Tektronix TDS5054, User Manual

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Creating and Using Math Waveforms
3- 162
TDS5000 Series User Manual
Table 3- 13: Window characteristics (Cont.)
Window Coefficients
Zero phase
reference Nearest
side lobe
Scallop loss 3 dB BW in bins
Blackman-Harris
1.92 0.81 dB --92 dB 50% 0.35875, 0.48829, 0.14128,
0.01168
Gaussian
2.0 0.76 dB --79 dB 50% a = 3.75 (not cosine series)
Flattop2
3.8 0.0065 dB --90 dB 50% 0.213348, --0.206985,
0.139512, --0.043084,
0.003745
Tek Exponential
1.42 0.60 dB --67 dB 20% na
3 dB BW in Bins: This is the bandwidth of the filter response of the spectral
analyzer to a sine wave input for a given window function. It is given in units of
bins. A bin is the interval between spectral samples when the interpolation ratio
due to FFT zero fill is one. The bandwidth is measured between the points on the
lobe that are 3 dB down from the peak of the lobe. The bandwidth in hertz may
be computed by dividing the BW in bins by the gate duration in seconds. This is
also referred to as resolution bandwidth (RBW).
Coherent gain: The gain factor normally associated with different window
functions is correctly scaled into the magnitude spectrum output. Therefore, the
magnitudes in the output spectrum do not change as different windows are
selected. (An exception is if the analyzer settings are such that no zero fill is used
in the FFT. Under this condition different scallop loss factors of different
windows might cause slight variations in magnitudes. Most of the time zero fill
is in effect and scallop is not present.)
Scallop Loss: This is the magnitude error of the spectral analyzer when the
frequency of the observed signal is exactly half way between two frequency
samples of the spectrum when the interpolation ratio due to zero fill of the FFT
is one. When zero fill is in effect, scallop loss is essentially eliminated because
of the interpolation in the frequency domain due to zero fill. If you work with
span settings less than full and you work with larger resolution bandwidth
settings, zero fill is in effect most of the time.