intro to electronic comm part2

Chapter 1

Introduction to Electronic

Communication Part II

Chapter 1 : Introduction to

Communication Systems

BEKC 3633 Communication Systems

Faculty of Electrical Engineering 1

Chapter 1 : Introduction to Communication Systems



1.7 Noise Representation, types & source

Definition any undesirable electrical energy that falls within the passband of the signal.

Effect of noise on the electrical signal :

2 general categories of noise :

Correlated noise noise that exists only when a signal is present.

Uncorrelated noise noise that presents all the time whether there is a signal or not




1.7.1 Uncorrelated noise

2 general categories of uncorrelated noise : external noise and internal noise 1. External noise noise that generated outside the device or circuit.

Atmospheric noise

- naturally occurring electrical disturbances that originate within earths atmosphere such as lightning.

- also known as static electricity.

Extraterrestrial noise

- consists of electrical signal that originate from outside earths atmosphere and therefore also known as deep-space noise.

- 2 categories of extraterrestrial noise.

i solar noise noise that generated directly from the suns heat.

ii cosmic noise / black-body noise noise that is distributed throughout the galaxies.

Man-made noise

- noise that is produced by mankind.

- source : spark-producing mechanism (commutators in electrical motors, automobile ignition

systems, ac power generating/switching equipment, fluorescent lights).





2. Internal noise noise that generated within the device or circuit. Three primary kinds of internal noise:

Shot noise

- caused by the random arrival of carriers (holes and electrons) at the output element of an electronic device.

- shot noise is randomly varying and is superimposed onto any signal present.

Transit-time noise

- irregular, random variation due to any modification to a stream of carriers as they pass from the input to the output of a device.

- this noise become noticeable when the time delay takes for a carrier to propagate through a device is excessive.

Thermal / Random noise

- it is associated with the rapid movement of electron within a conductor due to thermal disturbance

- thermal noise is present in all electronic components and communication systems.

- It is also known as white noise which uniformly distributed across the entire electromagnetic frequency spectrum





Thermal / random noise

- associated with the rapid and random movement of electrons within a conductor due to thermal agitation (disturbance)

- also known as Brownian noise, Johnson noise and white noise.

- uniformly distributed across the entire electromagnetic spectrum.

- a form of additive noise, meaning that it cannot be eliminated, and it increase in intensity with the number of devices and with circuit length.

- the most significant of all noise sources

- thermal noise power can be defined as follow :

(2.1)

where N : noise power (watts)

B : bandwidth (hertz)

T : absolute temperature (kelvin) .......... T = C + 273

K : Boltzmanns proportionality constant (1.38 x 10-23 joules per kelvin)

KTBN

Example 1.10

Find the noise power in dBm at temperature of 17oC.

The bandwidth is 1 Hz for this temperature.




Solution:

i. Convert the temperature unit to Kelvin

T = C + 273 = 17oC + 273 = 290oK

ii. Noise power in dBm

N(dBm) = 10 log10 (KTB /0.001)

= 10 log10 (KT/0.001) + 10 log10 B

= 10 log10 (1.38 x 10-23 * 290/0.001) + 10 log10 1

= - 174 dBm








Thermal / random noise - equivalent circuit for a thermal noise source when the internal resistance of the source R1 is

in series with the rms noise voltage VN

- for a worst case and maximum transfer of noise power, the load resistance R is made equal to the internal resistance. Thus the noise power developed across the load resistor :

(2.2)

thus rms noise voltage can be define as

(2.3)

R

V

R

VKTBN

NN

4

2/ 22

RKTBVN 4

Example 1-11

For an electronic device operating at a temperature of

17oC with a bandwidth of 10 kHz, determine:

i. Thermal noise power in watts and dBm

ii. rms noise voltage for a 100 internal resistance and a 100 load

resistance.




Solution: 1-11 (continue)

i. T = C + 273 = 17oC + 273 = 290oK

N = KTB = (1.38 x 10-23 x 290 x 1 x 104) = 4 x 10-17 W

N(dBm) = 10 log10 (4 x 10-17 / 0.001) = -134 dBm

ii. rms noise voltage for a 100 internal resistance and a 100

load resistance.

VN = 4RKTB

= (4 x 100 x (4 x 10-17 )

= 0.1265 V







1.7.2 Correlated noise

a form of internal noise that is correlated(mutually related) to the signal and

cannot be present in a circuit unless there is a signal.

produced by a nonlinear amplification resulting in nonlinear distortion.

there are 2 types of nonlinear distortion that create unwanted frequencies that

interfere with the signal and degrade the performance :

1. Harmonic distortion

occurs when unwanted harmonics of a signal are produced through nonlinear

amplification.

harmonics are integer multiples of the original signal. The original signal is the

first harmonic (fundamental harmonic), a frequency two times the fundamental

frequency is the second harmonic, three times is the third harmonic and so on.

Distortion measurements :





1. Harmonic distortion

distortion measurements :

- Nth harmonic distortion = ratio of the rms amplitude of Nth harmonic to the rms amplitude

of the fundamental.

- Total Harmonic Distortion (THD)

(2.4)

where

all in rms value.

100% lfundamenta

higher

v

vTHD

2 2 4 22 43 ....higher nv v v v v

Example 1-12

Determine

i. 2nd, 3rd and 12th harmonics for 1 kHz repetitive

wave

ii. Percent 2nd order, 3rd order and total harmonic of

distortion for a fundamental frequency with an

amplitude of 8 Vrms, a 2nd harmonic amplitude of

0.2 Vrms, and a 3rd harmonic amplitude of 0.1

Vrms.




Solution:

i. 2nd, 3rd and 12th harmonics for 1 kHz repetitive wave

2nd harmonic = 2 x fundamental = 2 x 1 kHz = 2 kHz

3rd harmonic = 3 x fundamental = 3 x 1 kHz = 3 kHz

12th harmonic = 12 x fundamental = 12 x 1 kHz = 12 kHz

ii. Percent 2nd order, 3rd order and total harmonic of distortion for a

fundamental frequency

a. % 2nd order = V2/V1 x 100 = 0.2/8 x 100 = 2.5%

b. % 3rd order = V2/V1 x 100 = 0.1/8 x 100 = 1.25%

c. % THD = (((0.2)2 + (0.1)2)1/2 / 8) x 100% = 2.795 %

Chapter 1 : Introduction to Communication Systems BEKC 3633 Communication Systems






2. Intermodulation distortion

Intermodulation distortion is the generation of unwanted sum and difference

frequencies produced when two or more signals mix in a nonlinear device (cross

products).

Mathematically, the sum and difference frequencies are:

Cross products = mf1 nf2 (2.5)

where f1 and f2 are fundamental frequencies, where f1 > f2, and m and n are

positive integers between one and infinity.

Example 1-13

For a non-linear amplifier with two input frequencies,

3kHz and 8kHz, determine

i. 1st three harmonics present in the output for

each input frequency.

ii. Cross product frequencies produced for values

of m and n of 1 and 2.




Solution: i. The first three harmonics include the two original frequencies 3kHz

and 8kHz:

- two times each of original frequencies

2 * 3kHz = 6kHz; 2 * 8kHz = 16kHz

- three times each of original frequencies

3 * 3kHz = 9kHz; 3 * 8kHz = 24kHz

ii. The cross products for values of m and n of 1 and 2 are determined

by equation (2.5)




m n

1 1 8kHz 3kHz = 5kHz and 11kHz

1 2 8kHz 6kHz = 2 kHz and 14kHz






1.7.3 Other type of noise

1. Impulse noise

characterized by high amplitude peaks of short duration (sudden burst of irregularly

shaped pulses) in the total noise spectrum.

common source of impulse noise : transient produced from electromechanical

switches (relays and solenoids), electric motors, appliances, electric lights, power

lines, poor-quality solder joints and lightning.

2. Interference

electrical interference occurs when information signals from one source produces

frequencies that fall outside their allocated bandwidth and interfere with information

signal from another source.

most occurs in the radio frequency spectrum.




1.8 Noise Parameters

1.8.1 Signal-to-noise Power Ratio

signal-to-noise power ratio (S/N) is the ratio of the signal power level to the noise power level and can be expressed as

(2.6)

in logarithmic function

(2.7)

in terms of voltages and resistance

(2.8)

in the case Rin = Rout, (2.8) can be reduced to

(2.9)

n

s

P

P

N

S

n

s

P

PdB

N

Slog10)(

outn

ins

RV

RVdB

N

S

/

/log10)(

2

2

n

s

V

VdB

N

Slog20)(




1.9 Examples

Ex 3 : For an amplifier with an output signal power of 10 W and output noise

power of 0.01 W, determine the signal-to-noise power ratio and dB.

Solution




100001.0

10

n

s

P

P

N

S

1. Using equation 2.6

2. Convert to dB, using equation 2.7

dBdBN

S301000log10)( 10




1.9 Examples

For an amplifier with an output signal voltage of 4V, an output noise voltage

0.005 V and an input and output resistance of 50 , determine the signal-to-noise

power ratio.

Solution




1. Using equation 2.8

dBV

VdB

N

S

n

s06.58

005.0

4log20log20)( 1010




1.8.2 Noise Factor and Noise Figure

Noise factor is the ratio of input signal-to-noise ratio to output signal-to-noise ratio

(2.10)

Noise figure is the noise factor stated in dB and is a parameter to indicate the

quality of a receiver

(2.11)

Noise Figure in Ideal and Non-ideal Amplifiers

- an electronic circuit amplifies signal and noise within its passband equally well

- in the case of ideal/noiseless amplifier, the input signal and the noise are

amplified equally.

- meaning that, signal-to-noise ratio at input = signal-to-noise ratio at output

out

in

NS

NSF

)/(

)/(

out

in

NS

NSFNF

)/(

)/(log10log10





Noise Figure in Ideal and Non-ideal Amplifiers (continue) - in reality, amplifiers are not ideal, adds internally generated noise to the

waveform, reducing the overall signal-to-noise ratio.

- in figure (a), the input and output S/N ratios are equal.

i

i

iP

iP

out

out

N

S

NA

SA

N

S

(2.12)





Noise Figure in Ideal and Non-ideal Amplifiers (continue) - in figure (b), the circuits add internally generated noise Nd to the waveform,

causing the output signal-to-noise ratio to be less than the input signal-to-noise

ratio.

)/( Pdi

i

diP

iP

out

out

ANN

S

NNA

SA

N

S

(2.13)




1.9 Examples

Ex 5 : For a non-ideal amplifier with a following parameters, determine

a. input S/N ratio (dB)

b. output S/N ratio (dB)

c. noise factor and noise figure

Input signal power = 2 x 10-10 W

Input noise power = 2 x 10-18 W

Power gain = 1000000

Internal noise Nd = 6 x 10-12 W

Solution:

a) Using equation 6.5

Change to dB

a) Using equation 6.12




8

18

10

10x1000,000,10010x2

10x2

in

in

N

S

000,000,2510x8

10x200

10x6)10x2(1000000

)10x2(100000012

6

1218

10

diP

iP

out

out

NNA

SA

N

S

dBN

S

dBin

in 8010x2

10x2log10

18

10

10

Solution:

Change to dB




dBN

S

dBout

out 74000,000,25log10 10

c. Noise Factor:

dBNFeNoiseFigur

N

S

N

SFFactorNoise

out

out

in

in

64log10

4000,000,25

000,000,100/

10





Noise Figure in Cascaded Amplifier - when two or more amplifiers are cascaded as shown in the following figure,

the total noise factor is the accumulation of the individual noise factors.

- Friss formula is used to calculate the total noise factor of several cascade

amplifiers

(2.14)

N

NT

AAA

F

AA

F

A

FFF

...

1...

11

2121

3

1

21





Noise Figure in Cascaded Amplifier (continue) - the Total Noise Figure

(2.15)

When using Friss formula, the noise figures must

be converted to noise factors !!!

TT FNF log10




1.9 Examples

Ex 6 : For 3 cascaded amplifier stages, each with a noise figures of 3 dB and

power gain of 10dB, determine the total noise factor and noise figure.

Solution




21

3

1

2

1

11

AA

F

A

FFFT

11.2100

12

10

122

Total Noise Figure , NFT = 10 log10 (2.11) =3.24 dB

intro to electronic comm part2

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