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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/
14) +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 which 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
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. For the HM5012/
5014 these distortions are typically 70dB below the input signal
level for signal levels not exceeding –27dBm 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 exeed –27dBm, or the
analyzer distortion products may exceed the specified 70dB range.
This 70dB 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
Subject to change without notice
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