Table of Contents
Advertisement
Always use the smallest possible SPAN when measuring close to the 0Hz
spike in order to reduce the noise floor:
0Hz noise floor
Big SPAN = high noise (smoother
filter cut off)
Small SPAN = low noise
(sharper filter cut off)
Try to avoid measurements near the 0Hz limit whenever possible.
Try not to use 0Hz as starting frequency.
If possible, try to measure from 1kHz or, even better, 10kHz to 100kHz instead
of 0Hz to 100kHz. Spectrum analyzers try to dampen the noise floor close to the
0Hz spike when doing large sweeps that start close to 0Hz. This has the side-
effect of a very low sensitivity in the "lower frequency range". Thus, do not expect
a 50Hz signal to be measured correctly when using a sweep of 0Hz to 100kHz.
However, should you despite of this wish to measure close to the 0Hz spike (for
example, 50Hz mains or 16.7Hz traction power), you should use the smallest
possible filters (0,3Hz, 1Hz or 3Hz). However, this naturally leads to an extre-
mely slow sweep. As this is not very practical, we have developed a very fast
sweep mode for exactly this case: the DFT sweep. Basically, it uses FFT, but
avoids its typical aliases. DFT mode is automatically activated if both of the fol-
lowing apply:
a) The 0,3Hz, 1Hz or 3Hz filter is used
AND
b) a SPAN smaller than 200Hz, but at least 10Hz is chosen
In the current software revision, there are still two further restrictions:
c) only use SPANs which are multiples of 15, like 15Hz, 30Hz, 45Hz etc.
d) always use a SPAN which is SMALLER than the difference between 0Hz and
the chosen starting frequency "flow". Otherwise, you will get erroneous results.
34
Firmware V 1.0 / © 2005-2013 by Aaronia AG, D-54597 Euscheid, www.aaronia.com
The SPAN itself also influences the strength of
the noise floor due to the steepness of the fil-
ters. These become smoother with a bigger
SPAN and also start capturing "neighbouring"
signals.
While this hardly matters with "higher" fre-
100Hz SPAN
quencies, the difference much more significant
30Hz
near the 0Hz spike. In spite of an identical star-
SPAN
ting frequency setting, the noise floor is much
stronger with a bigger SPAN than when using
a small SPAN.
8.0 Correct measurement
Advertisement
Table of Contents
