A] Noise figure: Noise factor (F) when expressed in logarithmic scale (decibels) is called as Noise Figure.

Noise Figure = 10 log (F)

B] Noise factor :

The ratio of SNR at i/p to SNR at o/p is called as Noise factor (F).

$F = \frac{S/N (i/p)}{S/N (o/p)}$

C] Noise temperature : (Teq)

Noise temperature (Teq) is the temperature at which noisy resistor has to be maintained so that by connecting it to a noiseless system, the system produces o/p as that by actual noisy system.

Teq = To (F-1)

D] S/N ratio : SNR is the ratio of signal power to Noise power. Higher the SNR, better is the system's performance.

$\frac{S}{N} = \frac{Signal \ power}{Noise \ power}$