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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 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.

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.

Tektronix CSA7404B User Manual

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