C-1
Annex C: Fundamentals of FFT
In Spectrum FFT mode, spectrum analysis is performed using a Fast
Fourier Transformation
(FFT). This is based on recording of the signal
shape (“sampling”), following by computation.
The individual steps are as follows:
Sampling
The signal shape is monitored for each active measurement axis (isotropic
or single channel) over a certain time period (“time window”) and internally
represented through ongoing sampling.
The sampling is free-running, i.e. it does not require any trigger signal
derived from the measurement signal or external trigger signal.
Transformation (FFT)
Based on the samples of the time-domain signal available internally, the
frequency spectrum is computed (“transformation”). The internal resolution
of individual spectral lines is automatically determined by the length of the
time window.
During selection of all measurement axes (isotropic), three separate spectra
are available internally.
Computation of the resulting spectrum
Upon selection of all three measurement channels (isotropic), the three
individual spectra are put together to form a resulting spectrum.
In the ACT RMS and AVG RMS detection modes, the three corresponding
spectral lines of each measurement channel are put together through
quadratic addition (RSS) to form an internal spectral line. The measurement
result is independent of the polarization of the field and allows direct read-
off of the RMS value.
In the VECT PEAK detection mode, the three corresponding spectral lines
for each measurement channel are formed through vectorial addition of
their peak values. This result is basically dependent on the field polarization.
The measured value can thus be up to 3 dB greater than the result in
ACT
RMS detection mode. The peak value of an individual spectral
component can be read off.
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