CHAPTER 4

CHAPTER 4NOISE IN COMMUNICATION SYSTEMS

By Prof. Shrikant Ganorkar SRCOE, Pune

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CONTENTSIntroductionSources Of NoiseTypes of NoiseWhite NoiseThermal NoiseShot NoisePartition NoiseLow Frequency or Flicker NoiseBurst or Popcorn NoiseAvalanche NoiseSignal - to - Noise RatioSNR Of Tandem Connection Noise Factor Noise FigureNoise TemperatureFrisss Formula For Noise FigureNoise Bandwidth

LIST OF BOOKSText Books :

T1. B. P. Lathi , Modern Digital and Analog. Communication Systems, 3rd Edition, Oxford University PressT2. Dennis Roddy & John Coolen, Electronic Communications, 4th Edition, Prentice Hall

Reference Books :

R1. Simon Haykin, Communication Systems, 4th Edition, John Wiley & SonsR2. Taub & Schilling, Principles of Communication Systems, Tata McGraw-HillR3. George Kennedy, Electronic Communication Systems 5th Edition, McGraw-HillR4. Frenzel, Principles of Electronic Communication Systems3rd Edition, Tata McGraw-Hill

Additional References:

Communication system :Analog & Digital by Singh, Sapre, TMH: 4.1 to 4.13

Lect.No.Unit No.Name of Topic ReferenceBit No. & page no.Remark1

4Sources of Noise, Types of Noise, White Noise,R2R3R17.1(315)2.1(15-20)1.9(61,62)Thermal noise, shot noise, partition noiseT24.2(119-129)4.3(130-131)4.4(131)Numericals are expected from topic Thermal Noise23Low frequency or flicker noise, burst noise, avalanche noise,T24.5,(131)4.6(132)4.7(132)4Signal to Noise Ratio ,SNR of tandem connection, T24.11(135-136)4.12(137)Numericals are expected from SNR, tandem connection and Noise Figure5Noise Figure, Noise TemperatureT24.13(137-141)4.14(144-145)6Friss formula for Noise Figure,Noise Bandwidth.T2R24.13(141)7.10(333)

INTRODUCTIONNoise is a general term which is used to describe an unwanted signal which affects a wanted signal.

These unwanted signals arise from a variety of sources which may be considered in one of two main categories:

Interference, usually from a human source (man made)Naturally occurring random noise

Interference Interference arises for example, from other communication systems (cross talk), 50 Hz supplies (hum) and harmonics, switched mode power supplies, thyristor circuits, ignition (car spark plugs) motors, etc. Natural Noise Naturally occurring external noise sources include atmosphere disturbance (e.g. electric storms, lighting, ionospheric effect etc), so called Sky Noise or Cosmic noise which includes noise from galaxy, solar noise and hot spot due to oxygen and water vapour resonance in the earths atmosphere.

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

Electrical noise any undesirable that falls within the pass-band of the signal.

Noise is random energy that interfere with the information signal.

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

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Noise can limit the range of systems.

Noise may sometimes even force a reduction in the bandwidth of system. It affects the sensitivity of the receiver.

2 general categoriesCorrelated 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.

SOURCES OF NOISENOISEEXTERNALINTERNALATMOSPHERIC NOISEEXTRATERRESTRIALNOISEINDUSTRIAL NOISETHERMALNOISESHOTNOISE

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EXTERNAL NOISEThe various forms of noise created outside the receiver.

2.1 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.

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2.2 EXTRATERRESTRIAL NOISESOLAR NOISE:Under normal condition, there is a constant noise radiation from the sun, simply because it is a large body at a very high temperature (over 6000 centigrade).Radiates over a very broad frequency spectrum which includes frequencies used for communications.

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.

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2.3 INDUSTRIAL NOISEBetween 1 to 600 MHz [ In urban, semi urban & other industrial areas], the intensity noise made by humans easily outstrips that created by any other source, internal or external to the receiver.

Sources such as: automobile, aircraft ignition, electric motors and other heavy machines, leakages from high voltage lines, florescent lights etc

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The nature of industrial noise is so variable that it is difficult to analyze it on any basis other than the statistical.

Industrial noise obeys the general principle that received noise increases as the receiver bandwidth is increased.

INTERNAL NOISEIt is the noise created by any of the active or passive devices in the receivers. Such noise is random, easy to observe & describe statistically. Random noise power is proportional to the bandwidth of measurement.

THERMAL NOISE

Is associated with the rapid and random movement of electrons within a conductor due to thermal agitation.Present in all electronic components and communications systems.Referred as white noise or Johnson Noise.Is a form of additive noise, cannot be eliminated.It increases in intensity with the number of devices in a circuit.This type of noise is generated by all resistances

Where k = Boltzmanns constant = 1.38 x 10-23 Joules per KT = absolute temperature B = bandwidth noise measured in (Hz)R = resistance (ohms)Experimental results (by Johnson) and theoretical studies (by Nyquist) give the mean square noise voltage as

The law relating noise power, N, to the temperature and bandwidth is, N = k TB watts

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Thermal noise is often referred to as white noise because it has a uniform spectral density.

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NOISE VOLTAGE

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 :

Figure 4.2 : Noise source equivalent circuit

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ExampleR.F. amplifier is saving an input resistor of 8Kohm and works in the frequency range of 12 to 15.5 MHz Calculate the rms noise voltage at the input to this amplifier at an ambient temperature of 17oC?Solution:

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Assume that R1 at temperature T1 and R2 at temperature T2, then i.e. The resistor in series at same temperature behave as a single resistor

Resistors in Series

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Resistance in Parallel

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Example Two resistor of 20k and 50 k are at room temperature (290K). For a bandwidth of 100kHz, calculate the thermal noise voltage generated by

each resistor the two resistor in series the two resistor in parallel

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WHITE NOISEThermal noise falls in to the category of power signals & hence it has a spectral density. The bandwidth B is assumed flat i.e. independent of frequency as shown in the figure. This thermal noise is referred to as white noise.

Noise is assumed to have a uniform noise power spectral density, given that the noise is not band limited by some filter bandwidth.

If we assume the bandwidth W is infinite (Idealization), then the autocorrelation of the noise is;

Where we use a subscript w to emphasize that the noise is white.Note that w(t) is uncorrelated with for any

The noise power in bandwidth W is;

SHOT NOISEShot noise is a random fluctuation that accompanies any direct current crossing a potential barrier.

Caused 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.

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The mean square noise component is proportional to the dc flowing & shot noise current is given by;

LOW FREQUENCY OR FLICKER NOISEActive devices, integrated circuit, diodes, transistors etc also exhibits a low frequency noise, which is frequency dependent (i.e. non uniform) known as flicker noise. The spectral density of the noise increases as the frequency decreases (1/f noise).

Flicker noise is due to impurities in the material which in turn cause charge carrier fluctuations.

In semiconductors Flicker noise arises due to the fluctuations in carrier densities.

BURST NOISE OR POPCORN NOISE

Low Frequency Noise observed in BJT is known as Burst Noise.Noise appears as a series of bursts at two or more levels.When present in an audio system , the noise produces popping sounds.Its Spectral density increases as frequency decreasesIt is also low frequency noise.

PARTITION NOISEIt occurs whenever current has to divide between 2 or more electrodes and results from the random fluctuations in the division.

The spectrum of Partition Noise is flat.

Diode is said to be less noisy than transistor for the same reason( third electrode makes more noisy).

AVALANCHE NOISEIt occurs due to avalanche i.e. ionizing where additional holes and electrons are produced , which in turn contribute to the ionization process.Large Noise spikes are present in the avalanche current.The spectral density of Avalanche Noise is flat. In Zener diodes avalanche noise is nuisance to be avoided.

SIGNAL TO NOISE RATIONoise is usually expressed as a power because the received signal is also expressed in terms of power.

Q. A receiver has an input signal power of l.2W. The noise power is 0.80W. The signal to noise ratio isSignal to Noise Ratio = 10 Log (1.2/0.8) = 10 log 1.5 = 10 (0.176) = 1.76 dB

SNR OF TANDEM CONNECTIONIf Power loss of a line is L then gain of amplifier is chosen such that LG=1

Power O/p at each repeater = Ps (as LG=1)But Total Noise is additive..

Q. Calculate the output signal-to-noise ratio in decibels for three identical links, given that the signal-to-noise ratio for any one link is 60 dB.

NOISE FACTOR- NOISE FIGUREConsider the network shown below,

The amount of noise added by the network is embodied in the Noise Factor F, which is defined by

Noise factor F =

37 F equals to 1 for noiseless network and in general F > 1. The noise figure in the noise factor quoted in dBi.e. Noise Figure F dB = 10 log10 FF 0 dB The noise figure / factor is the measure of how much a network degrades the (S/N)IN, the lower the value of F, the better the network.

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Q

Noise Factor of Amplifiers in Cascade/ Friiss Formula

Available noise power at input of amplifier 2

For additional stages of amplifiers equation can be written as:This is knows as Frisss Formula .

By making G1 large , contribution of other stages can be made negligible.

Q.

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Noise Temperature

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Frisss Formula can be expressed in terms of Noise temperature. Where, Te sys is overall noise temperature of the system and Te1, Te2 are noise temperature of individual stages:

Noise BandwidthSince any communication system has a front end filter. The noise bandwidth of interest is the bandwidth of this filter Receiver filter transfer function is H(f) . Ideally it should be rectangular as shown to transmit the same power as that of real filter. Noise bandwidth Bn is adjusted to get the same Bn= (/2) * fc Where fc= 3-db bandwidth

Thank You!!!

VN/2VN/2VNRRINoise Source