Video Filtering; Spectrum Analyzer Sensitivity - Hameg HM5010 Manual

Video Filtering

Spectrum Analyzer Sensitivity

Subject to change without notice
results in a 10dB decrease in noise level and consequently 10dB
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 non-
thermal 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
bandwith 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.
Measuring small signals can be difficult when they are
approximately 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.
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 3dB above the noise. When the signal power is added to
the average noise power, the power level on the CRT is doubled
(increased by 3dB) because the signal power=average noise power.
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