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infinitely attenuated. The wind
FFT processing.
In the same way as one would choose a particular camera lens for taking a pic
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 thu
harmonics present in the signal. Because the rectangular window decays as a (sinx)/x function in
domain, slight attenuation will be induced. Alternative functions with less attenuation (Flat Top an
Harris) provide maximum amplitude at the expense of frequency resolution; whereas, Ha
are good for general purpose use with continuous waveforms.
Window Type
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
Rectangular
multiple of the fundamental frequency of the window. Signals other than these types will sho
varying amounts of spectral leakage and scallop loss, which can be corrected by selecting
another type of window.
Hanning (Von
These reduce leakage and improve amplitude accuracy. However, frequency
Hann)
also
These reduce leakage and improve amplitude accuracy. However, frequency resolution is
Hamming
also reduced.
This window provides excellent amplitude accuracy with moderate reduction of leakage, but
Flat Top
with reduced frequency resolution.
Blackman–Harris It reduces the leakage to a minimum, but with reduced frequency resolution.
Window Type
Rectangular
von Hann
Hamming
Flat Top
Blackman-Harris
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 an
transient signals. The SNR increase is con
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 meaningfu
conclusions are to be arrived at when using FFT.
WRXi-OM-E Rev C
ow type defines the bandwidth and shape of the equivalent filter to be used in the
reduced.
FFT Window Filter Parameters
Highest Side Lobe
(dB)
-13
-32
-43
-44
-67
ditioned by the cut-off frequency of the ERES low-pass filter and the
s useful for estimating the type of
Applications and Limitations
Scallop Loss
(dB)
3.92
1.42
1.78
0.01
1.13
O
'
PERATOR
S
ture, some experimenting is
the spectral
d Blackman-
mming and Von Hann
resolution is
ENBW
Coherent Gain
(bins)
1.0
0.0
1.5
-6.02
1.37
-5.35
2.96
-11.05
1.71
-7.53
M
ANUAL
w
(dB)
d
l
137
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