O
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ANUAL
WRXi-OM-E Rev B
137
infinitely attenuated. The window type defines the bandwidth and shape of the equivalent filter to be used in the
FFT processing.
In the same way as one would choose a particular camera lens for taking a picture, some experimenting is
generally necessary to determine which window is most suitable. However, the following general guidelines
should help.
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.
FFT Window Filter Parameters
Window Type
Highest Side Lobe
(dB)
Scallop Loss
(dB)
ENBW
(bins)
Coherent Gain
(dB)
Rectangular
-13 3.92
1.0
0.0
von Hann
-32 1.42
1.5
-6.02
Hamming
-43 1.78
1.37
-5.35
Flat Top
-44 0.01
2.96
-11.05
Blackman-Harris
-67 1.13
1.71
-7.53
Improving Dynamic Range
Enhanced resolution uses a low-pass filtering technique that can potentially provide for three additional bits (18
dB) if the signal noise is uniformly distributed (white). Low-pass filtering should be considered when high
frequency components are irrelevant. A distinct advantage of this technique is that it works for both repetitive and
transient signals. The SNR increase is conditioned by the cut-off frequency of the ERES low-pass filter and the
noise shape (frequency distribution).
LeCroy digital oscilloscopes employ FIR digital filters so that a constant phase shift is maintained. The phase
information is therefore not distorted by the filtering action.
Record Length
Because of its versatility, FFT analysis has become a popular analysis tool. However, some care must be taken
with it. In most instances, incorrect positioning of the signal within the display grid will significantly alter the
spectrum. Effects such as leakage and aliasing that distort the spectrum must be understood if meaningful
conclusions are to be arrived at when using FFT.
