Preamplifier for noise measurements

Let's consider the various correction factors to calculate the total correction
for each averaging mode:

Many of today's microprocessor-controlled analyzers allow us to activate a
noise marker. When we do so, the microprocessor switches the analyzer into
the power (rms) averaging mode, computes the mean value of a number of
display points about the marker 10, normalizes and corrects the value to a
1 Hz noise-power bandwidth, and displays the normalized value.

The analyzer does the hard part. It is easy to convert the noise-marker value
to other bandwidths. For example, if we want to know the total noise in a
4 MHz communication channel, we add 10 log (4,000,000/1), or 66 dB to the
noise-marker value 11.

Preamplifier for noise measurements
Since noise signals are typically low-level signals, we often need a preamplifier
to have sufficient sensitivity to measure them. However, we must recalculate
sensitivity of our analyzer first. We previously defined sensitivity as the
level of a sinusoidal signal that is equal to the displayed average noise floor.
Since the analyzer is calibrated to show the proper amplitude of a sinusoid,
no correction for the signal was needed. But noise is displayed 2.5 dB too low,
so an input noise signal must be 2.5 dB above the analyzer's displayed noise
floor to be at the same level by the time it reaches the display. The input and
internal noise signals add to raise the displayed noise by 3 dB, a factor of
two in power. So we can define the noise figure of our analyzer for a noise
signal as:

10. For example, the ESA and PSA Series compute the
mean over half a division, regardless of the number
of display points.
11. Most modern spectrum analyzers make this
calculation even easier with the Channel Power
function. The user enters the integration bandwidth
of the channel and centers the signal on the
analyzer display. The Channel Power function then
calculates the total signal power in the channel.