User’s Manual
32
8.4.5.3
X-Y-Scale Direct Modification
Another way to zoom a graph is to directly change its scale. To do that, simply click on the value for left-
most or right-most marker of the X-scale, and select its value. Type a new value to replace the old one.
The scale is adjusted so that that new value is used at the beginning or end of the scale.
The same technique can be applied to the bottom-most or top-most values of the Y-scale of each
graph.
8.5
Spectrum Tab
The
Spectrum
tab is shown on
. 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 17
8.5.1
Resolution
The resolution is specified in terms of frequency separation between two adjacent spectrum points. The
smaller the frequency resolution, the longer the time window will be. The actual resolution is chosen as
the closest achievable value.
8.5.2
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.
8.5.3
Weighting
The weighting 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.