Choosing The Right Filter (Rbw) - Aaronia NF-5030S Manual

8.0 Correct measurement
Examples:
flow=30Hz and fhigh=60Hz (SPAN=30Hz): OK (ex.: 50Hz mains).
flow=15Hz and fhigh=30Hz (SPAN=15Hz): OK (ex.: 16.7Hz traction power).
flow=29Hz and fhigh=59Hz: WRONG. The distance to 0Hz is too small compa-
red to the SPAN. It is 29Hz, but the SPAN is set to 30Hz. The distance to 0Hz
should be therefore at least 30Hz.
flow=16Hz and fhigh=30Hz: WRONG. The SPAN is not a multiple of 15 (as it is
set to 14Hz), resulting in WRONG measurement results.
Also see the chapter on "DFT mode (rapid measurement with 1-3Hz RBW)"

8.3 Choosing the right filter (RBW)

The correct filter setting depends on various factors, among them the selected
SPAN. Usually, the filter should be SMALLER than the selected SPAN, as other-
wise, signals outside the (visible) SPAN will be also received and erroneously
displayed, which is only tolerable in very rare cases. Most of the time, a filter at
least 10 times smaller than the SPAN should be chosen. For example:
SPAN=100Hz; set filter to 10Hz (or smaller)
SPAN=500kHz; set filter to 100kHz or 30kHz (or smaller)
However, note that the selected filter directly influences the sweep speed and
display accuracy: the larger the selected filter, the faster the sweep but the less
accurate the display.
The following illustrations explain the filter problem, assuming a sweep from 50
to 60 kHz. The grid lines show the visible range:
10µT
signal at
46kHz
Graphical approximation of 30kHz filter
10µT
signal at
46kHz
Firmware V 1.0 / © 2005-2013 by Aaronia AG, D-54597 Euscheid, www.aaronia.com
Graphical appro-
ximation of the
10 1kHz filter
1µT signal at
50-60kHz
1µT signal at
50-60kHz
Good visible:
The 46kHz signal outside the visible
sweep range is not taken into
account, as the 1kHz filter only
"catches" a small frequency band
outside the visible 50-60kHz sweep
58kHz
range. The display would show
the right value of 1µT.
In contrast, when using a 30kHz fil-
ter, the 46kHz signal outside the
visible sweep range will be caught
as well, as the wide 30kHz filter will
also encompass a wide range outsi-
de the visible 50-60kHz sweep. The
58kHz
display would show a wrong
value of 10µT.
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