Download EXPERIMENT EMC2: RF EMISSION REDUCTION TECHNIQUES

EXPERIMENT EMC2: EMI EMISSION FROM PCB
OBJECTIVE:
To evaluate the EMI emission from different layouts of electronic circuits and
design layout guidelines for printed circuit board.
APPARATUS
1.
2.
3.
4.
Experiment Board (EMI Radiation)
DC Power Supply
Spectrum Analyzer
Close Field Probe
INTRODUCTION
Electronic products must meet both emission and immunity requirements of the
international Electromagnetic Compatibility (EMC) regulation in order to gain access to
the world’s markets. RF emission can be radiated from an electronic product through air
space or conducted through power lines or data lines and disturb the operation of a nearby
electronic device. Immunity (or susceptibility) refers to the ability of an electronic product
to tolerate inflicted electromagnetic fields from an emission source through air or through
the interconnecting cables.
For an electromagnetic interference (EMI) to take place, there must be a generator
or source, a coupling path, and a receptor. A radiated emission consists of a time-varying
electric (E) and magnetic (H) fields. The ratio E/H is called the wave impedance. At a
place far from the source (known as far field), the value of E/H is a constant 377 . At
the near field (distance < /2), however, E/H may be much greater or less than 377 . If
E/H is much greater than 377 , then any interference is more likely to be due to the Efield and the coupling to the receptor is primarily capacitive. The emission may be
radiated from binding posts or long pins (acting as dipole antennas) on a printed circuit
board (PCB). Conversely, if E/H is much less than 377 , then the H-field dominates and
the coupling to another circuit is inductive. In such a case, possible noise sources are
current loops which form radiating loop antennas.
One method to overcome the EMI is to isolate the noise source with a shielded
enclosure. The shielding material dissipates the field energy before it reaches the receptor.
For capacitive coupling (associated to E-field), a grounded Faraday cage made of high
conductivity material can be used to obtain an effective isolation of the source. The Efield parallel to the metal shield surface is essentially zero and all EMI fields are confined
inside the enclosure. For inductive coupling, however, the shielding enclosure is effective
only if the material has high conductivity and also high magnetic permeability to provide
sufficient absorption losses to the interfering H-field. Generally, high permeability
materials are not among those with the highest conductivity. Iron or steel is commonly
used as a good compromise.
Experiment EMC2
The effectiveness of the shielding is also affected by the presence of any
discontinuity in the enclosure. Openings or holes in the shielding material may result in a
large leakage of EMI fields. Currents induced around the holes may reradiate all the EMI
noises toward a focussed direction.
Shielding is usually too expensive to implement, especially in the competitive
market of consumer products. It is often considered as the last resource to solve EMC
problems. Observance of the proper layout and grounding principles during the design
stage is necessary in order to avoid this costly “bandage”. Coupling between circuits is
minimized when a current returns via a path as near as possible to the incident path. On a
PCB, ideally the return path should be laid in parallel to the incident path on the opposite
side of the PCB to form a stripline (see Figure 1). If the conductor width w is much
greater than the dielectric thickness t, the emitted noise is in general proportional to the
t/w ratio, wire length, and the frequency and amplitude of the signal current. This
implies that the conductor must be reasonably wide and the interconnection being kept as
short as possible. Sharp corners must be avoided to maintain constant characteristic
impedance of the transmission line and minimize reflection and radiation.
I
+
t
E
I
signal
H
w
(a)
E
ground
I
(b)
Figure 1: (a) Stripline and (b) fields near a surface mounted component.
When there are more than one return path (for example, two ground conductors
connected to a circuit), it is often difficult or impossible to force the signal current to
return via the desired path. The current may split between the two ground conductors or
add with a current from another circuit. The result is an unbalanced incident and return
currents, and the stray fields emitted from the circuit will not completely cancel each
other. This will greatly reduce the effectiveness of the stripline. A solid conductive
ground plane on the entire underside of the PCB is one method to minimize the unbalance
condition. The plane forms an equipotential surface for both the electric scalar potential V
and the magnetic vector potential A. An “image” of the incident path is created below the
ground plane to carry the return current in the opposite direction.
For the case of a single-sided PCB, where the return stripline or ground plane is
not available, the emitted noise can be minimized using a pair of parallel transmission
lines on the same plane. The separation between the two conductors must be kept as close
as possible. Since the conductors’ separation is now larger than the thickness of the
conductors, the effectiveness of this arrangement is less profound than a stripline.
Experiment EMC2
The RF emission from a circuit may interfere the operation of another circuit on
the same board, as well as any other equipment in the vicinity of that source. In the EMC
regulations, the radiated emission is measured at far field on an Open Area Test Site
(OATS) or a Semi-anechoic Chamber. If the radiation is above the regulatory limit, it
must go back to the bench for further diagnosis and repair. A close-field probe connected
to a spectrum analyzer is commonly used to find the problem frequencies and the
locations of the EMI sources. Subsequently, some circuit changes may be made to reduce
the emissions. The close-field probe can be used to re-measure the radiation and compare
with the earlier result. We can assume that an improvement indicated by the close field
measurement will give a proportional improvement when the product is tested at far field
on a standard OATS.
PRECAUTION
Measure the DC Power Supply voltage using a multimeter before connecting the voltage
to the Experiment Board. Make sure the polarity is correct.
Make sure you are familiar with the operation of a spectrum analyzer prior to the lab
experiment.
PROCEDURE
1. Study the wiring layout of the experiment board. Draw the outline of the PCB on a
paper and divide it into several sections using grid lines.
2. Connect the close-field probe to the RF input of a Spectrum Analyzer.
3. Set the Spectrum Analyser to a known state by pressing [Preset][Preset All].
4. Select the frequency range: press [Frequency][Start Freq][60][MHz] [Stop Freq]
[400][MHz].
5. Set the Amplitude unit to dBuV.
6. Set the DC Power Supply to 5 V and connect the supply to the experiment board.
Caution: make sure the polarity is correct.
7. Turn S1:1 of the DIP-Switch on. (The rest of the switches shall be off.) The highfrequency clock signal will be transmitted through the Long Microstrip.
8. Place the close-field probe near to the Long Microstrip. Measure the frequency of the
fundamental spectral line ([Measure][Freq Count][Count On]). Record the reading
and turn off the frequency count function: [Measure][Freq Count][Count Off].
9. Move the close-field probe slowly over the entire PCB column-by-column.
Throughout the experiment, keep the close-field probe at almost the same height
above the PCB.
10. For every column, record the highest magnitudes of the fundamental and 3rd
harmonic spectral lines. Record also the position (X-mm and Y-mm from the top lefthand corner of PCB) where the highest field magnitude is encountered.
11. Repeat step 9 and 10 for signal transmission using Straight Microstrip, Parallel Line,
and Long Wire by setting the DIP-Switch accordingly.
12. Show the results to your supervisor.
Experiment EMC2
DISCUSSION
1. Evaluate the EMI emissions level from different circuit layouts.
2. Analyze the trace layouts and justify why certain layout produces higher radiation
compared to the other layouts.
3. Conclude which layout generates the highest radiation and which layout produces the
lowest radiation.
4. How would you design the layout of your printed circuit board (PCB) on (i) single
sided PCB and (ii) double sided PCB in order to minimize EMI emission?
MARKING SCHEME
1. Experiment Results – 10%
2. Discussion – 12%
3. Conclusion – 3%
Experiment EMC2
Figure 2: PCB layout for top layer.
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Figure 3: PCB layout for bottom layer.
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Figure 4: Schematic diagram of the circuit.
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