noise

Question 1

dominant factors that limit the performance

Answer 1

additive noise generated by electronic devices

signal attenuation due to channel losses

Question 2

channel with additive noise and attenuation coefficient alfa:

Answer 2

r(t) = alfa*s*t) + n(t), 
where power spectral density of noise: 
Sn(f) = N0/2 = kT/2
T - temperature 
k - 1.38 - Boltzmann constant

Question 3

how to compensate the signal attenuation ?

Answer 3

using amplifier to boost signal level, but it introduces thermal noise - it is also called noise figure

Question 4

amount of noise power at the output

Answer 4

Pno = integral Sn(f) * |H(f)|^2 df = N0/2 * integral H(f) ^ 2 = N0G * Bneq
G - H(f) ^2 max - the maximum available power gain
Bneq = 1/2G * integral H(f)^2 - the noise equivalent bandwidth

Question 5

effective noise temperature of device with noise equivalent bandwidth Bneq and max available power gain G

Answer 5

Te = Pni / GkBneq

where Pni is additional noise due to generated noise

Question 6

sir at the output of the amplifier

Answer 6

snro = Pso/Pno = GPsi/(GkTBneq*(1 + Te/T)) = 1/(1+Te/T) * snri
Pso - power of the source signal at output
Psi - power of the input of the source signal

Question 7

noise figure, the factor by which the input to output sir degrades at room temperature

Answer 7

F = (1 + Te/T)

Question 8

friis formula - the overall noise of cascade of K amplifiers with gains Gi and noise figure Fi is

Answer 8

F = F1 + (F2-1)/G1 + (F3 - 1)/G1G2 + … + (Fk - 1)/G1G2*G3…Gk-1