Communication Engineering - Chapter 6 - Noise

CHAPTER 6CHAPTER 6CHAPTER 6CHAPTER 6

NOISENOISE

INTRODUCTION

Noise is random energy that interfere with the information signal.

Noise may be defined as any unwanted introduction of energy tending to interfere with the proper reception and reproduction of transmitted signal.

In radio receiver, noise may produce hiss in the loudspeaker output.

Noise can limit the range of systems. It affects the sensitivity of the receiver.

NOISE

Electrical noise – any undesirable that falls within the passband of the signal.

Figure 4 show the effect of noise on electrical noise.

2 general categories

• Correlated noise – implies relationship between the signal and the noise, exist only when signal is present.

• Uncorrelated noise – present at all time, whether there is signal or not.

Classification of Uncorrelated Noise

NOISE

EXTERNAL INTERNAL

ATMOSPHERIC NOISE

EXTRATERRESTRIALNOISE

INDUSTRIAL NOISE

THERMALNOISE

SHOTNOISE

Figure 6.1

ATMOSPHERIC NOISE Caused by lightning discharges in thunderstorms and

other natural electric disturbances occurring in the atmosphere.

Consist of spurious radio signal with components distributed over a wide range of frequencies.

It propagates over the earth in the same way as ordinary radio waves of the same frequencies.

Become less severe at frequencies above 30MHz because:• The higher frequencies are limited to line-of-sight

propagation.• Nature of the mechanism generating this noise is

such that very little of it is created in the VHF range and above.

EXTRATERRESTRIAL NOISE SOLAR NOISE:

• Normal condition, there is a constant noise radiation from the sun, simply because large body at a very high frequency.

• Radiates over a very broad frequency spectrum. COSMIC NOISE:

• Stars radiate RF noise in the same manner of sun.• The noise received is called thermal noise and

distributed fairly uniformly over the entire sky.

INDUSTRIAL NOISEBetween 1 to 600 MHz, the intensity

noise made by humans easily outstrips that created by any other source to the receiver.

Sources such as: automobile, aircraft, electric motors and other heavy machine.

The nature of industrial noise is so variable that it is difficult to analyze.

SHOT NOISECaused by the random arrival of

carriers at the output element of an electronic device.

First observed in the anode current of a vacuum-tube amplifier.

The current carriers are not moving in continuous steady flow.

Randomly varying and superimposed onto any signal present.

Sometimes called transistor noise.

THERMAL NOISE Is associated with the rapid and random

movement of electrons within a conductor due to thermal agitation.

Present in all electronic component and communications systems.

Referred as white noise. Is a form of additive noise, cannot be

eliminated. It increases in intensity with the

number of devices in a circuit.

Thermal noise power is proportional to the product of bandwidth and temperature.

Mathematically, noise power is

N=KTB

N = noise power, K=Boltzmann’s constant (1.38x10-23 J/K) B = bandwidth, T = absolute temperature (Kelvin)(17oC or 290K)

NOISE VOLTAGE

4NV RkTB

VN/2

VN/2VN R

RI

Noise Source• Figure 4.2 shows the equivalent circuit for a thermal noise source.

• Internal resistance RI in series with the rms noise voltage VN.

• For the worst condition, the load resistance R = RI , noise voltage dropped across R = half the noise source (VR=VN/2) and

• From the final equation The noise power PN , developed across the load resistor = KTB

The mathematical expression :

RKTBV

RKTBV

R

V

R

VKTBN

N

N

NN

4

4

4

2/

2

22

Figure 6.2 : Noise source equivalent circuit

Example 1Convert the following temperatures to kelvin:a) 100°Cb) 0°Cc) -10°C

T=a°C+273°C

Example 2

Calculate the thermal noise power available from any resistor at room temperature (290K) for a bandwidth of 1 MHz. Calculate also the corresponding noise voltage, given that R = 50.

Example 3

For an electronic device operating at a temperature of 17oC with a bandwidth of 10 kHz, determine a)Thermal noise power in watts and

dBmb)rms noise noise voltage for a 100

internal resistance and 100 load resistance.

Example 4

Two resistor of 20k and 50 k are at room temperature (290K). For a bandwidth of 100kHz, calculate the thermal noise voltage generated by1. each resistor2. the two resistor in series3. the two resistor in parallel

Correlated NoiseForm of internal noise that is correlated

to the signal and cannot be present in a circuit unless there is a signal.

Produced by nonlinear amplification.All circuits are nonlinear therefore, they

all produce nonlinear distortion.Nonlinear distortion creates unwanted

frequencies that interfere with the signal and degrade performance.

Intermodulation DistortionGeneration of unwanted sum and

difference frequencies produced when two or more signals mix in a nonlinear device.

The sum and difference frequencies are called cross products.

Unwanted cross products can interfere with the information signal.

Cross products are produced when harmonics as well as fundamental frequency mix in a nonlinear device.

Cont.. Cross products = mf1±nf2.F1 and f2 are fundamental

frequency.F1>f2M and n are positive integer.

Correlated Noise-Intermodulation Distortion

f1 f2

V1 V2

f1 f2f1-f2 f1+f2

V1 V2

Vdifference Vsum

Input frequency spectrum Output frequency spectrum

Figure 6.4

Example 6For a nonlinear amplifier with 2

input frequencies, 3kHz and 8kHz, determine:

a) First 3 harmonics present in the output for each input frequency.

b) Cross-product frequencies produced for values of m and n of 1 and 2.

Interference Form of external noise. Means to disturb or detract from. Electrical interference is when

information signals from one source produce frequencies that fall outside their allocated bandwidth and interfere with information signals form another source.

Most interference occur when harmonics frequencies from one source fall into the passband of a neighboring channel.

Review NotesGainAttenuation

• Both has the ratio output to the input.

in

outV V

V

input

outputA

Figure 6.5

Gain Ratio output to the input. Output has greater amplitude than the input

Most amplifiers are power amplifier, the same procedure can be used to calculate power gain, Ap.

Ap = Pout/Pin

Figure 6.6

AttenuationRefers to loss introduced

by a circuit.Output is less than input.

For cascade circuit, total attenuation is, AT=A1 x A2 x A3 …..

Voltage divider network may introduce attenuation.

in

out

V

VAnAttenuatio

Figure 4.7 Voltage divider introduces attenuation

Attenuation can be offset by introducing gain.

Figure 6.8 Total attenuation in cascaded network

Figure 6.9 Gain offsets the attenuation

Figure 6.10 Total gain is the product of the individual stage gains and attenuation

Example 7What is the gain of an amplifier that produces an output of 750 mV for 30 V input?

Example 8The power output of an amplifier is 6 W. The power gain is 80. What is the input power?

Example 9Three cascade amplifier have power gains of 5,2, and 17. The input power is 40 mW. What is the output power?

Signal to Noise Ratio (SNR)

Ratio of the signal power level to the noise power level.

Express in logarithmic function:

n

s

P

PSNR

n

s

P

PdBSNR log10)(

Example 101 For an amplifier with an output signal

power of 10W and an output noise power of 0.01W, determine the SNR.

2 For an amplifier with an output signal voltage of 4V, an output noise voltage of 0.005V and an input and output resistance of 50Ω, determine the SNR.

Noise Factor (F) and Noise Figure (NF)

Figures of merit used to indicate how much the SNR deteriorates as a signal passes through a circuit.

Noise factor is simply a ratio of input SNR to output SNR.

SNRoutput

SNRinputF

Cont..

NF is noise factor stated in dB.Used to indicate the quality of a receiver.

SNRoutput

SNRinputdBNF log10)(

FdBNF log10)(

Ideal Noiseless Amplifier

i

i

N

S

inpowerNoise

inpowerSignal

Ideal Noiseless AmplifierAp=power gain

i

i

ip

ip

N

S

NA

SA

inpowerSignal

outpowerSignal

Figure 6.11

Non ideal amplifier

i

i

N

S

inpowerNoise

inpowerSignal

Nonideal amplifierAp=power gain

Nd=internally generated noise

p

di

i

dip

ip

ANN

S

NNA

SA

inpowerSignal

outpowerSignal

Figure 6.12

Example 11For a nonlinear amplifier and the

following parameter, determine:a) Input SNR(dB)b) Output SNR(dB)c) Noise Factor and Noise Figure

Input signal power=2x10-10WInput Noise power=2x10-18WPower gain=1,000,000Internal noise (Nd)=6x10-12W

Noise Figure of Cascaded Amplifier

i

i

N

S

Ti

i

o

o NFN

S

N

S

Ap1NF1

Ap2NF2

Ap3NF3

Input Output

i

i

N

S

Figure 6.13

Cont..

Total noise factor is the accumulation of the individual noise factor.

Friiss’s formula is used to calculate the total noise factor of several cascaded amplifiers.

n

nT AAA

F

AA

F

A

FFF

2121

3

1

21

111

Example 12For 3 cascaded amplifier stages,

each with noise figure of 3 dB and power gain of 10 dB, determine the total noise figure.

Equivalent Noise Temperature (Te)

Hypothetical value that cannot be directly measured.

To indicates the reduction in the SNR a signal undergoes as it propagates through a receiver.

The lower Te is the better quality of a receiver. 1 FTTe

T

TF e1

Example 13

Determine:a) Noise Figure for an equivalent

noise temperature of 75K.b) Equivalent noise temperature for

a noise figure of 6dB.

Example 14

A voltage divider shown in Figure 6.9 has values of R1 = 10k and R2 = 47k.

1.What is the attenuation?2.What amplifier gain would you need

to offset the loss for an overall gain of 1?

Example 15

An amplifier has gain of 45,000, which is too much for the amplification. With an input voltage of 20 V, what attenuation factor is needed to keep the output voltage from exceeding 100mV?. Let A1= amplifier gain = 45,000; A2 = attenuation factor; AT = total gain.

Example 16A RF sine wave generator whose output impedance is 50 is connected to a 50 load using 50 coaxial cable. The generator’s output amplitude level is set to + 3 dBm. An rms voltmeter is used to measure the effective voltage, and an oscilloscope is used to display the sine wave. Compute the following:

1. The rms voltage measure by the rms voltmeter2. The peak voltage, Vp of the sine wave that should be

displayed on the oscilloscope.3. The peak-to-peak voltage, Vp-p of the sine wave that

should be displayed on the oscilloscope

Example 17

The input signal to a telecommunications receiver consists of 100W of signal power and 1W of noise power. The receiver contributes an additional 80W of noise, ND, and has a power gain of 20 dB. Compute the input SNR, the output SNR and the receiver’s noise figure.