BTEC HNC - Electronics - Investigate Signals and Noise

8/2/2019 BTEC HNC - Electronics - Investigate Signals and Noise

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Investigate Signals and Noise

ElectronicsBy Brendan Burr


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Brendan Burr BTEC Higher National Certificate in ElectronicsInvestigate Signals and Noise

Table of Contents

TABLE OF CONTENTS ......................................................... 2

TASK 1 .............................................................................. 4

1.1 Using the appropriate formulae convert the followingvalues of gain to decibels:- ................................................ 4

(a) Power ratio of 100 ........................................................ 4

(b) Voltage ratio of 20 ....................................................... 4

(c) Current ratio of 10 ........................................................ 4

1.2 Using the appropriate formulae convert the followingvalues of attenuation to decibels:- ..................................... 5

(a) Power ratio of 0.01 ....................................................... 5

(b) Voltage ratio of 0.05 .................................................... 5

(c) Current ratio of 0.1 ....................................................... 5

1.3 Express the following power levels in dBm :- ................. 6

(a) 100mW ........................................................................ 6

(b) 2W ............................................................................ 6

(c) 5W .............................................................................. 6

(d) 1mW ........................................................................... 6

TASK 2 .............................................................................. 7

2.1 Discuss, using supporting diagrams where appropriate, 3of the following types and sources of external noise found inelectronic systems. ........................................................... 7

(a) Mains hum ................................................................... 7

(b) Switching or contact noise ........................................... 8

(c) Crosstalk ..................................................................... 9

2.2 Discuss, using supporting diagrams where appropriate 3of the following types and sources of internal noise found inelectronic systems . ......................................................... 10

(a) Thermal Noise ........................................................... 10

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(b) Shot Noise ................................................................ 11

(c) Flicker noise ............................................................. 12

2.3 Discuss, using supporting diagrams where appropriate 2

different methods used for noise reduction. ...................... 13

TASK 3 ........................................................................... 15

3.1 Explain and define the following concepts used todescribe the effects of noise and its measurement inelectronic systems. .......................................................... 15

(a) Signal to Noise Ratio. ................................................ 15

(b) Noise figure or factor. ................................................ 16

3.2 Explain with supporting diagrams where appropriatehow Noise can be categorised using colours. ..................... 18

EVALUATION ................................................................... 20

CONCLUSION ................................................................... 20

BIBLIOGRAPHY ................................................................ 21

Books ............................................................................. 21

Catalogues ...................................................................... 21

Websites ......................................................................... 21

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Task 1

1.1 Using the appropriate formulae convert the following values ofgain todecibels:-

(a) Power ratio of 100

dBP

PLOGA

IN

OUT

P

=10

( )dBLOGAP

10010=

dBAP 20

=

(b) Voltage ratio of 20

dBV

VLOGA

IN

OUT

V

=20

( )dBLOGAV

2020=

dBAV

26

=

(c) Current ratio of 10

dBI

ILOGA

IN

OUT

I

=20

( )dBLOGAI

1020=

dBAI

20=

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1.2 Using the appropriate formulae convert the following values ofattenuation to decibels:-

(a) Power ratio of 0.01

dBP

PLOGA

IN

OUT

P

=10

( )dBLOGAP

01.010=

dBAP

20=

(b) Voltage ratio of 0.05

dBV

VLOGA

IN

OUT

V

=20

( )dBLOGAV

05.020=

dBAV

26=

(c) Current ratio of 0.1

dBI

ILOGA

IN

OUT

I

=20

( )dBLOGAI

1.020=

dBAI

20=

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1.3 Express the following power levels in dBm :-

(a) 100mW

=mW

PLOGdBm

OUT

110

= 3101

1.010LOGdBm

dBm20=

(b) 2W

=mW

PLOGdBm

OUT

110

=

3

6

101

10210LOGdBm

dBm27=

(c) 5W

=mW

PLOGdBm

OUT

110

= 3101

510LOGdBm

dBm37=

(d) 1mW

=mW

PLOGdBm

OUT

110

=

3

3

101

10110LOGdBm

dBm0=

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Task 2

2.1 Discuss, usingsupporting diagrams where appropriate, 3 of thefollowingtypes and sources ofexternal noise found in electronicsystems.

(a) Mains hum

Mains hum is an audible oscillation of Alternating Current.It can be caused by minute changes of the material in which ACCurrent is travelling, which occurs because of the presence ofsomething called magnetostriction in ferromagnetic materials. Thistype of hum is mainly noticed around large transformers.It can also be caused when susceptible audio equipment is placed

close to mains powered devices, such as power transformers, wherethe alternating electromagnetic field causes interference with thedevice, this is then heard through loudspeakers and amplifiers.

Electric hum is common around large transformers due to the magneticcore.Mains Hum can be alleviated by using a band-stop filter, however itshigher harmonics may still remain.

A common example of audible Mains Hum is under an electricity pylon,where there is a constant low frequency noise.

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(b) Switching or contact noise

An example of when Switching/Contact Noise occurs is when usingSwitching Regulators in Circuits. They are used in circuits to change aDC Current into an AC, amplify it and then convert it back to DC. Theprocess can cause a noise which is can affect other parts of the circuit.When the switch is transferred to a different state there is a suddenspike in the waveform caused by the current bridging the closing gapbetween the two contacts, just before contact.The voltage/current spike can overload the other components in thecircuit and can cause them to become damaged and faulty.

On the above image the you can see two waveforms, the lowerwaveform shows the voltage of a circuit that automatically switcheson/off approximately every 0.3 seconds. On the upper waveform youcan see that every time the circuit switched on, there is a noise whichis picked up and increases the lower waveform (switcher output) by0.2V.

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(c) Crosstalk

This is where a signal transmitted in one circuit can affect anothercircuit.In terms of electrical circuits in aircraft, crosstalk is tested for by theEMC Department, who test for susceptibility over multiple circuits,ensuring there is no conflict on the Flight Safety Critical Systems.If a high powered signal is sent through one cable and next to it therewas another cable which was sending a low powered signal, therecould be interference between the two signals, causing a false readingon a display. This could be extremely important when it comes tomeasurement instruments such as the Radar Altimeter on aircraft, afalse reading here could cause a catastrophic accident due to the pilotbelieving he/she has a larger distance between the landing gear and

the ground.

An purely acoustic example of crosstalk is an electric guitar and amicrophone, where the microphone picks up the amplified guitar andthen amplifies again creating a high pitched sound.

The image below shows a twisted cable that has a test meterconnected. An oscillation is sent through a similar cable, located nextto the first cable. Due to crosstalk you can see a slight reading on thetest meter, which should be recording 0A/0V.

Crosstalk can also occur in wireless communications where informationis send via the same channel resulting in the receiver becomingconfused.

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2.2 Discuss, usingsupporting diagrams where appropriate 3 of thefollowing types and sources of internal noise found in electronicsystems .

(a) Thermal Noise

Thermal noise is also known as White Noise. Circuits naturally heat upduring operation, due to the current flowing through them. As thecircuit gets warmer, signals from the circuit begin to differ than whenthe circuit was cold. This is the reason why there are operatingparameters for the various ICs on their datasheets, due to thefluctuations of its outputs.

As the circuit heats up the heat generated can affect other components

in the circuit, causing them to malfunction or alter their output signal.

Circuits which are designed to receive very low levels of signal, suchas radio telescopes intercepting signals from stars, use front endcircuits which are cooled to a very low temperature. This ensures thatthe amount of thermal noise is kept to an absolute minimum.

Thermal noise is always in resistors, where the device lowers currentand voltage by removing some of the energy and dissipating it as heatenergy.

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(b) Shot Noise

Understanding Shot Noise is important in Electronics,Telecommunications and fundamental Physics. It is where a smallamount of electrons that carry energy in an electronic circuit cause afluctuation in the measurement data.

Shot noise is normally only problematic when it comes to smallcurrents in electronic circuits. This is because the magnitude of thenoise increases with the average magnitude of the current, howeverthe magnitude of the average signal increases even greater than that,so the stronger the signal the weaker the shot noise becomes.

The above graph is a simulation of an ideal shot noise waveform.

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(c) Flicker noise

It is a type of electronic noise that is found in the pink spectrum andtherefore is referred to as Pink Noise. This type of electronic noiseoccurs in almost all electronic devices, and is always related to DC,resulting in a variety of effects such as impurities in conductivechannels or generation and recombination noise in a transistorbecause of the base current.

With electronic devices, flicker noise forms under low frequencyconditions due to higher frequency conditions being over shadowed bywhite noise.

Flicker noise is characterised by the corner frequency fc.

In resistors there are various levels of flicker noise depending on thecomposition of the resistor. In Carbon Composition Resistors theflicker noise increases the overall noise level, however in Wire-WoundResistors the flicker noise is much smaller and so doesnt affect itsignificantly. This is only where the DC levels are high, as when theyare low the Thermal Noise will be predominant and therefore theFlicker Noise will be less affected regardless of the type of resistorused.

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http://upload.wikimedia.org/wikipedia/en/b/b2/2Cin-2pi-x.jpeg


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2.3 Discuss, using supporting diagrams where appropriate 2 differentmethods used for noise reduction.

Noise occurs in many various forms however the noise reductiontechniques are very similar.

Screened cable is used in aircraft to prevent wires which are bundledtogether in a loom, becoming susceptible to cross feed.Screened cable works by having a wire mesh around the cables inside(indicated by the red arrow).

This wire mesh captures all and any of the cross feed and takes itdown to ground. In aircraft the wire mesh is connected to the plugwhich would be mounted on an aluminium mounting and that would beconnected to the airframe of the aircraft.

By doing this the signal is free to travel through the wire without havingthe potential of being interrupted by the noise.

Screening of cables is extremely important when it comes to wiringsensitive equipment such as computer displays. This is because smallamounts of voltage/current fluctuation can result in substantialinaccuracies in the operation of these devices.

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Another way to reduce the noise is by ground lifting. This method iscommonly used to reduce the affects of mains hum on musicalequipment.

The hum occurs because of the loop created between the earthing pinand various pieces of equipment.

Firstly the piece of equipment that is causing the mains hum needs tobe identified, this can be done by disconnecting all of the equipmentand then reintroducing it piece by piece.

Once established this piece of equipmentcan be isolated from the other pieces byremoving it from the earthed loop.

The image to the right is of a PublicAnnouncement System which has aGround Lift circuit in.

When a current is introduced to theshielded section of the cable, the crossfeed causes the disturbance in thetravelling signal. By increasing theresistance in the shielded part of the cablethe current is limited and therefore the

problem of the ground loop is nearlyeliminated.

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Task 3

3.1 Explain and define the following concepts used to describe theeffects of noise and its measurementin electronic systems.

(a) Signal to Noise Ratio.

This is defined as the ratio of a signal power to the noise powercorrupting the signal.Signal-to-noise ratio compares the level of a desired signal (such asmusic) to the level of background noise. The higher the ratio, the lessobtrusive the background noise is.

As you can see on the above diagram the Signal Strength is strongerthan the Noise Level, so it would be possible to clearly see/hear thesignal. This would be shown as a ratio higher than 1:1.If the noise level was higher than the signal strength then the signalwould become inaudible and is would be corrupted as it would beunclear. This would be shown as a ration lower than 1:1.If there was a ratio of 1:1 then the noise level would be just as strongas the signal and would result in the signal being difficult to hear/see.

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(b) Noise figure or factor.

This is a measure of degradation of the Signal to Noise Ratio. It canbe calculated by:

out

in

SNR

SNRNF =

The Noise Figure is the ratio of actual output noise compared to thenoise that the device itself introduces. This makes it suitable inmeasuring the performance of radio receivers.

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The above image is a graph which shows the increasing Noise Figurein Decibels in terms of the Noise Temperature in degrees Kelvin.

As the temperature reaches an operating temperature of around 17degrees Celsius the Noise Figure is 3dBs.

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3.2 Explain with supporting diagrams where appropriate howNoise canbe categorised usingcolours.

Noise has been categorised into many different colours depending ontheir characteristics, such as:

Black Noise This is the name of the noise that is commonly silent buthas occasional spikes.

Blue Noise Has more energy as the frequency increases. It is similarto Pink Noise, where density is proportional to the frequency.

Brown Noise This noise has more energy at lower frequencies andmimics the signal noise produced by Brownian Motion. Brown Noisesounds similar to White Noise to the human ear.

Gray Noise This is also similar but not the same as White Noise,however it has been filtered so that the sound level appears constant atall frequencies to the human ear.

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Green Noise This is basically Natural Noise, and involves all of thebackground noises of the world. It is not actually a recognised term.

Orange Noise This is a description of a noise which has beenstripped of harmonic frequencies. It relates to musical scales.

Pink Noise This type of noise is also known as Flicker Noise. It issimilar to White Noise however it contains an equal amount of energyin each octave band.Sound Engineers use Pink Noise to see if a system has a flatfrequency response.

Purple Noise As the frequency increases in Purple Noise there ismore energy. It is similar to brown noise, however the power densityincreases 6dB every octave as the frequency increases.

Red Noise This noise is the name given to ambient underwater noisecaused from distant sources.

White Noise This noise is random and contains an equal amount ofenergy in all frequency bands.This noise is made up of all audio frequencies. It is used in ElectronicMusic to create synthesized sounds.

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Evaluation

In this assignment I focused on researching various topics of Signals and

Noise. I tried to include images to clarify my answers however in some it wasdifficult to find a relevant image. I find that when explaining something it isoften easier with the image. For example in explaining the Thermal Noise Ifound it harder to explain things as I couldnt find a relevant image forexplanation.

I found that converting the gain to decibels in the first question was straightforward as we have touched upon this subject in one of last yearsassignments. The information was also readily available from the notes takenfrom class.

When it came to the research I began to slow up a bit due to the lengthyprocess of browsing the almost unlimited pages of the internet.

Conclusion

I found this assignment tested my existing knowledge of Signals and Noise aswell as expanding it to a broader range of categories.I struggled to find information from internet sources for Switching/ContactNoise for some time, however eventually managed to find relevant informationrequired for this assignment.I think I can remember touching on different types of noise which exist in

previous years at college, however never delved deeper than generaldiscussion. I have been pushed towards researching new informationbuilding on these techniques and now realise that there is much moreinformation available than just on the first page of Google.Datasheets provide a good source of information for descriptions ofcomponents, which are included in circuits that I have never seen before, so Iwas able to understand how they work.Overall I have learned a lot from this research and assignment and amlooking forward to the next one.

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Bibliography

Through guidance from my lecturer, the following text books, catalogues and

websites I was able to complete this assignment:

Books

BTEC National Engineering (Mike Tooley & Lloyd Dingle)ISBN: 978-0-7506-8521-4Success in Electronics (Tom Duncan & John Murray)ISBN: 0-7195-4015-1Higher Engineering Mathematics (John Bird)ISBN: 0-7506-8152-7

CataloguesN/A

Websites

http://en.wikipedia.org/wiki/Mains_humhttp://www.mediacollege.com/audio/noise/http://en.wikipedia.org/wiki/Noise_(electronics)http://en.wikipedia.org/wiki/Shot_noisehttp://en.wikipedia.org/wiki/Flicker_noisehttp://en.wikipedia.org/wiki/Crosstalk

http://en.wikipedia.org/wiki/Band-stop_filterhttp://www.datacottage.com/nch/images/next.gifhttp://www.wisegeek.com/what-is-a-switching-regulator.htmhttp://www.tapr.org/images/dsp4.figure_5.jpghttp://scien.stanford.edu/class/psych221/projects/05/gregng/fig-var-sim-shotnoise.pnghttp://www.cisco.com/en/US/i/100001-200000/110001-120000/119001-120000/119129.jpghttp://www.microwaves101.com/encyclopedia/images/Noise%20figure/noisetemperature.jpg
http://en.wikipedia.org/wiki/Mains_humhttp://www.mediacollege.com/audio/noise/http://en.wikipedia.org/wiki/Noise_(electronics)http://en.wikipedia.org/wiki/Shot_noisehttp://en.wikipedia.org/wiki/Flicker_noisehttp://en.wikipedia.org/wiki/Crosstalkhttp://en.wikipedia.org/wiki/Band-stop_filterhttp://www.datacottage.com/nch/images/next.gifhttp://www.wisegeek.com/what-is-a-switching-regulator.htmhttp://www.tapr.org/images/dsp4.figure_5.jpghttp://scien.stanford.edu/class/psych221/projects/05/gregng/fig-var-sim-shotnoise.pnghttp://scien.stanford.edu/class/psych221/projects/05/gregng/fig-var-sim-shotnoise.pnghttp://www.cisco.com/en/US/i/100001-200000/110001-120000/119001-120000/119129.jpghttp://www.cisco.com/en/US/i/100001-200000/110001-120000/119001-120000/119129.jpghttp://www.microwaves101.com/encyclopedia/images/Noise%20figure/noisetemperature.jpghttp://www.microwaves101.com/encyclopedia/images/Noise%20figure/noisetemperature.jpghttp://en.wikipedia.org/wiki/Mains_humhttp://www.mediacollege.com/audio/noise/http://en.wikipedia.org/wiki/Noise_(electronics)http://en.wikipedia.org/wiki/Shot_noisehttp://en.wikipedia.org/wiki/Flicker_noisehttp://en.wikipedia.org/wiki/Crosstalkhttp://en.wikipedia.org/wiki/Band-stop_filterhttp://www.datacottage.com/nch/images/next.gifhttp://www.wisegeek.com/what-is-a-switching-regulator.htmhttp://www.tapr.org/images/dsp4.figure_5.jpghttp://scien.stanford.edu/class/psych221/projects/05/gregng/fig-var-sim-shotnoise.pnghttp://scien.stanford.edu/class/psych221/projects/05/gregng/fig-var-sim-shotnoise.pnghttp://www.cisco.com/en/US/i/100001-200000/110001-120000/119001-120000/119129.jpghttp://www.cisco.com/en/US/i/100001-200000/110001-120000/119001-120000/119129.jpghttp://www.microwaves101.com/encyclopedia/images/Noise%20figure/noisetemperature.jpghttp://www.microwaves101.com/encyclopedia/images/Noise%20figure/noisetemperature.jpg

BTEC HNC - Electronics - Investigate Signals and Noise

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