Annex C: Fundamentals of FFT EFA-200/-300
C-2
Averaging of the resulting spectrum
In the ACT RMS and VECT PEAK detection modes, each computed
spectrum is used for evaluation.
In the AVG RMS detection mode, a selectable number (averaging
parameter NAVG) of successive power spectra are averaged. Larger
averaging parameters result in a reduction of the noise shown on the screen
and allow you to read off smaller measurement results.
For the frequency range 5 Hz…2 kHz, the averaging time can be computed
exactly (averaging time = NAVG x 0.25 s) so that the averaging time of 1 s
provided in many standards can be set.
Display of the resulting spectrum
The large number of spectral lines present internally can be displayed in the
logarithmic screen only after compressing multiple lines. This compression
is important particularly in the upper screen area in which many internal
spectral lines have to be compressed into just a few screen spectral lines.
With linear scale division, the full resolution is also available on the screen.
Evaluation with the marker
High-resolution evaluation of the spectrum with the marker is possible at
any time since all spectral lines are available internally. When using the
marker, the frequency resolution and the accuracy of the field strength
measurement are improved by at least a factor of ten through an
interpolation technique.
Overlapping real-time operation
For the frequency range 5 Hz…2 kHz, the recording of the field (sampling)
goes on with no interruptions in an overlapping fashion during the ongoing
computation of the spectrum. This is unique since it prevents the device
from becoming blind to field changes, as occurs during sequential
processing.
In this setting, it is also possible to analyze brief fields (pulsed alternating
fields 5
Hz…2 kHz) or fields that vary over time. It is recommended to use
the AVG RMS or ACT RMS detection mode in combination with MAX
display mode, which keeps the spectrum in the screen for subsequent
monitoring.
For the frequency range 40 Hz…32 kHz, brief field changes such as pulsed
fields cannot be recorded. A stationary field is required to produce correct
measurement results.
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