LeCroy DDA 7 Zi series, Руководство оператора

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WavePro 7Zi
153
WP700Zi-OM-E-RevA
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