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Wire and Wireless Communication Applications Chap. 8
where
f 0 +B/2
P ao = 2 ao (f) df
L
f 0 -B/2
is the measured available output in a bandwidth B hertz wide centered on a frequency of
f 0 and T 0 = 290 K.
If the available gain is constant over the band so that G a ( f ) = G a over the frequency
interval (f 0 - B2) f (f 0 + B2), the noise figure becomes
F =
(8–21a)
kT 0 BG a
The noise figure is often measured by using the Y-factor method. This technique is illustrated
by Prob. 8–19. The Agilent N8973A noise figure analyzer uses the Y-factor method for measuring
the noise figure of devices.
The noise figure can also be specified in decibel units: †
F dB = 10 log(F) = 10 log a b
(8–21b)
kT 0 BG a
For example, suppose that the manufacturer of an RF preamplifier specifies that an RF
preamp has a 2-dB noise figure. This means that the actual noise power at the output is 1.58
times the power that would occur due to amplification of thermal noise from the input. The
other figure of merit for evaluating the noise performance of a linear device is the effective
input-noise temperature. This is illustrated in Fig. 8–20.
P ao
DEFINITION. The spot effective input-noise temperature, T es ( f ), of a linear device is
the additional temperature required for an input source, which is driving an ideal
(noise-free) device, to produce the same available PSD at the ideal device output as is
P ao
Model of
actual device
k
p as (f)=– T
2 i
R S
+
k
p(f)=– [T
2 i +T es (f)]
+
n(t)
G a (f)
Noise-free
linear device
p ao (f)
R L =R o
Thermal
source
Excess noise
source with PSD,
p
k
x (f)=– T
2 es (f)
Figure 8–20
Another noise model for an actual device.
† Some authors call F the noise factor and F dB the noise figure.