Agilent Spectrum Analysis Basics - Agilent Technologies

Finding a preamplifier that will give us better sensitivity without costing
us measurement range dictates that we must meet the second of the above
criteria; that is, the sum of its gain and noise figure must be at least 10 dB
less than the noise figure of the spectrum analyzer. In this case the displayed
noise floor will not change noticeably when we connect the preamplifier,
so although we shift the whole measurement range down by the gain of the
preamplifier, we end up with the same overall range that we started with.
To choose the correct preamplifier, we must look at our measurement needs.
If we want absolutely the best sensitivity and are not concerned about
measurement range, we would choose a high-gain, low-noise-figure
preamplifier so that our system would take on the noise figure of the
preamplifier, less 2.5 dB. If we want better sensitivity but cannot afford to
give up any measurement range, we must choose a lower-gain preamplifier.
Interestingly enough, we can use the input attenuator of the spectrum analyzer
to effectively degrade the noise figure (or reduce the gain of the preamplifier,
if you prefer). For example, if we need slightly better sensitivity but cannot
afford to give up any measurement range, we can use the above preamplifier
with 30 dB of RF input attenuation on the spectrum analyzer. This attenuation
increases the noise figure of the analyzer from 24 to 54 dB. Now the gain plus
noise figure of the preamplifier (36 + 8) is 10 dB less than the noise figure of
the analyzer, and we have met the conditions of the second criterion above.
The noise figure of the system is now:
NFsys
= NF SA – G PRE
= 54 dB – 36 dB
= 18 dB
This represents a 6 dB improvement over the noise figure of the analyzer
alone with 0 dB of input attenuation. So we have improved sensitivity by 6 dB
and given up virtually no measurement range.
Of course, there are preamplifiers that fall in between the extremes.
Figure 5-5 enables us to determine system noise figure from a knowledge
of the noise figures of the spectrum analyzer and preamplifier and the
gain of the amplifier. We enter the graph of Figure 5-5 by determining
NF PRE + G PRE – NF SA . If the value is less than zero, we find the corresponding
point on the dashed curve and read system noise figure as the left ordinate
in terms of dB above NF SA – G PRE . If NF PRE + G PRE – NF SA is a positive value,
we find the corresponding point on the solid curve and read system noise
figure as the right ordinate in terms of dB above NF PRE .
NF SA – G pre + 3 dB
NF pre + 3 dB
System Noise
Figure (dB)
NF SA – G pre + 2 dB
NF SA – G pre + 1 dB
NF pre + 2 dB
NF pre + 1 dB
NF SA – G pre
NF pre
NF pre – 1 dB
NF pre – 2 dB
–10 –5 0 +5 +10
NF pre + G pre – NF SA (dB)
NF pre – 2.5 dB
Figure 5-5. System noise figure for sinusoidal signals
64