Mumbai University > Electronics Engineering > Sem4 > Fundamentals of Communication Engineering

Marks: 5M

Year: May 2014

• Noise factor expressed in decibels is called as Noise figure.

• Noise figure is given as:

  Noise figure = $F_{dB}$ = 10 log F

  = 10 log⁡(SNR at input) - log⁡(SNR at output)

• Noise factor and noise figure is a measure of degradation of the signal to noise ratio due to the noise added in the system.

• Noise figure can be used to determine the equivalent noise temperature by the given equation below:

  $T_e = T_0 (F-1)$

  Where $T_0$ = 290K

• The ideal value of noise figure is 0 dB.

• The components used for the amplifier and mixer stages must produce low noise in order to improve noise figure of a receiver. Diodes and FETs are therefore preferred.

• Using high gain amplifiers also improves noise figure.
• Noise figure in cascaded system can be calculated as below for a two port network with the help of the Fig1.

Where,

$F_1$ and $F_2$ are the noise figures of the two networks and $G_1$ and $G_2$ are the respective power gains. $N_1$ and $N_2$ are the noise power of the networks.

The output noise power, considering noise contributed by the source and network, is equal to $F_1G_1N_1G_2 + (F_2 - 1) N_1G_2.$

The overall noise figure for the cascaded system is given below:

$F = \frac{F_1 G_1 N_1 G_2 + (F_2-1)N_1 G_1}{N_1 G_1 G_2 } \\ = F_1 +\frac{F_2-1}{G_1} $