EMC 1 Lab Report

8/3/2019 EMC 1 Lab Report

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OBJECTIVE

The objective is to analyze the conducted electromagnetic interference in between the circuit 1

and 2 on the printed circuit board. Besides that, we also enquiry to use different EMI reduction

techniques such as decoupling capacitor and also L-network filters.

INTRODUCTION

Digital signals usually have some built-in tolerance for a varying ground potential. However, a

"clean" ground has to be maintained for any sensitive analogue circuits on board and in system

with mixed analogue and digital signals.

One of the most important parts is that current must flow in a loop because we want to provide a

return path for the current to return to the signal source terminal. Besides that, most analogue

circuits are recommended to have a 0.01 to 0.1uF of capacitor in parallel with a larger 1-uF

capacitor connected close to the supply pins to ground. The capacitor that use for this situation is

often called as decoupling capacitor. This capacitor actually act 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. This method will help in reducing the noise and

prevents the voltage fluctuation at the voltage supply pins. Other than this, we have to make sure

that the lead of the decoupling capacitor must be kept as short as possible in order to minimize

any parasitic inductance.

Noise is one of our concerns in this experiment and so with the PCB interference. This is because

interference of intra system is commonly happen due to the high current inducing a voltage in a

nearby low-voltage circuit. So, this stated that a circuit that carrying a high current should not

place nearby of a sensitive circuit. Analogue circuit are usually place with a gap in between to

avoid this kind of high current interference.

When trying to design a circuit with high current being return to the source, we also need to

make sure that the current that flow back from a circuit will not flow into the ground of another


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circuit. So, among one of the solution to solve this problem is by separated the ground for

different circuit. Other than this, the distance between the junction point which act as a

transmission line and also the load is very important. If the distance is big, then a decoupling

capacitor will be an ideal solution to absorb the sum of the transient currents incurred by each

varying load.

An L-network filter may be necessary on each load. 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.

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 layouts 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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RESULT

Circuit 1: (PCB with single point common grounding layout)

Step 6:

Figure 1: Sinewave of analog circuit 1

Step 8-9:

Figure 2: Noise Signal riding on the sinewave


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Step 10:

Figure 3: Noise level after plugging the BNC connector in and out

Step 12:

During the motor turn on and off, the circuit sinewave vibrate and expand with the motor status,

the wave alter not too much as the motor size wasnt that large. Things can be severe enough

imagine if we were using a very large motor that will draw much serious impact to the system.

The waveform alter due to the switching happened everytime when motor turning on and off, it

will draw large amount of current and causes shortage of supply as well.


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Step 14:

Ground-trace:

Figure 4: Grounding noise from motor driver circuit 1

Figure 5: Grounding noise from digital circuit 1


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Figure 6: Grounding noise from analog circuit 1

DC-value of 5V input:

Figure 7: 5V input DC value taken at motor driver circuit 1


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Figure 8: 5V input DC value taken at digital circuit 1

Figure 9: 5V input DC value taken at analog circuit 1


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AC-value (fluctuation noise) of 5V input:

Figure 10: Noise riding on the 5V input taken at motor driver circuit 1

Figure 11: Noise riding on the 5V input taken at digital circuit 1


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Figure 12: Noise riding on 5V input taken at analog circuit 1

Step 17:

Modify on digital circuit 1:

Figure 13: Resistor added on digital circuit 1


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Figure 14: Together resistor and capacitor added on digital circuit 1

Modify on analog circuit 1:

Figure 15: Resistor added on analog circuit 1


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Figure 16: Both resistor and capacitors added on analog circuit 1

Modify on both analog and digital circuit 1:

Figure 17: Added decoupling network and L-network on circuit 1

Observation:

The noise amplitude gets higher as the distance from the main supply gets larger.

Combination of both decoupling capacitor and L-network filter give better result.


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Circuit 2: (PCB with single point separate grounding layout)

Step 6:

Figure 18: Sinewave of analog circuit 2

Step 8-9:

Figure 19: Noise signal riding on sinewave (circuit 2)


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Step 10:

Figure 20; Noise level after plugging the BNC connector in and out (circuit 2)

Step 12:

The same output goes for this circuit 2, the waveform itself vibrate and expand a little bit every

time when the motor turns activated.

Step 14:

Step 14 wasnt performed in circuit 2 due to shortage in time, and since circuit 2 use separate

point grounding layout, we can directly know that taking measurement at different points will

not give us significant results like circuit 1 with common point grounding layout do.


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Modify on analog circuit 2:

Figure 23: Added resistor on analog circuit 2

Figure 24: Added capacitors to analog circuit 2


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Modify on both digital and analog circuit 2:

Figure 25: Added decoupling capacitors and L-network to both digital and analog circuit 2

Observation:

The sinewave we got is better due to separate grounding method.

Combination of both decoupling capacitor and L-network filter still the best noise reduction

configuration.


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DISCUSSION

1.

Turning on the digital circuit do affect the sine waveform as digital circuit contains widerange of frequencies with harmonic spread, this wide range (high) frequency will affect

and interfere the performance of analog circuit.

2. Lets considered both analog and digital and circuit are turned on, the noise amplitudewill be different when the oscilloscope connected only to analog circuit compared when

the oscilloscope connected to both analog and digital circuit. Connected both analog and

digital circuit to the oscilloscope tends to give us higher noise amplitude because there

will be some signal sending in form the system to the oscilloscope via CH2, and this will

interfere the performance of the oscilloscope comparing if we only take the measurement

from analog circuit (CH1) only.

3. When the motor turning on and off, the waveform shows transient change due to thestatus of the motor, when the motor turning on and off, it eventually simulate a switching

on and off, this switching will draw large amount of current and causes noise to the

system.

4. 5V supply is not constant DC at various Vcc pins on the experiment board is due to thelayout of the PCB, as the supply signal goes far apart from the main supply, it will

encounter with some parasitic elements such as resistance, also with the return path, the

return path get longer if we take measurement at point that far apart from the main supply.

This parasitic element and longer return path will cause reduction on the travel signal as

well as brining in noise to the travelling signal.

5. Circuit 1 and circuit 2 showed different levels of noise amplitudes as circuit 1 use singlepoint common grounding layout while circuit 2 use separate grounding layout. Circuit 2

with separate grounding give us better result because the layout use cross coupling

between ground lead, some more in the returning path, the signal wouldnt interfere with


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each other along their return path due to the separate grounding layout, this will further

minimize the noise voltage. The same explanation goes for the power (supply) line.

Circuit 1 with common grounding and supply give rise to higher noise amplitude because

the supply and return signal share the same path, further increase the interference in

between the signal.

6. The decoupling capacitor is actually act as a charge reservoir to supply the requiredtransient current and allows this current to flow through the decoupling capacitor instaed

of propagating along the power line. While for L-network, it is actually a low pass filter

that will block high frequency content. This will make the supply voltage stable. The

configuration would definitely be the implementation of both decoupling capacitors and

L-network filter together.

7. Circuit 2 can achieve lower noise amplitude. Circuit 1 give rise to higher noise amplitudebecause the system use shared power and ground line, so, circuit 2 still the better

compared to circuit 1.

8. Different circuits have their own best circuit layout and also the grounding method.However, in general, multiple grounding layout will provide better performance followed

by separate grounding then common grounding. But its hard to implement multiple

grounding layout in analog circuit because it is very difficult to keep 50/60Hz

interference out of the circuit.

In term of noise reduction methods, still the implementation of both decoupling

capacitors with L-network filters will be the best configuration.

9. For a single sided PCB, of course it will be best to minimize the travelling signal path,keep every path as short as possible will helps to reduce the potential difference. Also, I

will implement the decoupling capacitors to minimize the noises and prevent the voltage

fluctuation, also the capacitors lead must be short in order to minimize lesser parasitic


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inductance. Depends on the situation, L-network filters will be only added if the

decoupling capacitors may not be able to provide sufficient suppression. For sure,

separate grounding layout will be used.

10.To design the same circuit on double-sided PCB, I will introduce the multiple groundinglayout to the PCB itself, however, if the multiple grounding isnt that suitable, we can

have hybrid grounding layout so the PCB will be suitable for different frequencies levels.

CONCLUSION

Noise and EMI are always presented in our every circuits, proper circuit layout designshould be considered from the beginning stages of circuit design. The control of noise

and EMI is essential, not only because they are mandatory requirements, but also for the

proper functionality of the entire system. I have come across with the circuit layout

design issues and the noise reduction methods from this laboratory experiment, its not

only the circuit layout to be considered but also the proper noise reduction methods to be

introduced to every circuit to give us better and more guaranteed performances.

Documents

EMC 1 Lab Report