NOISE

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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?
• Todays telecom networks handle enormous volume
  of data
• The switching equipment needs to handle high
  traffic volumes as well
• 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?
• Special encoding and decoding techniques used to
  optimize the recovery of the signal
• Transmission medium is chosen based on the
  bandwidth, end to end reliability requirements,
  anticipated surrounding noise levels and the
  distance to destination
• 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 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
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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)/(Vi2/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
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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
• If Vi 2 mV then Vo ?
• A v(dB) 20 Log Av
• Av 10 Av(dB)/20
• Av 25.11
• 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 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 v1 mW (10
(Ap(dBm)/10)) x R (where R standardized
value
obtained 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
• rms voltage measured by rms voltmeter
• Peak voltage Vp of sine wave that should be
  displayed on the oscilloscope
• Peak-to-peak voltage of sinewave that should be
  displayed on the oscilloscope
• a) Vrms v 1mW x 10 Ap(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 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 receivers 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
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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 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
• Input SNR 10 Log (150 µW/ 1.2 µW) 125
  20.97 dB
• Power gain of 20 dB means Ap 100 (why?)
• So Si x Ap 150x100 15mW
• Output noise No Ni x Ap Nr 1.2 µW x 100
  48 µW 168 µW
• So/No 15 mW/168 µW 89.3
• 10 Log 89.3 19.5 dB
• 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 Extraterrestrial noise
• Gaussian Noise
• Crosstalk
• Impulse Noise

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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
  earths atmosphere.

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• 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 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 at the output
electrodes of semiconductor and vaccum tube
devices. Noise current associated with shot
noise can be computed as In v 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
• Using twisted pair of wires
• Use of shielding to prevent signals from
  radiating in to other conductors
• Transmitted and received signals over long
  distance are physically separated
• and shielded
• 4) 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
  few Microseconds to several hundred milliseconds.
• What causes Impulse noise?
• Electromechanical switching relays at the C.O.
• Electrical motors and appliances, ignition
  systems
• Lightning