possible time. Each of the
examples and projects can be completed
in no more than a couple of hours using
T
hose of you that are as old
as me will doubtless recall the
seemingly endless ‘smog’ that
we had to live with in the 1950s.
‘Smoke fog’ was an unpleasant
consequence of the UK’s increasing
post-war reliance on coal as a fuel,
leading eventually to the introduction
of the United Kingdom Clean Air
Act in 1956. Less visible, but still
undesirable, is the ‘electronic smog’
that surrounds the growing number
of electronic devices that we rely on
in our everyday lives. And, while
you might be blissfully unaware of
‘electronic smog’, it can still have a
serious impact on the environment
in which it is placed. Consequently,
electromagnetic compatibility
(EMC) has become an important
consideration in the design and use
of any item of electronic or electrical
equipment – but, before delving
deeper into this important and often
misunderstood subject, it’s perhaps
worth exploring what we mean by
‘electromagnetic compatibility’ and
how it relates to the two associated
terms ‘electromagnetic interference’
(EMI) and ‘electromagnetic
susceptibility’ (EMS).
Put simply, EMS is a measure of how
a device reacts to the electromagnetic
environment in which it is used, while
EMI is an indicator of how much a
device impacts its electromagnetic
environment. Thus, EMI and EMS are
both important when considering an
equipment’s compatibility with the
environment in which it is used.
Why is all this significant? Simply
this, when designing, constructing and
using electronic equipment we need to
consider how the equipment will interact
with the electromagnetic environment
in which it is placed. Furthermore,
failure to observe EMC precautions
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‘off-the-shelf’ parts. As well as briefly
explaining the underlying principles and
technology used, the series will provide
you with a variety of representative
solutions and examples, along with just
enough information to be able to adapt
and extend them for your own use.
This fifth KickStart instalment explains
the importance of electromagnetic
compatibility (EMC) with some practical
hints on how to reduce electromagnetic
interference (EMI) and electromagnetic
susceptibility (EMS) in your PCBs, circuits
and projects.
can have serious consequences. In
the commercial world and in most,
but not all countries, emissions
that cause potential interference
to other apparatus and services are
strictly regulated. It is also worth
noting that EMC regulations normally
specify limits on acceptable levels of
interference, and often specify tests to
be carried out to verify compliance.
any item of electronic equipment.
Emissions may be radiated in the
form of electromagnetic (radio) waves,
or they may be conducted from the
equipment along power lines and other
wiring. In most countries, limits on
the magnitude of these emissions are
imposed by regulatory bodies. These
limits are designed to protect all users
of the radio spectrum from the effects
of harmful or annoying interference
which may degrade the performance
of other nearby electronic equipment.
Common EMI sources include:
Sources of EMI
Electromagnetic emissions can (and
do) arise from various sources within
Fig.5.1. Spectral analysis of the MW band in the author’s workshop.
Fig.5.2. Spectral analysis repeated in the vicinity of an EMI-producing SMPS.
Practical Electronics | October | 2021
Fig.5.3. Waveform of the output of the EMI-producing SMPS.
Active (radiating) antennas and
their associated feeders when poorly
matched or inadequately screened
Fundamental and harmonic
components of repetitive waveforms
such as clock signals or switching
waveforms used in an SMPS
(switched-mode power supply)
Unwanted ‘parasitic’ oscillation
resulting from poor circuit design
and/or layout
Transients caused by short-duration
pulses
Radiation of wideband noise from
magnetic components.
To put this into a practical context,
Fig.5.1 shows the normal frequency
spectrum and waterfall display of radio
signals in the medium wave (MW) band
in the author’s workshop. The spectrum
display shows signal amplitude over
the range extending from 500kHz to
1.8MHz, while the waterfall shows
how signal amplitude varies with time.
Individual broadcast MW signals can be
clearly seen, and the noise floor is at
around –100dB. Compare this spectrum
with that shown in Fig.5.2 when a lowcost 5V switched-mode power supply
(SMPS) is operated in the vicinity
of the spectrum analyser’s antenna.
Note how the noise floor has risen by
about 18dB with broadband spurious
harmonic components masking all but
the strongest broadcast signals.
The waveform at the output of the
EMI-producing 5V SMPS is shown
in Fig.5.3. This reveals a switching
transient with an amplitude of 100mV
and a repetition frequency of around
20kHz (the power supply’s switching
frequency). Additional filtering and
screening are urgently needed to
improve performance and comply with
the relevant EMC regulations!
A quick check procedure
Accurate methods of investigating
EMI involve the use of specialist test
equipment (spectrum analysers, test
Practical Electronics | October | 2021
Fig.5.4. Effective grounding to a metal enclosure can be
important to comply with EMC regulations.
fixtures and calibrated antennas) that
is usually only available in test labs
(see Going further at the end). Despite
this, a quick check on the level of EMI
produced by a suspect item of electronic
equipment (such as an SMPS) can be
carried out using nothing more than a
portable MW AM radio receiver. Placing
the receiver close to the equipment
under investigation and sweeping it
across the MW band (around 1MHz)
will usually reveal some radiated
noise. Then, moving it away from the
equipment following the connecting
cables and supply leads should reveal
the presence of any conducted noise.
Using this technique, it is possible
to obtain a quick assessment of the
performance of EMI improvement
methods, such as screening, grounding
and filtering.
Improving EMC performance
There are various ways of improving
EMC performance to reduce the amount
of EMI produced by a ‘noisy’ item of
electronic equipment. Measures can
also be taken to prevent the ingress of
noise to reduce susceptibility to EMI
experienced by nearby equipment.
1. Shielding and grounding
EMC performance is significantly
improved by effective shielding
(screening) and grounding (earthing).
Not only can this help to reduce radiation
from noise-producing circuitry, but it
will also help reduce the ingress of
noise experienced by equipment that
might be suffering the effects of EMI. An
effective low impedance path to ground
is an essential pre-requisite. This helps
bypass noise currents to ground (zero
potential) thereby reducing the noise
potential that might exist between a
component’s 0V pin and the supply
ground. To eliminate unwanted ground
loops a single common ground point
is often used for internal wiring (see
Fig.5.4). This helps to avoid the
situation in which small noise voltages
enter the signal path between multiple
ground points.
Screening is important to prevent
unwanted signals from being radiated
from equipment. Screening should be
a continuous grounded metal surface.
This is often aluminium, steel or
tinplate of an appropriate gauge which
must be grounded at several points.
It’s important to ensure that adequate
ventilation is included in the screening
design – equipment that produces
heat should not be totally enclosed
for obvious reasons! Wire mesh and/
or small ventilation holes are usually
permissible unless the frequencies
concerned are high, in which case
primary internal as well as secondary
outer screening is needed.
2. PCB considerations
The PCB design can be crucial in
optimising EMC performance, and it is
essential that the design process takes
this into consideration. Stages in an
EMC-compliant design process might
involve the following:
Physical parameters such as size,
shape and the number and function
of PCB layers
Selection and placement of individual
components and off-board connectors
Track layout and routing (including
power supply rails)
Grounding of common 0V tracks and
decoupling of power rails
Shielding of signal leads and
interconnecting wiring.
Critical areas of the circuit need to be
identified to ensure that that circuitry
is kept within functional groups. For
example, a pre-amplifier should be
positioned well away from a power
supply, a sensitive detector away
from a switching converter, and so on.
Similarly, components that are likely to
carry appreciable currents need to be
positioned close to the rails or grounds
that connect them. PCB tracks should
be kept as wide as possible to minimise
65
Fig.5.5. An example of EMC design considerations for good EMC performance.
voltage drops and dedicated copper ‘land’ used to extend
the ground area around HF (high-frequency) and RF (radiofrequency) circuits – see Fig.5.5.
EMC-compliant PCB design techniques are gained with
experience, but in the meantime it’s well worth examining
any boards that you have to hand in order to gain an insight
into professional layout techniques.
Sensitive analogue circuitry should always be kept apart
from digital circuitry (where rapidly changing logic levels
of several volts may be present). Not only should analogue
and digital circuitry be physically separate, but also, they
should be electrically separate and the key to this is the use
of entirely separate supplies with separate decoupling and
ground points.
Crosstalk can occur between adjacent signal lines or PCB
tracks. The amount of crosstalk depends on the shared
reactance – capacitive or inductive – between the two signal
paths. Capacitively induced crosstalk occurs when mutual
capacitance links two signal paths whereas inductively
induced crosstalk occurs when adjacent signal paths are
linked by mutual inductance (a similar effect to a pair of
coupled windings on a transformer). Note that, in many cases,
both effects may be present at the same time. The effects
of crosstalk can be minimised by reducing the coupling
(capacitive or inductive) between the conductors.
Transmission lines and bus lines should always be matched
or terminated with their specified characteristic impedance.
Fig.5.6. Typical single-stage and two-stage supply filters.
Fig.5.7. Circuit of the single-stage filter shown in Fig.5.6.
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Practical Electronics | October | 2021
Fig.5.8. Circuit of the two-stage supply filter shown in Fig.5.6.
This helps to reduce the presence
of standing waves and ‘ringing’. A
substantial ground or ‘land’ area may
also be required for VHF and UHF
PCB applications. ‘Guard’ areas where
high-impedance signal tracks and pins
are surrounded by grounded copper
track may also be required to minimise
capacitive coupling and protect highimpedance points.
3. Supply filters
Supply filters are usually fitted at or
close to power connectors and in some
cases, filters can also be present within
I/O connectors (Ethernet connectors
often incorporate common-mode
filters). Fig.5.6 shows two commercial
pi-section supply filters. The smaller
filter comprises a single inductor
with two Class-X and two Class-Y
capacitors, as shown in Fig.5.7. The
larger filter has two cascaded stages
with two inductors, three Class-X and
two Class-Y capacitors.
This filter also incorporates a 1MΩ
discharge resistor, R1 (not present in the
smaller single-stage filter). Note that, in
both cases the two inductor windings
are wound in opposite directions and
Fig.5.9. An EMI filter fitted in an IEC
mains connector.
so they tend to cancel the commonmode currents.
The two filters shown in Fig.5.7
combine two different network
topologies. The Class-X capacitors are
designed to filter differential-mode
currents, while the common-mode
inductor and Class-Y capacitors are
designed to filter common-mode noise.
The resistor shown is usually 100kΩ
to 200kΩ and its purpose is to provide
a discharge path for any residual line
voltage stored on the capacitors.
Measurements carried out by the
author reveal that the single-stage filter
has a rejection of greater than 30dB
at 1MHz while the two-stage filter
provides a rejection in excess of 60dB
at the same frequency. Fig.5.9 shows
an EMI filter fitted inside an IEC mains
Fig.5.10. A simple home-constructed pisection filter offering a rejection of 40dB
at 1MHz.
Fig.5.13. Test circuit used to obtain the characteristic shown in
Fig.5.12.
Fig.5.11. Circuit of the home-constructed
filter shown in Fig.5.10.
Fig.5.12. Frequency response of the home-constructed filter
shown in Fig.5.10.
Practical Electronics | October | 2021
Fig.5.14. Ferrite core filters suitable for clamping onto cables.
67
Table 5.1: Going Further with EMC
Topic
Source
EMC
regulations
and directives
Notes
The UK Electromagnetic Compatibility Regulations 2016 can be
found at: https://bit.ly/pe-oct21-ks1
The EU Electromagnetic Compatibility (EMC) Directive can be found
at: https://bit.ly/pe-oct21-ks2
The Slovenian Institute of Quality and Metrology (SIQ) provides a
short introduction to EMC at:
https://youtu.be/8SbegmFJ_WM
YouTube
videos
Power Electronics Basics provides a detailed discussion of EMC
and EMI at: https://youtu.be/C2UDpUAgoSQ
The short SIQ video also provides a glimpse into a
modern EMC testing laboratory. By contrast, Andy
Lawson’s comprehensive lecture lasts nearly an hour.
Andy Lawson’s Beginner’s Guide to EMC can be found at: https://
youtu.be/n4tB2UqZN6c
Part 8 of Electronics Teach-in 4 (available from PE magazine)
provides a general introduction to analogue circuit applications
including passive and active filters.
Filters
The Laird Technology website has a useful guide to ferrite EMI cable
cores: https://bit.ly/pe-oct21-ks3
Ferrite cores
The Coilcraft website provides a useful introduction to inductors and
chokes at: https://bit.ly/pe-oct2-ks4
A similar guide is available from TDK at: https://bit.ly/pe-oct21-ks5
Your best bet since MAPLIN
Chock-a-Block with Stock
Visit: www.cricklewoodelectronics.com
Or phone our friendly knowledgeable staff on 020 8452 0161
Components • Audio • Video • Connectors • Cables
Arduino • Test Equipment etc, etc
The Coilcraft website (see below) provides some useful
information in the form of application notes on the design of
L-C filters.
The Laird guide a useful “rule of thumb” guide to
selecting ferrite cores for use in EMI suppression.
The TDK guide shows test equipment set-up used for
measuring the performance of ferrite clamp-on filters.
A wide range of clamp-on ferrite cores is available from
Mouser Electronics: www.mouser.co.uk
connector. Note that the filter is completely enclosed in an
aluminium case which must be properly earthed to ensure
satisfactory performance (see earlier).
A simple home-constructed pi-section filter for a lowvoltage DC supply (up to 50V at 2A) is shown in Fig.5.10.
The circuit of this filter is shown in Fig.5.11 and its measured
frequency response is shown in Fig.5.12. The rejection at
1MHz was measured at 40dB. Fig.5.13 shows the simple test
circuit used. Finally, radiation from cables can be significantly
reduced by fitting clamp-on ferrite cores (see Fig.5.14).
Clamp-on ferrite cores consist of two semi-circular cores
that can be quickly and easily snapped onto cable in a single
operation and without the need to cut the cable or remove any
existing connectors. They are available in various grades and
dimensions and are usually more effective when mounted close
to the incoming supply connectors of EMI-producing equipment.
When selecting a clamp-on core for a particular application
it is advisable to check manufacturers’ data sheet specifications
for their recommended frequency range and attenuation
characteristic. It is also worth noting that the effectiveness of most
cores increases with frequency (up to a maximum for the core
in question) and attenuation can be significantly improved by
winding several turns through the core. For example, a popular
Fair-Rite clamp-on ferrite core exhibits an impedance of 200Ω
at 50MHz with only a single pass-through conductor but this
increases to 800Ω and 1.9kΩ with two and three turns respectively.
Going further
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020 8452 0161
68
Visit our shop at:
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London NW2 3ET
This section (shown above) details a variety of sources
that will help you locate the component parts and further
information that will allow you to understand EMC and
improve the EMI and EMS performance of your electronic
designs and equipment. It also provides links to relevant
regulations and underpinning knowledge.
Practical Electronics | October | 2021
