Noise

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Instructor: Sam Nanavaty

NOISE

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Noise is the UNDESIRABLE portion of an

electrical signal that interferes

with the intelligence

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Why is it important to study the effects of Noise?

a) Todays telecom networks handle enormous volume of datab) The switching equipment needs to handle high traffic volumes as

well

c) our ability to recover the required data without error is inversely

proportional to the magnitude of noise

What steps are taken to minimize the effects of noise?

a) Special encoding and decoding techniques used to optimize the

recovery of the signal

b) Transmission medium is chosen based on the bandwidth, end to

end reliability requirements, anticipated surrounding noise levelsand the distance to destination

c) Elaborate error detection and correction mechanisms utilized in the

communications systems

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The decibel(abbreviated dB) is the unit used to measure

the intensity of a sound.! The smallest audible sound

(near total silence) is 0 dB. A sound 10 times morepowerful is 10 dB. A sound 1,000 times more powerful

than near total silence is 30 dB.

Here are some common sounds and their decibel ratings:

Normal conversation - 60 dB

A rock concert - 120 dB

It takes approximate 4 hours of exposure to a 120-dBsound to cause damage to your ears, however 140-dB

sound can result in an immediate damage

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Relativepower gain of a device can be expressed as

Ap= Po/Pi (Power levels are expressed in Watts)

Relativepower gain of a device in decibels is

Ap(dB)= 10 Log Ap = 10 Log Po/Pi

Alternatively, the above equations can be represented as

Ap(dB) = 10 Log (Vo2/Ro)/(Vi

2/Ri)

If (Ro= Ri)

Av(dB) = 10 Log (Vo/Vi)2= 20 Log (Vo/Vi) = 20 Log Av

Poand Pican be substituted with Pfinand Pinitas in

Final and initial values of power source

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Ap is a relative power gain

Ap is not necessarily the power gain between output and input

Ap can be computed for comparing any two different power levels

e.g., You may be asked to compute a relative power gain ratio of an

amplifier which has been redesigned so that the maximum output power

has increased from .25W and 5W

Ap= 5/.25 = 20 and Ap(dB)= 10 x Log Ap= 10 x Log 20 = 13.01

If Ap(dB)= 20 dB and Po= 550mW, compute Pi

Ap = 10 Ap(dB)/10 = 100

Po/Pi = 100

Pi = 550/100 = 5.5 mW

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A preamp has a voltage gain of 28dB. Compute the following:

a) If Vi = 2 mV then Vo = ?

A v(dB)= 20 Log Av

Av = 10Av(dB)/20

Av = 25.11

a) If Vi increases from 2 to 5 mV, how many dB has the signal increased?

Av(dB) = 20 Log (Vfin/Vinit) = 20 Log (5/2) = 20 Log 2.5 = 7.95 dB

b) If Vi drops from 2 to 1 mV, how many dB has the signal dropped?

Av(dB) = 20 Log (Vfin/Vinit) = 20 Log (1/2) = 20 Log .5 = -6 dB

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The absolute power gain is defined as A unit of gain or loss expressed

as an absolute value based on 1 mW of standard reference

Ap(dBm) = 10 Log (P/ 1 mW)

This represents an absolute Power gain based on a standard input level of

1 mW in to 50, 600 or 900 depending on the impedance of theTransmission media.

P represents Power level which can then be computed as follows:

P = 1 mW (10 (Ap(dBm)/10))

In terms of voltage, the above equation can then be represented as

(Vrms

2/R) = 1 mW (10 (Ap(dBm)/10))

Vrms= 1 mW (10 (Ap(dBm)/10)) x R (where R = standardized valueobtained from the manufacturer)

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1) If Signal level of 30 MHz test tone measures -30dBm on a spectrum

analyzer, Compute the power level P of signal.

P = 1mW x 10 Ap(dBm)/10 = 1 mW x 10 -30dBm/10= 1 mW x 10 -3= 1 uW

2) An rf sinewave generator with o/p impedance of 50 is connected to 50

Load using a 50 coaxial cable. The generators output amplitude level is set to

-12 dBm. An rms voltmeter is used to measure the effective voltage and an

oscilloscope is used to display the sine wave. Compute the following:

a) rms voltage measured by rms voltmeter

b) Peak voltage Vp of sine wave that should be displayed on the oscilloscope

c) Peak-to-peak voltage of sinewave that should be displayed on the oscilloscope

a) Vrms = 1mW x 10Ap(dBm)x 50 = 56.17 mV

b) Vp = Vrms/.707 = 79.45 mV

c) Vp-p = 2x Vp = 158.9 mV

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Signal to Noise ratio: It is a ratio of signal power to

Noise power at some point in a Telecom system

expressed in decibels (dB)

It is typically measured at the receiving end of the

communications system BEFORE the detection of

signal.

SNR = 10 Log (Signal power/ Noise power) dB

SNR = 10 Log (Vs/VN)2 = 20 Log (Vs/VN)

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1) The noise power at the output of receivers IF stage ismeasured at 45 W. With receiver tuned to test signal, output

power increases to 3.58 mW. Compute the SNR

SNR = 10 Log (Signal power/ Noise power) dB

= 10 Log (3.58 mW/ 45 W) = 19 dB

2) A 1 kHZ test tone measured with an oscilloscope at the input of

receivers FM detector stage. Its peak to peak voltage is 3V. With test toneat transmitter turned off, the noise at same test point is measure with an

rms voltmeter. Its value is 640 mV. Compute SNR in dB.

SNR = 20 Log (Vs/Vn) = 20 Log ((.707 x Vp-p/2)/Vn)

= 20 Log (1.06V/640 mV)= 4.39 dB

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Noise Factor (F) : It is a measure of How

Noisy A Device Is

Noise figure (NF) = Noise factor expressed

in dB

F = (Si/Ni) / (So/No)

NF = 10 Log F

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An input signal of repeater is made of 150 W of input power and 1.2 W

of Noise power. The repeater contributes an additional 48 W of noise

and has a power gain of 20 dB. Compute

a) Input SNR : 10 Log (150 W/ 1.2 W) = 125 = 20.97 dB

b) Power gain of 20 dB means Ap = 100 (why?)

c) So = Si x Ap = 150x100 = 15mW

d) Output noise = No = Ni x Ap + Nr = 1.2 W x 100 + 48 W = 168 W

e) So/No = 15 mW/168 W = 89.3

10 Log 89.3 = 19.5 dB

f) Noise factor = 10 log ((Si/Ni) / (So/No)) = 10 log (125/89.3) = 10 log 1.4

Noise factor = 1.46 dB

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Bit Error Rate: Number of bits that are

Corrupted or destroyed during transmission

E.g., BER of 10-5 indicates that 1 bit out of every

100000 is destroyed during transmission.

The factors governing BER are:

B/W, SNR, transmission media, Environment surrounding

The media, Transmission distance and the transmitter and

Receiver performance

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Noise Types

Atmospheric and Extraterrestrialnoise

Gaussian Noise Crosstalk

Impulse Noise

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Atmospheric and Extraterrestrial

Noise

Lightning: The static discharge generatesa wide range of frequencies

Solar Noise: Ionised gases of SUNproduce a wide range of frequencies as

well.

Cosmic Noise: Distant stars radiateintense level of noise at frequencies that

penetrate the earths atmosphere.

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Gaussian Noise: The cumulative effect of allrandom noise generated over a period of time (itincludes all frequencies).

Thermal Noise: generated by random motion offree electrons and molecular vibrations inresistive components. The power associated

with thermal noise is proportional to bothtemperature and bandwidth

Pn = K x T x BW

K = Boltzmanns constant 1.38x10 -23

T = Absolute temperature of device

BW = Circuit bandwidth

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Shot Noise:Results from the random arrival rate of discrete

current carriers atthe output electrodes of semiconductor and vaccum tube

devices.

Noise current associated with shot noise can be computed as

In= 2qIf

In = Shot noise current in rms

q = charge of an electron

I = DC current flowing through the device

f = system bandwidth (Hz)

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Crosstalk:electrical noise or interference caused by inductive

and capacitive coupling of signals from adjacent channels

In LANs, the crosstalk noise has greater effect on system

Performance than any other types of noise

Problem remedied by using UTP or STP. By twisting the cable

pairs together, the EMF surrounding the wires cancel out each

other.

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Near end crosstalk: Occurs at transmitting station when strong signals radiating

from transmitting pair of wires are coupled in to adjacent weak received signals

traveling in opposite direction.

Far end crosstalk: Occurs at the far end receiver as a result of adjacent signals

traveling in the same direction.

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Minimizing crosstalk in telecom systems

1) Using twisted pair of wires

2) Use of shielding to prevent signals from radiating in to other conductors

3) Transmitted and received signals over long distance are physically

separated

and shielded4) Differential amplifiers and receivers are used to reject common-mode

signals

5) Balanced transformers are used with twisted pair media to cancel crosstalk

signals coupled equally in both lines

6) Maximum channels used within a cable are limited to a certain value

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Impulse Noise:Noise consisting of sudden bursts of

irregularly shaped pulses and lasting for a fewMicroseconds to several hundred milliseconds.

What causes Impulse noise?

a) Electromechanical switching relays at the C.O.

b) Electrical motors and appliances, ignition systems

c) Lightning

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