Introduction to Radar Basics - MATLAB Simulations for Radar Systems Design - page 67

Noise Figure

Appendix 1B

1B.1. Noise Figure
Any signal other than the target returns in the radar receiver is considered to
be noise. This includes interfering signals from outside the radar and thermal
noise generated within the receiver itself. Thermal noise (thermal agitation of
electrons) and shot noise (variation in carrier density of a semiconductor) are
the two main internal noise sources within a radar receiver.

The power spectral density of thermal noise is given by

ω h

S n () = --- --- -------------------------------------- --

(1B.1)

ω h
2π kT

 

-------------

π exp – 1

 

where is the absolute value of the frequency in radians per second, is the ω T

temperature of the conducting medium in degrees Kelvin, is BoltzmanÓs k

– 34

constant, and is PlankÓs constant ( ). When

h h = 6.625 × 10 joule sec onds

the condition is true, it can be shown that Eq. (1B.1) is approxi-

ω 2π kT « ⁄ h

mated by

S n () 2 kT ≈ (1B.2)

This approximation is widely accepted, since, in practice, radar systems oper-
ate at frequencies less than ; and, for example, if , then 100 GHz T = 290 K

. 2π kT ⁄ h ≈ 6000 GHz