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Creating and Using Math Waveforms
CSA7000 Series, TDS7000 Series, & TDS6000 Series Instruments User Manual
3- 215
In the time domain a window is a bell-shaped function equal in length to the gate
duration. For most windows this function tapers to zero at both ends of the gate
region. Before computation of the spectral transform, the window is multiplied,
sample by sample, times the input data in the gate region. The window function
affects the shape of the spectral analyzer response in the frequency domain. The
window functions affect the ability to resolve frequency in the output spectrum
and can affect the accuracy of the magnitude and phase measurements.
Figure 3--51 shows how the time domain record is processed.
Source
waveform
Waveform after
windowing
Window
function
(Hanning)
FFT
×
=
Point-by-point
multiply
With windowing
Waveform
data points
Zero fill
Figure 3- 51: Windowing the time domain r ecord
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.