Tektronix TDS5054, Руководство пользователя

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Creating and Using Math Waveforms
TDS5000 Series User Manual
3- 161
Source
waveform
Waveform after
windowing
Window
function
(Hanning)
FFT
×
=
Point-by-point
multiply
With windowing
Waveform
data points
Figure 3- 48: Windowing the time domain record
Accurate magnitude measurements require that the input source waveform be
stationary within the gated region. This means that waveform parameters such as
frequency and amplitude should 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 gated region.
Choice of a window: Your choice of window function will depend on the input
source characteristics that you want to observe and the characteristics of the
window function. The window characteristics are shown in Table 3--13.
Table 3- 13: Window characteristics
Window 3 dB BW in bins Scallop loss
Nearest
side lobe Zero phase
reference
Coefficients
Rectangular
0.89 3.96 dB --13 dB 50% 1.0
Hamming
1.3 1.78 dB --43 dB 50% 0.543478, 0.456522
Hanning
1.44 1.42 dB --32 dB 50% 0.5, 0.5
Kaiser-Bessel
1.72 1.02 dB --69 dB 50% 0.40243, 0.49804, 0.09831,
0.00122