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Noise 2.0
 

Noise 2.0

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different types of internal and external noises, s/n ratio, s/n ratio of a tandem connection, noise factor, amplifier input noise, noise factor of amplifiers in cascade (friss's formula).

different types of internal and external noises, s/n ratio, s/n ratio of a tandem connection, noise factor, amplifier input noise, noise factor of amplifiers in cascade (friss's formula).

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  • On a worldwide scale, eight million lightning flashes occur daily. This is about 100 lightning flashes per second. The sum of all these lightning flashes results in atmospheric noise.At low frequencies (below approximately 2 MHz or so) radio signals travel as ground waves, which follow the Earth's curvature due to diffractionwith the layers of atmosphere. This enables AM radio signals in low-noise environments to be received well after the transmitting antenna has dropped below the horizon. Additionally, frequencies between approximately 1 and 30 MHz, can be reflected by the F1/F2 Layer, thus giving radio transmissions in this range a potentially global reach (see shortwave radio), again along multiply deflected straight lines. The effects of multiple diffraction or reflection lead to macroscopically "quasi-curved paths".However, at higher frequencies and in lower levels of the atmosphere, neither of these effects are significant. Thus any obstruction between the transmitting antenna and the receiving antenna will block the signal, just like the light that the eye may sense. Therefore, since the ability to visually see a transmitting antenna (disregarding the limitations of the eye's resolution) roughly corresponds to the ability to receive a radio signal from it, the propagation characteristic of high-frequency radio is called "line-of-sight". The farthest possible point of propagation is referred to as the "radio horizon".
  • Space (or Extraterrestrial noise) is observable at frequencies in the range of about 8Mhz to 1.43 Ghz, the latter frequency corresponding to the 21cm hydrogen line. Apart from man made noise it is the strongest component over the range of about 20 to 120Mhz. Not very much of it below 20Mhz penetrates down through the ionosphere, and it also disappears at frequencies in the excess of 1.5Ghz because of the mechnisms generating it.
  • The telephone cables of analog telephone system is divided into number of line sections as shown, and each line section has some amount of power loss which is compensated by adding amplifiers known as repeaters. So if the power loss of a line section is L then amplifier of power Gain G is chosen such that LG=1.TheEach amplifier in every section adds its own noise and so the noise accumulates as the signal travels along the system.Input signal to first section of line is Ps and input noise at this point is assumed to be negligible. This signal power remain same at the output of 1st section since the loss l in this section is being compensated by gain G of the amp .The noise at the output of 1st repeater is Pn1 and consists of the noise added by the line section and the amplifier.As the signal progresses along the link, the power output at each repeater remains the same i.e Ps because LG =1 for each link. But the total noise at the output of Mth link will be Pn1 + Pn2 +….If the links are identical and each link contributes to Pn, the total noise power becomes PnM = MPn
  • Amplifier noise is generated in many components throughout the amplifier , but it proves convenient to imagine it to originate from some Equivalent power source at the input of the amplifier .
  • Consider first two amplifiers in cascade . The problem is to determine the overall noise factor F in terms of individual noise factors and available power gains .

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