COMMUNICATION SYSTEM EECB353Chapter 4

NOISE ANALYSIS

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Noise Analysis

Noise is any undesired signal that ultimately appears in the output of a communications system.

Electrical noise is defined as any undesirable electrical energy that falls within the passband of the signal.

Electrical noise may be said to be the introduction of any unwanted energy, which tend to interfere with the proper reception and reproduction of transmitted signals.

2 types of noise: Correlated – exist when a signal is present. Uncorrelated – exist regardless of whether there is a

signal present or not.

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Uncorrelated Noise

Subdivided into 2 categories: External Noise

Present in a received radio signal that has been introduced in the transmitting medium.

Source - atmospheric, extraterrestrial and man-made

Internal Noise Introduced by the receiver itself. Electrical interference generated within a device

i.e create from the communication equipment. Type – shot, transit time and thermal.

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External Noise1. Atmospheric Noise

Caused by naturally occurring disturbances in the earth’s atmosphere, with lighting discharges being the most prominent contributors.

It is often in the form of impulse that spread energy throughout a wide range of freq.

2. Extraterrestrial Noise (Space Noise) Originates from outside earth’s atmosphere (outer space), also call

deep-space noise. Sub-divided into 2 categories:

Solar noise – generated from the sun’s heat. The sun radiates a broad spectrum of freq, including those which are used for broadcasting.

Cosmic noise – originating from stars other than the sun.

3. Human-made Noise Produce by mankind. Generated by equipment that produces

sparks. Eg. Automobile engine, switching equipment, fluorescent light.

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External Noise

4. Impulse Noise (spikes) – characterized by high amplitude peaks of short duration in the total noise spectrum. Consists of sudden burst of irregularly shaped pulses that

generally last between a few milliseconds. Some of the sources of impulse noise are voltage changes in

adjacent lines, lightning flashes during thunderstorms and fluorescent lights.

5. Interference – form of external noise. Electrical interference occurs when info signals from one source

produce freqs outside their allocated BW and interfere with other info signal.

Most interference occurs when harmonics or cross-product freq from one source fall into the passband of a neighboring channel.

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Internal Noise

1. Shot Noise Produced in active devices such as transistors. Caused by a random arrival of carriers (holes & electrons) in the

pn junctions of semiconductor. The carrier is not moving in continuous and steady flow i.e it

moves in a random path of motion.

2. Transit-time Noise Noise produced in semiconductors when the transit time of the

carriers crossing a junction is close to the signal's period and some of the carriers diffuse back to the source or emitter of the semiconductor.

i.e Due to any modification to a stream of carriers as they pass form input to the output of a device (from emitter to collector).

Time taken for the carrier to propagate through a device produces irregular and random variation of noise.

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Internal Noise

3. Thermal Noise (White Noise or Johnson Noise) Generated by the agitation and interaction of electrons in a

conductor due to heat. Thermal Noise Power, N = KTB

where N = noise power (W)K = Boltzmann’s contant (1.38 x 10-23 Joules/Kelvin)T = absolute temperature (Kelvin), and T= C + 273B = bandwidth (Hz)

Thermal Noise Power in dBm, Thermal Noise – dependent on temperature. White Noise – another name for thermal noise because its frequency

content is uniform across spectrum. Johnson Noise – another name for thermal noise, first studied by J.B.

Johnson.

dBmKTB

NdBm 001.0log10

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Internal Noise

For worst case and max noise power transfer,RI = R. Thus, VR = VN/2 = VL

The rms noise voltage, VN that appears across a resistor at temperature T is:

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Example 1

For an electronic device operating at a temperature of 27C over a 1 MHz frequency range, determine

a. thermal noise power in watts and dBmb. rms noise voltage for a 100 resistance

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Example 2

The noise produced by a resistor is to be amplified by a

noiseless amplifier having a voltage gain of 75 and a

bandwidth of 100 kHz. A sensitive meter at the output

reads 240 µV rms. Assuming operation at 37°C,

calculate the resistor’s resistance. If the bandwidth were

cut to 25 kHz, determine the expected output meter

reading.

Solution

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Correlated Noise

It is a form of internal noise that is correlated to the signal and cannot be present in a circuit unless there is a signal i.e – NO SIGNAL, NO NOISE!

Produced by nonlinear amplification and includes harmonic and intermodulation distortion, both of which are forms of nonlinear distortion.

Nonlinear distortion creates unwanted frequencies that interfere with the signal and degrade performance

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Correlated Noise Harmonic distortion (Amplitude Distortion) occurs when

unwanted harmonics of a signal are produced through nonlinear amplification (nonlinear mixing). Harmonics are integer multiples of the original signal. The original signal = first harmonic = fundamental

frequency. 2 x the original signal freq is called the second harmonic, ..

n x original signal freq = nth harmonic.

Figure: Correlated Noise (Harmonic Distortion)Note – from Figure, the output spectrum contains the original input freq plus several

harmonics (2f1, 3f1, 4f1) that were not part of the original signal.

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Correlated Noise

Total Harmonic Distortion, THD is the ratio of the quadratic sum of the rms values of all the higher harmonics to the rms value of the fundamental.

%THD = 100lfundamenta

higher

v

v

224

23

22 ... nvvvv wher

e vhigher =

vfundamental = rms voltage of fundamental freq

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Example 3 – Determine

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

b. Percent second-order, third-order and total harmonic distortion for a fundamental frequency with an amplitude of 8Vrms, a second harmonic amplitude of 0.2Vrms, and a third harmonic amplitude of 0.1Vrms.

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Correlated Noise Intermodulation distortion is the generation of unwanted sum and difference

frequencies produced when two or more signals mix in a nonlinear device. The sum and difference freq are called cross product i.e mathematically

Cross product = where f1, f2 = fundamental frequencies, f1 > f2

m,n = positive integers Unwanted cross-product freq can interfere with the info signals in a cct or with the

info signal in other cct. 21 nfmf

Figure: Correlated Noise (Intermodulation Distortion)

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Example 4 - For a non linear amplifier with two input frequencies, 3 kHz and 8 kHz, determinea. First three harmonics present in the output for

each input frequency.b. Cross-product frequencies produced for values

of m and n of 1 and 2.

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INTERFERENCE Form of external noise “to disturb or to detract from” Electrical interference : When

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

Most interference occurs when harmonics or cross product frequencies from 1 source fall into the passband of a neighboring channel

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Noise Summary

NOISE

CORRELATED (Internal)

UNCORRELATED

Nonlinear distortion o Harmonic o Intermodulation

External o Atmospheric o Extraterrestrial

Solar Cosmic

o Man-made o Impulse o Interference

Internal o Thermal o Shot o Transient-time