Specifications

63
Using these expressions, we’ll see how a preamplifier affects our sensitivity.
Assume that our spectrum analyzer has a noise figure of 24 dB and the
preamplifier has a gain of 36 dB and a noise figure of 8 dB. All we need to
do is to compare the gain plus noise figure of the preamplifier to the noise
figure of the spectrum analyzer. The gain plus noise figure of the preamplifier
is 44 dB, more than 15 dB higher than the noise figure of the spectrum
analyzer, so the noise figure of the preamplifier/spectrum-analyzer
combination is that of the preamplifier less 2.5 dB, or 5.5 dB. In a 10 kHz
resolution bandwidth, our preamplifier/analyzer system has a sensitivity of:
kTB
B=1
+ 10 log(RBW/1) + NF
sys
= –174 + 40 + 5.5
= –128.5 dBm
This is an improvement of 18.5 dB over the –110 dBm noise floor without the
preamplifier.
There might, however, be a drawback to using this preamplifier, depending
upon our ultimate measurement objective. If we want the best sensitivity but
no loss of measurement range, then this preamplifier is not the right choice.
Figure 5-4 illustrates this point. A spectrum analyzer with a 24 dB noise
figure will have an average displayed noise level of –110 dBm in a 10 kHz
resolution bandwidth. If the 1 dB compression point
6
for that analyzer is
0 dBm, the measurement range is 110 dB. When we connect the preamplifier,
we must reduce the maximum input to the system by the gain of the
preamplifier to –36 dBm. However, when we connect the preamplifier,
the displayed average noise level will rise by about 17.5 dB because the
noise power out of the preamplifier is that much higher than the analyzer’s
own noise floor, even after accounting for the 2.5 dB factor. It is from this
higher noise level that we now subtract the gain of the preamplifier. With
the preamplifier in place, our measurement range is 92.5 dB, 17.5 dB less
than without the preamplifier. The loss in measurement range equals the
change in the displayed noise when the preamplifier is connected.
Spectrum analyzer
Spectrum analyzer and preamplifier
0 dBm
1 dB compression
–36 dBm
G
pre
System 1 dB compression
–110 dBm
–128.5 dBm
DANL
System sensitivity
G
pre
DANL
–92.5 dBm
110 dB
spectrum
analyzer
range
92.5 dB
system
range
Figure 5-4. If displayed noise goes up when a preamplifier is connected, measurement range is
diminished by the amount the noise changes
6. See the section titled “Mixer compression” in
Chapter 6.