noise sources and effects

8/8/2019 Noise Sources and Effects

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Why study noise?

WYPR-FM 88.1 MHz

Transmitted power 15,500 W

Received power

~0.00000000000001 W

~19 orders ofmagnitude

Noisesources and effects


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Why study noise?

Noise is one of the principle limiting factors inthe performance of communication systems.

Electrical noise is any undesired voltages or

currents that end up appearing at the receiveroutput. An example is static that is commonly

encountered on broadcast

AM radio.

In a practical system noise is always present.


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Types of noise

Noise is divided into two broad categories:

External noise is noise introduced in the

transmission channel.

Internal noise is noise introduced inside the

receiver itself.

Information or

intelligence

(audio, video,

data, etc.)

Recovered

information

orIntelligence

Free space (radio), wire,

fiber-optic cable, etc.

Transmitter(TX)

Communicationschannel or medium

Receiver(RX)

External

noise

Internal

noise


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External noise

Industrial noise is caused by human made

electrical sources (motors,

generators, ignition)

Atmospheric noise is due to naturally

occurring disturbances in earthsatmosphere such as lightening.


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External noise

Extraterrestrial noise (solar noise)is electrical

noise due to solar and cosmic activity.

Dominates at higher frequencies and can be aserious problem in satellite communications.


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Internal noise

Internal noise is noise introduced inside the

receiver.

Three types of internal noise we will consider Thermal noise

Semiconductor noise

Intermodulation distortion


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Thermal noise

Thermal noise is caused by random motion offree electrons and vibrating ions in a

conductor due to heat.

Noise Power in watts is directly proportionalto Bandwidth in Hz, and the temperature in

degrees Kelvin.Thermal noise voltage as a function of timeRandom electron motion due to heat


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Thermal noise

Thermal noise is also termed white noise or

Gaussian noise.

Just as white light contains all frequencies, white

noise contains an equal weighting of allfrequencies.

Frequency (Hz)

Frequency spectrum of thermal noise

Thermal noise voltage as a function of time


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N KTB!N= noise power (watts)

B =bandwidth (hertz)

K= Boltzmanns proportionality constant

(1.38 10-23 Joules per kelvin)

T= absolute temperature (kelvin)

10 log 10 log 10 log0.001 0.001

dBm

KTB KT N B! !

@ 290k 174 dBm 10 log

dBmN B!

Thermal noise


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2

224

4

N

N

N

V

V N KTB V RKTB

R R

! ! ! p !

2V

P VIR

! !

V IR!

The noise voltage produced by a resistorR over a bandwidthB can be calculated

Thermal noise voltage


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Thermal noise voltage

Thermal noise design considerations; Since

thermal noise is proportional to resistance,temperature, and bandwidth, receiver designsthat reduce these values will have superiorperformance.

23

4

where rms noise voltage

oltzman's constant (1.38 10 J/K)

temperature, K ( C 27

3)bandwidth, Hz

resistance,

n

n

v kTBR

v

k

T

B

R

!

!

! v

! r !

! ;


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Example Problem 1

The bandwidth of a receiver with a 75-; input resistance is 6

MHz. If the temperature is 29C, what is the input thermal noise

voltage?


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Semiconductor noise

The other major category of internal noise

originates from semiconductor devices such as

diodes and transistors.

Semiconductor noise is comprised of

Shot noise

Transit-time noise

Flicker noise


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Shot noise

Shot noise, the largest contributor to transistor

noise, is due to the random paths of the

current carriers flowing in semiconductors.

circuit symbol for a diode

diode


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Transit-time noise

Transit-time noise occurs at high frequencies

when transit time of charge carriers crossing

the semiconductors junction approaches the

signals period.

This type noise increases rapidly when

operating above the devices high-frequency

cutoff. Time required to crosspn-junction close to period (T)

of the signal.


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Flicker noise

Flicker noise results from minute variations in

resistance in semiconductor material.

Flicker noise is inversely proportional to

frequency and sometimes referred to as 1/fnoise or pink noise.

Flicker noise voltage as a function of time


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Flicker noise

Flicker noise is also found in resistors and

conductors.

Typeofresistor Noise voltagerange(QV)

Carbon-composition 0.1-3.0

Carbon-film 0.05-0.3

Metal-film 0.02-0.2

Wire-wound 0.01-0.2

Flicker noise for various types of resistors

metal-film

carbon-

composition wire-wound


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Semiconductor noise

Total noise voltage of semiconductor devices

varies with frequency with different types of

noise predominating in different regions.


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Measuring noise

To quantify the effect of noise on a signal, we

use the signal-to-noise (S/N) ratio or SNR.

A strong signal and weak noise results in a high

S/N ratio.

A weak signal and strong noise results in a low S/N

ratio.

signal without noise 20 d S/N ratio 10 d S/N ratio


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Signal-to-noise ratio

Signal to noise ratio can be expressed in terms

of voltage or power. signal voltage (rms)noise voltage (rms)

or here signal po er (W)

noise po er (W)

s

ns s

sn n

n

V

VS V S P

PN V N P

P

! !

! ! ! !

+

signal

(Vs orPs)

noise

(Vn

orPn)

signal plus noise


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Signal-to-noise (decibels)

Signal to noise ratio is most commonly

reported in decibels

/ ratio (d ) using voltage: d 20log

/ ratio (d ) using power: d 10log

s

n

s

n

V

S NV

PS N

P

!

!


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Noise ratio (NR)

We often need to quantify how much noise a

device adds to a signal as it passes through

the device.

One measure is the noise ratio (NR) which is

simply the ratio of input S/N to output S/N.

amplifier

output signalinput signal

/ inputNR

/ output

S N

S N!


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Noise figure (NF)

When the noise ratio (NR) is expressed in

decibels, its called the noise figure (NF).

Since the output S/N ratio will be less than theinput, the NR > 1 and NF > 0.

For an ideal device, NR = 1.0 and NF = 0 dB.

In practice, NF less than 2 dB is excellent.

amplifier


10log [dB]!


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Noise Ratio (NR) and Noise Figure (NF)

NF = 10 log (NR)

NF (dB) = (SNR)input (dB) (SNR)output (dB)

output

input

SNR

SNRNR !


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Example Problem 2

A transistor amplifier has a measured S/N power ratio of

10,000 at its input and 5,624 at its output.

(a) Calculate the NR.

(b) Calculate the NF.

amplifier



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Noise in cascaded stages

The total noise performance of a cascade ofamplifiers depends upon the noise ratio andpower gain of each stage.

The total noise performance of multiple stagesis given by Friis formula

whereAi the power gain andNR

inoise ratio of the i th stage.

signal input output

A1, NR1 A2, NR2 A3, NR3

2 31

1 1 2 1 2 1

NR 1 NR 1 NR 1NR NR n

n A A A A A A

! L

L


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Example Problem 3

The gain of the three stages of an amplifier are 8.45 dB, 10.79

dB, and 20 dB. The noise figures associated with these stages

are 2.04 dB, 3.0 dB, and 9.29 dB. What is the overall NR and NF

for this cascade of amplifiers?

signal input output

A1 =8.45 d

NF1 =2.04 d

A2 =10.79 d

NF2 =3.0 d

A3 =20.0 d

NF3 =9.29 d

NF


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Implications of Friis formula

The total noise performance of a receiver is

invariably determined by the very first stage.

Beyond the first and second stage, noise is nolonger a problem.

signal input output

A1, NR1A2, NR2 A3, NR3

2 31

1 1 2 1 2 1

NR 1 NR 1 NR 1NR NR n

n A A A A A A

! L

L


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Noise Temperature

N

N KTB T KB

! p !

T= environmental temperature (290 Kelvin)

N= noise power (watts)

K= Boltzmanns constant (1.38 10-23 J/K)

B = total noise factor (hertz)

1

e

T T F!

Te= equivalent noise temperature

T= environmental temperature (290 Kelvin)

F = noise factor (dimensionless)

1e

TF

T!

1F u


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Hartley-Shannon Theorem:

Significance of SNR

Hartley-Shannon Theorem (also called

Shannons Limit) states that the maximumdata rate for a communications channel is

determined by a channels bandwidth and

SNR.

A SNR of zero dB means that noise power

equals the signal power.


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Noise Effects on Communications

Data

May be satisfactory in the presence of whitenoise but impulse noise will destroy a data signal

BER (Bit Error Rate) is used as a performancemeasure in digital systems

Voice

White noise (continuous disturbance) can bebothersome to humans but impulse noise can beacceptable for speech communications

SNR (Signal-to-Noise Ratio) is used as aperformance measure in analog systems

noise sources and effects

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