3 Spectrum Analyzer Mode
3.9 SEM Measurement
the log scale is not synergistic, though; NFE with the power scale works a little
better than NFE with log averaging type.
The results from NFE with internal preamp can often be lower than the theoretical
noise in a signal source at room temperature, a noise density of -174 dBm/Hz. This
is expected and useful behavior, because NFE is designed to report the amount of
input signal that is in excess of the thermal noise, not the amount that includes the
thermal noise. This can be a useful behavior because thermal noise often interferes
with what you want to measure, instead of being part of what you want to measure.
Note that NFE is not adequately accurate to always be able to read below kTB.
Adaptive NFE provides an alternative to fully-on and fully-off NFE. Fully-on NFE
can, notably in cases with little or no averaging of the spectrum, result in a display
that is distractingly unfamiliar in the variability in response to low level signals.
Fully-off NFE fails to achieve the potential improvement in dynamic range and
associated accuracy of measurement of low-level signals. Adaptive NFE controls the
degree of potential improvement in the noise floor to give more improvement for
those instrument settings that can make good use of the potential improvement —
those settings with high degrees of variance reduction through some variant of
averaging. When the potential improvement is small, the display acts like the NFE-
off case, and when it is high, it acts like the fully-on case, and in-between,
application is a compromise between attractiveness and effectiveness.
On instruments with the NF2 license installed, the calibrated Noise Floor used by
Noise Floor Extension should be refreshed periodically. Keysight recommends that
the Characterize Noise Floor operation be performed after the first 500 hours of
operation, and once every calendar year. The control to perform this is located in the
System, Alignments, Advanced menu. If you have not done this yourself at the
recommended interval, then when you turn on Noise Floor Extension, the
instrument will prompt you to do so with a dialog that says:
“This action will take several minutes to perform. Please disconnect all cables from
the RF input and press Enter to proceed. Press ESC to cancel, or Postpone to
postpone for a week”
If you Cancel, you will be prompted again the next time you turn NFE on. If you
postpone, you will be prompted again after a week passes and you then turn NFE
on.
Enable Wideband IF for FFT
When OFF, the maximum FFT BW is limited to 40 MHz. When ON, FFT with more
wideband IF is supported depending on the instrument. For example, the max FFT
BW is 510 MHz with option B5X. When ON for R10/R20/R40, the max FFT BW is
1GHz.
Spectrum Analyzer Mode User's &Programmer's Reference 1721
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