COMMUNICATION SYSTEM EECB353 Chapter 4 NOISE ANALYSIS

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COMMUNICATION SYSTEM EECB353Chapter 4NOISE
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 Noise

• 1. Atmospheric Noise
• Caused by naturally occurring disturbances in the
  earths 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 earths atmosphere (outer
  space), also call deep-space noise.
• Sub-divided into 2 categories
• Solar noise generated from the suns 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

• 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.
• 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 Boltzmanns contant (1.38 x 10-23
  Joules/Kelvin)
• T absolute temperature (Kelvin), and T ?C
  273?
• B 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.

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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 27?C over a 1 MHz frequency range,
  determine
• thermal noise power in watts and dBm
• 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 37C, calculate the
  resistors 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
where vhigher
vfundamental rms voltage of fundamental freq
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• Example 3 Determine
• 2nd, 3rd and 12th harmonics for a 1 kHz
  repetitive wave.
• 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 gt
  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.

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, determine
• First three harmonics present in the output for
  each input frequency.
• 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