Tektronix CSA7404B, User Manual

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Creating and Using Math Waveforms
CSA7000B Series & TDS7000B Series Instruments User Manual
3- 193
Source
waveform
Waveform after
windowing
Window
function
(Hanning)
FFT
×
=
Point-by-point
multiply
With windowing
Waveform
data points
Zero fill
Figure 3- 53: Windowing the time domain record
Accurate magnitude measurements require that the input source waveform be
stationary within the gate region. This means that waveform parameters such as
frequency and amplitude do not change significantly as a function of time within
the gate region that is input to the spectral analyzer. Also, the gate width must be
greater than or equal to the period of the start frequency of the span of the
spectral analyzer, that is, there must be at least one cycle of the harmonic being
measured within the gate region.
H
Choice of a window. Your choice of window function will depend on the
input source characteristics which you want to observe and the characteris-
tics of the window function. The window characteristics are shown in
Table 3--13.
H
FFT length. The FFT length is controlled so that the gate width in samples is
never more than 0.8 of the FFT length. Thus, zero fill is always in effect.
This essentially eliminates scallop loss errors in magnitude that would occur
without zero fill.