Operators Manual Owner manual

Operator's Manual
l Least Prime (default) - a least primes algorithm that computes FFTs on transform sizes having
lengths that can be expressed as factors of 2
N
*5
K
. This is very compatible with the record
lengths encountered in the oscilloscope, which are often multiples of 1, 2, 4, 5, or 10.
l Power of 2 - a power of 2 algorithm where the record lengths are in the form of 2
N
. The power of
2 algorithm generally runs faster than the least primes algorithm. The price that is paid is a rec-
ord length that is not the same as the acquired signal. The power of 2 FFT truncates to the near-
est power of 2 less than record length (if truncate is chosen) or fill data to nearest power of 2
greater than the record length (if zero fill is selected).
8. Depending on your Output Type selection, you may also make selections for :
l Group Delay Shift
l Line Impedence - by default, the FFT function assumes that the oscilloscope is terminated in 50
Ohms. If an external terminator is being used, this setting can be changed to properly calculate
the FFT based on the new termination value.
Choosing a Window
The choice of a spectral window is dictated by the signal's characteristics. Weighting functions control
the filter response shape, and affect noise bandwidth as well as side lobe levels. Ideally, the main lobe
should be as narrow and flat as possible to effectively discriminate all spectral components, while all
side lobes should be infinitely attenuated. The window type defines the bandwidth and shape of the
equivalent filter to be used in the FFT processing.
Rectangular windows provide the highest frequency resolution and are thus useful for estimating the type
of harmonics present in the signal. Because the rectangular window decays as a (sinx)/x function in the
spectral domain, slight attenuation will be induced. Alternative functions with less attenuation (Flat Top
and Blackman-Harris) provide maximum amplitude at the expense of frequency resolution. Whereas,
Hamming and Von Hann are good for general purpose use with continuous waveforms.
Window Type Applications and Limitations
Rectangular These are normally used when the signal is transient (completely contained in the time-
domain window) or known to have a fundamental frequency component that is an integer
multiple of the fundamental frequency of the window. Signals other than these types will
show varying amounts of spectral leakage and scallop loss, which can be corrected by
selecting another type of window.
Hanning (Von Hann) These reduce leakage and improve amplitude accuracy. However, frequency resolution is
also reduced.
Hamming These reduce leakage and improve amplitude accuracy. However, frequency resolution is
also reduced.
Flat Top This window provides excellent amplitude accuracy with moderate reduction of leakage,
but with reduced frequency resolution.
Blackman-Harris It reduces the leakage to a minimum, but with reduced frequency resolution.
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