B+K precision 2165A, Instruction Manual, Page: 19 / 28

19
Introduction to Spectrum Analysis
Sensitivity
Sensitivity is a measure of the analyzers’ ability to detect
small signals. The maximum sensitivity of an analyzer is
limited by its internally generated noise. This noise is basi-
cally of two types: thermal (or Johnson) and nonthermal noise.
Thermal noise power can be expressed as:
PN = k x T x B
where:
PN = Noise power in watts
k
= Boltzmann’s Constant (1.38x10
-23
Joule/K)
T = absolute temperature, K
B = bandwidth of system in Hertz
As seen from this equation, the noise level is directly
proportional to bandwidth. Therefore, a decade decrease in
bandwidth results in a 10 dB decrease in noise level and
consequently 10 dB better sensitivity. Nonthermal noise
accounts for all noise produced within the analyzer that is not
temperature dependent. Spurious emissions due to
nonlinearities of active elements, impedance mismatch, etc.
are sources of nonthermal noise. A figure of merit, or noise
figure, is usually assigned to this nonthermal noise which
when added to the thermal noise gives the total noise of the
analyzer system. This system noise which is measured on the
CRT, determines the maximum sensitivity of the spectrum
analyzer.
Because noise level changes with bandwidth, it is important,
when comparing the sensitivity of two analyzers, to compare
sensitivity specifications for equal bandwidths.
A spectrum analyzer sweeps over a wide frequency range, but
is really a narrow band instrument. All of the signals that
appear in the frequency range of the analyzer are converted to
a single IF frequency which must pass through an IF filter; the
detector sees only this noise at any time. Therefore, the noise
displayed on the analyzer is only that which is contained in the
IF passband. When measuring discrete signals, maximum
sensitivity is obtained by using the narrowest IF bandwidth.
Video Filtering
Measuring small signals can be difficult when they are ap-
proximately the same amplitude as the average internal noise
level of the analyzer. To facilitate the measurement, it is best
to use video filtering. A video filter is a post-detection low
pass filter which averages the internal noise of the analyzer.
When the noise is averaged, the input signal may be seen. If
the resolution bandwidth is very narrow for the span, the video
filter should not be selected, as this will not allow the
amplitude of the analyzed signals to reach full amplitude due
to its video bandwidth limiting property.
Spectrum Analyzer Sensitivity
Specifying sensitivity on a spectrum analyzer is somewhat
arbitrary. One way of specifying sensitivity is to define it as
the signal level when signal power = average noise power.
The analyzer always measures signal plus noise. Therefore,
when the input signal is equal to the internal noise level, the
signal will appear 3 dB above the noise. When the signal
power is added to the average noise power, the power level on
the CRT is doubled (increased by 3 dB) because the signal
power = average noise power.
The maximum input level to the spectrum analyzer is the
damage level or burn-out level of the input circuit. This is +10
dBm for the input mixer and +20 dBm for the input attenuator.
Before reaching the damage level of the analyzer, the analyzer
will begin to gain compress the input signal. This gain
compression is not considered serious until it reaches 1 dB.
The maximum input signal level which will always result in
less than 1 dB gain compression is called the linear input level.
Above 1 dB gain compression the analyzer is considered to be
operating nonlinearly because the signal amplitude displayed
on the CRT is not an accurate measure of the input signal
level.
Whenever a signal is applied to the input of the analyzer,
distortions are produced within the analyzer itself. Most of
these are caused by the non-linear behavior of the input mixer.
These distortions are typically 70 dB below the input signal
level for signal levels not exceeding –27 dBm at the input of
the first mixer. To accommodate larger input signal levels, an
attenuator is placed in the input circuit before the first mixer.
The largest input signal that can be applied, at each setting of
the input attenuator, while maintaining the internally generated
distortions below a certain level, is called the optimum input
level of the analyzer. The signal is attenuated before the first
mixer because the input to the mixer must not exceed –27
dBm, or the analyzer distortion products may exceed the
specified 70 dB range. This 70 dB distortion-free range is
called the spurious-free dynamic range of the analyzer. The
display dynamic range is defined as the ratio of the largest
signal to the smallest signal that can be displayed
simultaneously with no analyzer distortions present.
Dynamic range requires several things then. The display range
must be adequate, no spurious or unidentified response can
occur, and the sensitivity must be sufficient to eliminate noise
from the displayed amplitude range.
The maximum dynamic range for a spectrum analyzer can be
easily determined from its specifications. First check the
distortion spec. For example, this might be “all spurious
products 70 dB down for –27 dBm at the input mixer”. Then,
determine that adequate sensitivity exists. For example, 70 dB
down from –27 dBm is –97 dB. This is the level we must be
able to detect, and the bandwidth required for this sensitivity
must not be too narrow or it will be useless. Last, the display
range must be adequate.