13-26 824 Reference Manual 9/29/09
fundamental frequencies above 2500Hz is to be measure,
choose a Bandwidth of 20kHz so that the 2nd through
4th harmonics are captured.
• Window -Setting for the FFT window type. Choices are
Rectangular, Hanning and Flat-top. Hanning is required
for THD and frequency calculations.
• Weighting -Frequency weighting setting for the FFT.
This setting controls an analog filter prior to analog to
digital conversions whereas the RTA/SLM and Pulse/FM
mode use Flat analog weighed electronics and digital
filtering. The default value is A weighting so that the
Linearity FFT display has the greatest noise rejection.
The Flatness FFT display overrides this setting and
forces the electronics to be Flat weighted.
Note: The Linearity FFT display when using A or C
weighting should not be used to measure frequency response
and is not good to use for absolute amplitude measurement.
Always zero the display at your reference point and use the
relative level to see changes in amplitude relative to that
reference. Note also that the FFT's A and C weighting filters
may have up to ±0.2dB variation compared to the FFT Flat
weighting (due to component variations and tolerance).
To make absolute or flat weighted measurements, ensure
that the Weighting setting is set to Flat.
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