instructor: sam nanavaty noise. instructor: sam nanavaty noise is the undesirable portion of an...

Instructor: Sam Nanavaty

NOISE


Noise is the UNDESIRABLE portion of an electrical signal that interferes with the intelligence


Why is it important to study the effects of Noise?

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

wellc) 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 levels and the distance to destination

c) Elaborate error detection and correction mechanisms utilized in the communications systems


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 more powerful 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-dB sound to cause damage to your ears, however 140-dB sound can result in an immediate damage


Relative power gain of a device can be expressed as

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

Relative power 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

Po and Pi can be substituted with Pfin and Pinit as in Final and initial values of power source


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


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 = 10 Av(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


The absolute power gain is defined as “A unit of gain or loss expressedas 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 of1 mW in to 50Ω , 600Ω or 900Ω depending on the impedance of the Transmission 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

(Vrms2/R) = 1 mW (10 (Ap(dBm)/10))

Vrms = √1 mW (10 (Ap(dBm)/10)) x R (where R = standardized value

obtained from the manufacturer)


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 50Ω is connected to 50ΩLoad using a 50Ω coaxial cable. The generator’s 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 voltmeterb) Peak voltage Vp of sine wave that should be displayed on the oscilloscopec) Peak-to-peak voltage of sinewave that should be displayed on the

oscilloscope

a) Vrms = √ 1mW x 10 Ap(dBm) x 50 = 56.17 mVb) Vp = Vrms/.707 = 79.45 mVc) Vp-p = 2x Vp = 158.9 mV


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)


1) The noise power at the output of receiver’s IF stage is measured at 45 µW. 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 µW) = 19 dB

2) A 1 kHZ test tone measured with an oscilloscope at the input of receiver’s FM detector stage. Its peak to peak voltage is 3V. With test tone at 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


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


An input signal of repeater is made of 150 µW of input power and 1.2 µW of Noise power. The repeater contributes an additional 48 µW of noise and has a power gain of 20 dB. Compute

a) Input SNR : 10 Log (150 µW/ 1.2 µW) = 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 µW x 100 + 48 µW = 168 µW

e) So/No = 15 mW/168 µW = 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 1.4Noise factor = 1.46 dB


Bit Error Rate: Number of bits that areCorrupted or destroyed during transmission

E.g., BER of 10-5 indicates that 1 bit out of every100000 is destroyed during transmission.

The factors governing BER are:

B/W, SNR, transmission media, Environment surrounding The media, Transmission distance and the transmitter andReceiver performance


Noise Types

• Atmospheric and Extraterrestrial noise

• Gaussian Noise

• Crosstalk

• Impulse Noise


Atmospheric and Extraterrestrial Noise

• Lightning: The static discharge generates a wide range of frequencies

• Solar Noise: Ionised gases of SUN produce a wide range of frequencies as well.

• Cosmic Noise: Distant stars radiate intense level of noise at frequencies that penetrate the earth’s atmosphere.


Gaussian Noise: The cumulative effect of all random noise generated over a period of time (it includes all frequencies).

Thermal Noise: generated by random motion of free electrons and molecular vibrations in resistive components. The power associated with thermal noise is proportional to both temperature and bandwidth

Pn = K x T x BW

K = Boltzmann’s constant 1.38x10 -23

T = Absolute temperature of deviceBW = Circuit bandwidth


Shot Noise: Results from the random arrival rate of discrete current carriers at the 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 rmsq = charge of an electronI = DC current flowing through the devicef = system bandwidth (Hz)


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 eachother.


Near end crosstalk: Occurs at transmitting station when strong signals radiatingfrom transmitting pair of wires are coupled in to adjacent weak received signalstraveling in opposite direction.

Far end crosstalk: Occurs at the far end receiver as a result of adjacent signals traveling in the same direction.


Minimizing crosstalk in telecom systems

1) Using twisted pair of wires2) Use of shielding to prevent signals from radiating in to other conductors3) Transmitted and received signals over long distance are physically

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

signals5) Balanced transformers are used with twisted pair media to cancel crosstalk

signals coupled equally in both lines6) Maximum channels used within a cable are limited to a certain value


Impulse Noise: Noise consisting of sudden bursts of irregularly shaped pulses and lasting for a few Microseconds to several hundred milliseconds.

What causes Impulse noise?

a) Electromechanical switching relays at the C.O. b) Electrical motors and appliances, ignition systemsc) Lightning

instructor: sam nanavaty noise. instructor: sam nanavaty noise is the undesirable portion of an...

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