Sensitivity; Video Filtering; Spectrum Analyzer Sensitivity - Hameg Instruments Hm5012-2 Handbuch

the resolution. They appear above the skirt of the IF filter
and reduce the off band rejection of the filter. This limits
the resolution when measuring signals of unequal
amplitude.
The resolution of the spectrum analyzer is limited by its
narrowest IF bandwidth. For example, if the narrowest
bandwidth is 9kHz then the nearest any two signals can be
and still be resolved is 9kHz. This is because the analyzer
traces out its own IF band pass shape as it sweeps through a
CW signal. Since the resolution of the analyzer is limited by
bandwidth, it seems that by reducing the IF bandwidth
indefinitely, infinite resolution will be achieved.
The fallacy here is that the usable IF bandwidth is limited by
the stability (residual FM) of the analyzer. If the internal
frequency deviation of the analyzer is 9kHz, then the narrowest
bandwidth that can be used to distinguish a single input signal
is 10kHz.
Any narrower IF-filter will result in more than one response or
an intermittent response for a single input frequency. A
practical limitation exists on the IF bandwidth as well, since
narrow filters have long time constants and would require
excessive scan time.

Sensitivity

Sensitivity is a measure of the analyzer's ability to detect small
signals. The maximum sensitivity of an analyzer is limited by
its internally generated noise. This noise is basically of two
types:
Thermal (or Johnson) and non thermal noise. Thermal noise
power can be expressed as:
PN = k x T x B
where:
PN = Noise power in watts
k = Boltzmanns Constant (1.38 x ?10-23 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 10dB decrease in noise level and
consequently 10dB better sensitivity. All noise produced
within the analyzer that is not temperature dependent is
known as non thermal noise. Spurious emissions due to
non linearities of active elements, impedance mismatch, etc.
are sources of non thermal 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
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 pass band. When
measuring discrete signals, maximum sensitivity is obtained
by using the narrowest IF bandwidth.
Subject to change without notice
Introduction to Spectrum Analysis

Video Filtering

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.

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 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.
The maximum input level to the spectrum analyzer is the
damage level or burn-out level of the input circuit. This is (for
the HM5012-2/5014-2) +10dBm for the input mixer and
+20dBm 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 1dB. The maximum input signal level
that will always result in less than 1dB gain compression is
called the linear input level. Above 1dB gain compression,
the analyzer is considered to be operating non linearly 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.
For the HM5012-2/5014-2 these distortions are typically
>75dB below the input signal level for signal levels not
exceeding –30dBm 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 –
30dBm, or the analyzer distortion products may exceed the
specified 75dB range. This 75dB 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 must 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 >75dB down for –30dBm at the input mixer". Then,
determine that adequate sensitivity exists. For example, 75dB
down from –30dBm is –105dB.
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