
33-11Noise Measurement
e noise gure is the noise factor expressed in dB and is given by
NF = 1
0logF
(3
3
.36
)
For
t
he
v
n
− i
n
amplier noise model, the noise factor is given by
F
v
kTR B
v v i Z i Z
kTR B
= = +
+ +
ni
s n
n n n n s
s n
s
*
2
2 2 2 2
2
4
1
2
4
Re( )
γ
(3
3
.37
)
where B
n
is the amplier noise bandwidth. e value of Z
s
that minimizes the noise gure is called the
op
timum source impedance
a
nd
i
s
g
iven
b
y
Z R jX j
v
i
so so so i i
n
n
= + = − −
1
2
2
2
γ γ
(3
3
.38
)
where
γ
i
= Im (γ). e corresponding value of F is denoted by F
0
and is given by
F
v i
kTB
j
0
2 2
2
1
2
1= + + −
(
)
n n
n
r i
γ γ
(33.39)
It follows that F can be expressed in terms of F
0
as follows:
F F
G
R
R R X