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Communication Engineering - Chapter 6 - Noise

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  • Audio recording – any unwanted signal that fall within audio frequency band of 0Hz to 15kHz will interfere with the music

Communication Engineering - Chapter 6 - Noise Communication Engineering - Chapter 6 - Noise Presentation Transcript

  • CHAPTER 6 NOISE
  • INTRODUCTION
    • Noise is random energy that interfere with the information signal.
    • Noise may be defined as any unwanted introduction of energy tending to interfere with the proper reception and reproduction of transmitted signal.
    • In radio receiver, noise may produce hiss in the loudspeaker output.
    • Noise can limit the range of systems.
    • It affects the sensitivity of the receiver.
  • NOISE
    • Electrical noise – any undesirable that falls within the passband of the signal.
    • Figure 4 show the effect of noise on electrical noise.
    • 2 general categories
      • Correlated noise – implies relationship between the signal and the noise, exist only when signal is present.
      • Uncorrelated noise – present at all time, whether there is signal or not.
  • Classification of Uncorrelated Noise NOISE EXTERNAL INTERNAL ATMOSPHERIC NOISE EXTRATERRESTRIAL NOISE INDUSTRIAL NOISE THERMAL NOISE SHOT NOISE Figure 6.1
  • ATMOSPHERIC NOISE
    • Caused by lightning discharges in thunderstorms and other natural electric disturbances occurring in the atmosphere.
    • Consist of spurious radio signal with components distributed over a wide range of frequencies.
    • It propagates over the earth in the same way as ordinary radio waves of the same frequencies.
    • Become less severe at frequencies above 30MHz because:
      • The higher frequencies are limited to line-of-sight propagation.
      • Nature of the mechanism generating this noise is such that very little of it is created in the VHF range and above.
  • EXTRATERRESTRIAL NOISE
    • SOLAR NOISE:
      • Normal condition, there is a constant noise radiation from the sun, simply because large body at a very high frequency.
      • Radiates over a very broad frequency spectrum.
    • COSMIC NOISE:
      • Stars radiate RF noise in the same manner of sun.
      • The noise received is called thermal noise and distributed fairly uniformly over the entire sky.
  • INDUSTRIAL NOISE
    • Between 1 to 600 MHz, the intensity noise made by humans easily outstrips that created by any other source to the receiver.
    • Sources such as: automobile, aircraft, electric motors and other heavy machine.
    • The nature of industrial noise is so variable that it is difficult to analyze.
  • SHOT NOISE
    • Caused by the random arrival of carriers at the output element of an electronic device.
    • First observed in the anode current of a vacuum-tube amplifier.
    • The current carriers are not moving in continuous steady flow.
    • Randomly varying and superimposed onto any signal present.
    • Sometimes called transistor noise.
  • THERMAL NOISE
    • Is associated with the rapid and random movement of electrons within a conductor due to thermal agitation.
    • Present in all electronic component and communications systems.
    • Referred as white noise.
    • Is a form of additive noise, cannot be eliminated.
    • It increases in intensity with the number of devices in a circuit.
    • Thermal noise power is proportional to the product of bandwidth and temperature.
    • Mathematically, noise power is
    • N=KTB
    • N = noise power,
    • K=Boltzmann’s constant ( 1.38x10 -23 J/K )
    • B = bandwidth,
    • T = absolute temperature (Kelvin)( 17 o C or 290K )
  • NOISE VOLTAGE
    • Figure 4.2 shows the equivalent circuit for a thermal noise source.
    • Internal resistance R I in series with the rms noise voltage V N .
    • For the worst condition, the load resistance R = R I , noise voltage dropped across R = half the noise source ( V R =V N / 2 ) and
    • From the final equation The noise power P N , developed across the load resistor = KTB
    The mathematical expression : Figure 6.2 : Noise source equivalent circuit
  • Example 1
    • Convert the following temperatures to kelvin:
    • a) 100°C
    • b) 0°C
    • c) -10°C
    • T=a°C+273°C
  • Example 2
    • Calculate the thermal noise power available from any resistor at room temperature (290K) for a bandwidth of 1 MHz. Calculate also the corresponding noise voltage, given that R = 50  .
  • Example 3
    • For an electronic device operating at a temperature of 17 o C with a bandwidth of 10 kHz, determine
      • Thermal noise power in watts and dBm
      • rms noise noise voltage for a 100  internal resistance and 100  load resistance.
  • Example 4
    • Two resistor of 20k  and 50 k  are at room temperature (290K). For a bandwidth of 100kHz, calculate the thermal noise voltage generated by
    • each resistor
    • the two resistor in series
    • the two resistor in parallel
  • Correlated Noise
    • Form of internal noise that is correlated to the signal and cannot be present in a circuit unless there is a signal.
    • Produced by nonlinear amplification.
    • All circuits are nonlinear therefore, they all produce nonlinear distortion .
    • Nonlinear distortion creates unwanted frequencies that interfere with the signal and degrade performance.
  • Intermodulation Distortion
    • Generation of unwanted sum and difference frequencies produced when two or more signals mix in a nonlinear device.
    • The sum and difference frequencies are called cross products.
    • Unwanted cross products can interfere with the information signal.
    • Cross products are produced when harmonics as well as fundamental frequency mix in a nonlinear device.
  • Cont..
    • Cross products = mf 1 ±nf 2 .
    • F1 and f2 are fundamental frequency.
    • F1>f2
    • M and n are positive integer.
  • Correlated Noise-Intermodulation Distortion f1 f2 V1 V2 f1 f2 f1-f2 f1+f2 V1 V2 V difference V sum Input frequency spectrum Output frequency spectrum Figure 6.4
  • Example 6
    • For a nonlinear amplifier with 2 input frequencies, 3kHz and 8kHz, determine:
    • First 3 harmonics present in the output for each input frequency.
    • Cross-product frequencies produced for values of m and n of 1 and 2.
  • Interference
    • Form of external noise.
    • Means to disturb or detract from.
    • Electrical interference is when information signals from one source produce frequencies that fall outside their allocated bandwidth and interfere with information signals form another source.
    • Most interference occur when harmonics frequencies from one source fall into the passband of a neighboring channel.
  • Review Notes
    • Gain
    • Attenuation
      • Both has the ratio output to the input.
    Figure 6.5
  • Gain
    • Ratio output to the input.
    • Output has greater amplitude than the input
    • Most amplifiers are power amplifier, the same procedure can be used to calculate power gain, A p .
    • A p = P out /P in
    Figure 6.6
  • Attenuation
    • Refers to loss introduced by a circuit.
    • Output is less than input.
    • For cascade circuit, total attenuation is, A T =A 1 x A 2 x A 3 …..
    • Voltage divider network may introduce attenuation.
    Figure 4.7 Voltage divider introduces attenuation
    • Attenuation can be offset by introducing gain.
    Figure 6.8 Total attenuation in cascaded network Figure 6.9 Gain offsets the attenuation
  • Figure 6.10 Total gain is the product of the individual stage gains and attenuation
    • Example 7
    • What is the gain of an amplifier that produces an output of 750 mV for 30  V input?
    • Example 8
    • The power output of an amplifier is 6 W. The power gain is 80. What is the input power?
    • Example 9
    • Three cascade amplifier have power gains of 5,2, and 17. The input power is 40 mW. What is the output power?
  • Signal to Noise Ratio (SNR)
    • Ratio of the signal power level to the noise power level.
    • Express in logarithmic function:
  • Example 10
    • For an amplifier with an output signal power of 10W and an output noise power of 0.01W, determine the SNR.
    • 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 Ω , determine the SNR.
  • Noise Factor (F) and Noise Figure (NF)
    • Figures of merit used to indicate how much the SNR deteriorates as a signal passes through a circuit.
    • Noise factor is simply a ratio of input SNR to output SNR.
  • Cont..
    • NF is noise factor stated in dB.
    • Used to indicate the quality of a receiver.
  • Ideal Noiseless Amplifier Ideal Noiseless Amplifier Ap=power gain Figure 6.11
  • Non ideal amplifier Nonideal amplifier Ap=power gain Nd=internally generated noise Figure 6.12
  • Example 11
    • For a nonlinear amplifier and the following parameter, determine:
    • a) Input SNR(dB)
    • b) Output SNR(dB)
    • c) Noise Factor and Noise Figure
    • Input signal power=2x10 -10 W
    • Input Noise power=2x10 -18 W
    • Power gain=1,000,000
    • Internal noise (Nd)=6x10 -12 W
  • Noise Figure of Cascaded Amplifier Ap1 NF1 Ap2 NF2 Ap3 NF3 Input Output Figure 6.13
  • Cont..
    • Total noise factor is the accumulation of the individual noise factor.
    • Friiss’s formula is used to calculate the total noise factor of several cascaded amplifiers.
  • Example 12
    • For 3 cascaded amplifier stages, each with noise figure of 3 dB and power gain of 10 dB, determine the total noise figure.
  • Equivalent Noise Temperature (T e )
    • Hypothetical value that cannot be directly measured.
    • To indicates the reduction in the SNR a signal undergoes as it propagates through a receiver.
    • The lower Te is the better quality of a receiver.
  • Example 13
    • Determine:
    • Noise Figure for an equivalent noise temperature of 75K.
    • Equivalent noise temperature for a noise figure of 6dB.
  • Example 14
    • A voltage divider shown in Figure 6.9 has values of R 1 = 10k  and R 2 = 47k  .
      • What is the attenuation?
      • What amplifier gain would you need to offset the loss for an overall gain of 1?
  • Example 15
    • An amplifier has gain of 45,000, which is too much for the amplification. With an input voltage of 20  V, what attenuation factor is needed to keep the output voltage from exceeding 100mV?. Let A 1 = amplifier gain = 45,000; A 2 = attenuation factor; A T = total gain.
  • Example 16
    • A RF sine wave generator whose output impedance is 50  is connected to a 50  load using 50  coaxial cable. The generator’s output amplitude level is set to + 3 dBm. An rms voltmeter is used to measure the effective voltage, and an oscilloscope is used to display the sine wave. Compute the following:
      • The rms voltage measure by the rms voltmeter
      • The peak voltage, V p of the sine wave that should be displayed on the oscilloscope.
      • The peak-to-peak voltage, V p-p of the sine wave that should be displayed on the oscilloscope
  • Example 17
    • The input signal to a telecommunications receiver consists of 100  W of signal power and 1  W of noise power. The receiver contributes an additional 80  W of noise, N D , and has a power gain of 20 dB. Compute the input SNR, the output SNR and the receiver’s noise figure.