Specifications

62
The 24 dB noise figure in our example tells us that a sinusoidal signal must
be 24 dB above kTB to be equal to the displayed average noise level on this
particular analyzer. Thus we can use noise figure to determine the DANL for
a given bandwidth or to compare DANLs of different analyzers on the same
bandwidth.
5
Preamplifiers
One reason for introducing noise figure is that it helps us determine how much
benefit we can derive from the use of a preamplifier. A 24 dB noise figure,
while good for a spectrum analyzer, is not so good for a dedicated receiver.
However, by placing an appropriate preamplifier in front of the spectrum
analyzer, we can obtain a system (preamplifier/spectrum analyzer) noise
figure that is lower than that of the spectrum analyzer alone. To the extent
that we lower the noise figure, we also improve the system sensitivity.
When we introduced noise figure in the previous discussion, we did so on
the basis of a sinusoidal input signal. We can examine the benefits of a
preamplifier on the same basis. However, a preamplifier also amplifies noise,
and this output noise can be higher than the effective input noise of the
analyzer. As we shall see in the “Noise as a signal” section later in this chapter,
a spectrum analyzer using log power averaging displays a random noise
signal 2.5 dB below its actual value. As we explore preamplifiers, we shall
account for this 2.5 dB factor where appropriate.
Rather than develop a lot of formulas to see what benefit we get from a
preamplifier, let us look at two extreme cases and see when each might apply.
First, if the noise power out of the preamplifier (in a bandwidth equal to
that of the spectrum analyzer) is at least 15 dB higher than the DANL of the
spectrum analyzer, then the noise figure of the system is approximately that
of the preamplifier less 2.5 dB. How can we tell if this is the case? Simply
connect the preamplifier to the analyzer and note what happens to the noise
on the display. If it goes up 15 dB or more, we have fulfilled this requirement.
On the other hand, if the noise power out of the preamplifier (again, in the
same bandwidth as that of the spectrum analyzer) is 10 dB or more lower
than the displayed average noise level on the analyzer, then the noise figure
of the system is that of the spectrum analyzer less the gain of the preamplifier.
Again we can test by inspection. Connect the preamplifier to the analyzer;
if the displayed noise does not change, we have fulfilled the requirement.
But testing by experiment means that we have the equipment at hand.
We do not need to worry about numbers. We simply connect the preamplifier
to the analyzer, note the average displayed noise level, and subtract the gain
of the preamplifier. Then we have the sensitivity of the system.
What we really want is to know ahead of time what a preamplifier will do
for us. We can state the two cases above as follows:
IfNF
pre
+ G
pre
≥ NF
sa
+ 15 dB,
Then NF
sys
=NF
pre
– 2.5 dB
And
IfNF
pre
+ G
pre
≤ NFsa – 10 dB,
Then NF
sys
=NF
sa
– G
pre
5. The noise figure computed in this manner cannot be
directly compared to that of a receiver because the
“measured noise” term in the equation understates
the actual noise by 2.5 dB. See the section titled
“Noise as a signal” later in this chapter.