Download EXPERIMENT EMC1: LAYOUT AND GROUNDING OF

Electromagnetic Interference
Multimedia University
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EXPERIMENT EMC1: LAYOUT AND GROUNDING OF
PRINTED CIRCUIT BOARD
OBJECTIVE:
To analyze the conducted electromagnetic interference between the circuits on
a printed circuit board, and evaluate various EMI reduction techniques, such as
decoupling capacitors and L-network filters.
APPARATUS
1.
2.
3.
4.
5.
6.
Experiment Board (Layout and Grounding)
DC Power Supply
Oscilloscope 100 MHz
Capacitors (0.01 F, 0.1 F, 10 F)
Inductors (100 H, 220 H)
Resistors (100 , 220 )
INTRODUCTION
The ideal role of ground is to act as the reference point for all other signals in
an electrical system. The integrity of this reference point is critical to the overall
design quality. Digital signals usually have some built-in tolerance for a varying
ground potential. However, a "clean" ground has to be maintained for any sensitive
analog circuits on board and in system with mixed analog and digital signals. As
frequency increases, the problems become more critical.
One must always keep in mind that current must flow in a loop. The practical
role of ground is to provide a return path for the current taken into an electrical load to
return to the signal source terminal. Finite impedance exists in the connecting wire
and ground path. Current flowing through this impedance causes voltage drop. This
voltage drop, which is the potential difference between the reference ground and the
local circuit’s operating ground, is the root of the problem (as illustrated in Figure 1).
As it is impossible to remove the current or make the impedance zero, tight control of
the current paths (length, width and distribution pattern) becomes the major task of the
wiring layout engineers. Most analog circuits are recommended to have a 0.01- to 0.1F capacitor in parallel with a larger 1-F capacitor connected close to the supply pins
to ground. These capacitors are referred to as decoupling capacitors. It acts as a charge
reservoir to supply the required transient current IT and allows this current to flow
through the decoupling capacitor Cd instead of propagating along the power line. In
this way, it minimizes the noise and prevents the voltage fluctuation at the VCC supply
pins. The lead of the decoupling capacitor must be kept as short as possible in order to
minimize any parasitic inductance.
Experiment EMC1
Electromagnetic Interference
Multimedia University
(IT+IDC)Z1
+
2
+
VS
+
Z1
-
Z2
VL=VS-(IT+IDC)(Z1+Z2)
IT
IDC
-
-
+
(IT+IDC)Z2
Figure 1: Equivalent circuit of a power distribution bus.
For a big current loop, the stray inductance may be sufficient to cause a large
cross-coupling of the signal to its neighboring loops. The most likely cause of
interference on the same PCB, however, is a high current inducing a voltage in a
nearby low-voltage circuit. This suggests that a circuit carrying a large current should
not be located near a sensitive circuit. Therefore, the various function circuits on a
PCB must be classified according to the magnitudes of the signal voltages and
currents. Different classes of circuits must be physically separated. Digital logic
circuits are grouped and located within an allocated area while the analog circuits are
grouped in another area of the PCB or system, and preferably with a sufficient spacing
between the different circuit types.
A power bus usually drives multiple loads, as shown in Figure 2. Each circuit
type should have a ground trace or ground plane as the current return path. It is
undesirable to allow the return current from one circuit type to flow in the ground
trace of another. This can be reliably prevented only by mean of separating the ground
traces. No ground trace of one circuit type should extend into the area of another
circuit type. The conductors between the junction point behave as transmission lines
to the individual loads. A decoupling capacitor is usually necessary at each load unless
the load is very close to the junction point. If the junction point is some distance from
the power source, another decoupling capacitor with a much larger value is also
needed at the junction point to absorb the sum of the transient currents incurred by
each varying load. In practice, a power amplifier is placed closest to the power
distribution junction to minimize the loop and the number of decoupling capacitors.
Experiment EMC1
Electromagnetic Interference
Multimedia University
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VL1
Cd1
VS
+
-
IT1
Idc1
Cd
VL2
Cd2
IT2
Idc2
Figure 2: Power bus driving two varying load.
In extreme cases, decoupling capacitors may not be able to provide enough
suppression at one or more frequencies. An L-network filter may be necessary on each
load, as shown in Figure 3. The series resistor does not reduce the change in the load
voltage VL due to the transient current of that circuit itself, but it does impede the
transient from propagating to other loads. The L-network forms a low-pass RC ladder
filter. The resistor may be replaced by an inductor if the dc voltage drop across the
resistor is too large.
Rd1
Cd1
VS
+
-
Cd
VL1
IT1
Idc1
Rd2
Cd2
VL2
IT2
Idc2
Figure 3: Power bus with L-network filter on each load.
Although it is easy to suggest ideal grounding and decoupling strategies for
individual components, the implementation is often very complex in large systems
which involve thousands of wires layout on a limited board size. The engineer will
have to make the most out of the less-than-ideal situation where he does not have
much control over the system ground and power supply connections.
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Electromagnetic Interference
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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 an oscilloscope prior to the lab
experiment.
PROCEDURE
1. Study the PCB layout of Circuit-1 and Circuit-2 and identify the differences.
2. Start the experiment without C102, C105, C107, C108, C202, C205, C207 and
C208.
3. Set the DC Power Supply voltage to 5 V.
4. Connect the 5-V supply to Circuit-1. Caution: make sure the polarity is correct.
5. Toggle SW103 to ON. The analog circuit is a sinewave oscillator. Connect the
sinewave output (CN103) to the oscilloscope CH1 input (AC coupling).
6. Set the oscilloscope properly (volt/div, time/div, trigger source, trigger level) to
obtain the sine waveform. Sketch the waveform on a graph paper.
7. Toggle SW102 to ON. The digital circuit is a clock generator. Connect the digital
output (CN102) to the oscilloscope CH2 input.
8. Select CH1 as the trigger source. Measure the peak-to-peak amplitude of the noise
riding on the sinewave signal (using proper volt/div setting).
9. Sketch the waveforms. Explain your observation. Show your result to the lab
supervisor.
10. Try disconnect and then connect the BNC connector on CH2 of the oscilloscope.
Observe carefully any changes in the noise level on the sinewave at the moment
you disconnect or connect the BNC connector. Compare the noise level and
explain your observation.
11. Turn the digital clock (SW102) OFF. Then, toggle SW101 to ON. The electric
motor should turn on and off at a rate of about 1 Hz.
12. Distinguish the changes on the sinewave. Explain your observations. Show your
result to the lab supervisor.
13. Turn the digital clock (SW102) ON.
14. Use a probe to examine the DC and AC voltage components on various points of
the 5 V-trace and ground-trace and Vcc pins of the ICs. Describe your observation.
15. Disconnect the power supply from Circuit-1, and connect it to Circuit-2.
16. Repeat steps 5 to 14 on Circuit-2. Compare the noise amplitude due to the
different layout of Circuit-1 and Circuit-2.
17. Use the available capacitors, inductors and resistors on the relevant sockets on the
PCB. Try various configurations of decoupling capacitors and L-network filters to
minimize the noise on the sinewave signal when the digital circuit is on. Try every
logical combination (use the reference number on the Experiment Board for
capacitors and inductors to record the combination) and record the different
magnitudes of improvement on the sinewave quality (for both Circuit-1 and
Circuit-2).
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Electromagnetic Interference
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DISCUSSION
1. Justify why the sine waveform become noisy when the digital circuit is turned on?
2. Evaluate the reason why the noise amplitude is different when the oscilloscope is
connected to both the analog circuit (CH1) and the digital circuit (CH2) compared
to the case when the oscilloscope is only connected to the analog circuit (CH1
only).
3. Why the sine waveform shows a transient change when the motor switches on-tooff or off-to-on?
4. Why the 5-V supply is not constant d.c. at various Vcc pins on the Experiment
Board?
5. Justify why the different layouts of Circuit-1 and Circuit-2 give rise to different
noise amplitudes?
6. Analyze the noise reduction performance between different combination of
decoupling capacitors and L-network filters, and conclude the best configuration
of decoupling capacitors and L-network filters that minimizes the noise amplitude
on the sinewave signal.
7. With the use of decoupling capacitors and L-network filters, conclude which
circuit (Circuit-1 or Circuit-2) can achieve the lowest noise amplitude?
8. Conclude the different layout and grounding methods, and noise reduction
methods.
9. Recommend how you would layout the printed wires on a single-sided PCB to get
a better performance.
10. How would you design the layout of the same circuit on a double-sided PCB?
MARKING SCHEME
1. Experiment Results – 10%
2. Discussion – 12%
3. Conclusion – 3%
ACKNOWLEDGEMENTS
This lab sheet was designed by Dr. Chung Boon Kuan.
Experiment EMC1
Electromagnetic Interference
Multimedia University
Schematic diagram of the circuit.
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Electromagnetic Interference
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Component placing on the experiment board.
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Electromagnetic Interference
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Wiring layout of the experiment board.
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