Page 145 - Fundamentals of Radar Signal Processing

the receiver bandwidth) at the output of a receiver is some value S , then an
n
equivalent temperature T′ of the noise source at its input is defined to be

(2.80)

so that S = kT′G and the total noise power is
n
s

(2.81)

The total output noise power at the receiver output is the primary quantity of
interest. In a radar system, the contributors to this noise include the external
noise, the intrinsic kT β thermal noise, and additional thermal noise due to
0 n
losses in the antenna structure and nonideal receivers. Detailed noise analyses
assign individual equivalent noise temperatures to each stage in the system; a
good introductory description is given in Curlander and McDonough (1991).

When considering the system as a whole, it is common to express the total
output noise power as the sum of the power that would be observed due to the
minimum noise density kT at the input and a second term that accounts for the
0
additional noise due to the nonideal system

(2.82)

In this equation, G is now the power gain of the complete receiver system,
s
including antenna loss effects. The equivalent temperature Te used to account
for noise above the theoretical minimum is called the effective temperature of
the system.
The noise temperature description of noise power is most useful for low-
noise receivers. An alternative description common in radar is based on the

idea of noise figure F , which is the ratio of the actual noise power at the output
n
of a system to the minimum power kT β G (Skolnik, 2001). As with noise
0 n
s
temperatures, various noise figures can be defined to include the effects of just
the receiver, or of the entire antenna and receiver system, and so forth. Here, the
term noise figure used without qualification will mean the noise figure of the
complete receiver system, so that

(2.83)

Equation (2.83) shows that knowledge of the noise equivalent bandwidth, gain,