Convergence Instruments NSRTW mk3, User Manual

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NSRTW_mk3 User’s Manual 24
7.5 Spectrum Tab
The Spectrum tab is shown on Figure 4 . When that tab is selected the time-domain signals captured
by the instrument are presented in the form of averaged power spectra. The processing is as follows:
Figure 9 (-> shows the defaults)
7.5.1 Window
The type of window chosen has an impact on the spectral resolution. The Hanning window is generally
a good choice that provides good spectral selectivity.
7.5.2 Weighting
The averaging can be exponential or linear.
•
Exponential Averaging is comparable to a low-pass time filtering of the spectrum. In
other words each spectral line is low-pass filtered so that the resulting spectrum is smoothed
out over time. In that case the
Nb_Avg represents an equivalent averaging time constant,
expressed in number of successive spectra. The averaging is continuous and does not stop.
Each successive spectrum is taken into account in the calculation. A larger value of
Nb_Avg
produces a spectrum that is more stable, but also averages-out short-lived transients.
•
Linear Averaging takes an equal-weight average of Nb_Avg successive
spectra. After the Nb_Avg spectra have been observed the result is frozen.
In both cases the Reset button discards the present display and restarts the averaging from zero.
The Averaging Mode can be chosen from several choices:
•
RMS Averaging RMS averaging reduces signal fluctuations but not the noise floor.
The noise floor is not reduced because RMS averaging averages the power of the signal.
RMS Averaging is the most common type.
•
Peak-Hold Peak hold averaging retains the peak levels of the measured spectra.
Peak hold averaging is performed at each frequency line separately, retaining peak levels
from one FFT record to the next.
Note: All the FFT calculations are performed on the PC. The raw time-signals are transferred from
the instrument to the PC, where the FFT and averaging calculations are performed.
7.5.3 Scale
The power spectra can be displayed in dB or linear (Pa) scale. Changing the scale does not restart the
averaging.