COMMUNICATION SYSTEM EEEB453 Chapter 4(II) NOISE ANALYSIS

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COMMUNICATION SYSTEM EEEB453Chapter 4(II)NOISE
ANALYSIS
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Gain and Attenuation

• Most circuits in are used to manipulate signals
  to produce a desired relectronic communication
  esult.
• All signal processing circuits involve
• Gain
• Attenuation

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Gain and Attenuation

• Gain means amplification. It is the ratio of a
  circuits output to its input.
• The number obtained by dividing the output by the
  input shows how much larger the output is than
  the input.
• A positive () dB value indicates that the output
  power is greater than the input power which
  indicates power gain.

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Gain and Attenuation

• Most amplifiers are also power amplifiers, so the
  same procedure can be used to calculate power
  gain AP where Pin is the power input and Pout is
  the power output.
• Power gain (Ap) Pout / Pin
• An amplifier is cascaded when two or more stages
  are connected together.
• The overall gain is the product of the individual
  circuit gains.

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Gain and Attenuation

• Example 1
• The power output of an amplifier is 6 watts (W).
  The power gain is 80. What is the input power?
• b) Three cascaded amplifiers have power gains of
  5, 2, and 17. The input power is 40 mW. What is
  the output power?

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Gain and Attenuation

• Attenuation means a power loss.
• Attenuation refers to a loss introduced by a
  circuit or component. If the output signal is
  lower in amplitude than the input, the circuit
  has loss or attenuation.
• The letter A is used to represent attenuation
• Attenuation A output/input Vout/Vin
• A negative (-) dB value indicates that the output
  power is less than the input power, which
  indicates power loss.
• i.e circuits that introduce attenuation have a
  gain that is less than 1.
• With cascaded circuits, the total attenuation is
  the product of the individual attenuations.

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Gain and Attenuation

• Example 2
• Calculate the total attenuation
• Calculate Vout

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Decibels

• The gain or loss (attenuation) of a circuit is
  usually expressed in decibels (dB).
• Total gain or attenuation is the algebraic sum of
  the individual stage gains in decibels.

AT A1 A2 A3
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Decibels

• Example
• Calculate the total gain.

A115dB
A2-20dB
A335dB
AT A1 A2 A3 AT 15 (-20) 35 30dB
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Signal to Noise Power Ratio (S/N or SNR)

• SNR is the ratio of the signal power level to the
  noise power level.

SNRdB
SNR
where Vs signal voltage Rin input resistance
Vn noise voltage Rout output resistance

• If Rin Rout, then

SNRdB

• dBm is an abbreviation for the power ratio in
  decibel (dB) of the measured power referenced to
  one milliwatt (mW).
• It should not be confused with dB, a
  dimensionless unit, which is used when measuring
  the ratio between two values, such as
  signal-to-noise ratio.

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• Example 3 For an amplifier with an output
  signal power of 10W and an output noise power of
  0.01W, determine the signal-to-noise power ratio.
• Example 4 For an amplifier with an output
  signal voltage of 4V, an output noise voltage of
  0.005V, and an input and output resistance of
  50?, determine the signal-to-noise power ratio.

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Noise Factor (F) and Noise Figure (NF)

• F and NF are figures of merit used to indicate
  how much the SNR weaken as a signal passes
  through a cct or series of ccts.
• Noise Factor, F
• Noise Figure, NF parameter to indicate the
  quality of a Rx. It is simply the noise factor, F
  stated in dB.
• NF 10log F dB
• NF indicates how much the SNR deteriorates (drop)
  as waveform propagates from input to the output
  of a cct.
• For eg. NF of 6dB means that the SNR at output is
  6dB less than the SNR at the input.
• If the cct. is noiseless, SNRoutput SNRinput ,
  thus F 1 and NF 0dB.

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Noise Factor (F) and Noise Figure (NF)

• An electronic cct amplify signal and noise within
  its passband equally well, ideally, the amplifier
  is noiseless.
• In reality, amplifier in not ideal. It produces
  internally generated noise to the waveform, thus
  reduce the overall SNR.
• For ideal noiseless amplifier,
• For non-ideal amplifier that generates an
  internal noise, Nd,

The total power gain is simply the product of the
individual gains. (If in dB, sum up the
individual gains)
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• Example 5 For a non-ideal amplifier and the
  following parameters,
• Input signal power 2x10-10W
• Power gain 1,000,000
• Input noise power 2x10-18W
• Internal noise(Nd) 6x10-12W
• determine
• Input S/N ratio (dB)
• Output S/N ratio (dB)
• Noise factor and noise figure

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Noise Factor (F) and Noise Figure (NF)

• When 2 or more amplifiers are cascaded, the total
  noise factor is the accumulation of the
  individual noise factor.
• Friss formula is used to calculate it,
  mathematically,
• For Friss, NF must be converted to F i.e NFT (dB)
  10log FT

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Example 6 Determine the total noise figure for
a three-cascaded amplifier of 3dB noise figure
and 10dB power gains each.
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Equivalent Noise Temperature (Te)

• Since noise produced from thermal agitation ?
  temperature, thermal noise can be expressed in
  degree/ watts/ dBm.
• From NKTB,then T Kelvin
• Te cannot be directly measured. It is used rather
  than noise factor in low-noise sophisticated
  VHF,UHF, microwave and satellite radio Rx.
• Te? F, indicates the reduction in SNR a signal
  undergoes as it propagates thro a Rx.
• The lower the Te, the better the quality of a Rx.
• Mathematically, Te at input Rx
• Te T(F-1)Kelvin
• Where TO environmental temperature, ref
  value of 290K
• F noise factor (unitless)
• Thus,

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• Example 7 Determine
• Noise figure for an equivalent noise temperature
  of 75K (use 290K for the ref temp).
• Equivalent noise temperature for a noise figure
  of 6dB

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