10–20
Zoom, Mathematics and Math Setup
Similarly, if the source waveform is a zoom trace, the frequency
resolution is the reciprocal of the displayed waveform’s duration.
Setting the Span The frequency span of the FFT is called the Nyquist frequency
and is related to the sampling frequency of the time domain
waveform. If the math memory size, set using the “for Math use
max points” menu (see page 10–8), is identical to the number of
samples in the acquired waveform (“TIMEBASE” menus,
Chapter 7), then the span will be half the sampling frequency.
But if this “max points for math” number is less than the number
of points, the waveform and in turn the FFT span will be
decimated. The relationship between the span of the FFT and
the sampling rate (1/∆T) is illustrated here.
TIME
FREQUENCY
AMPLITUDEAMPLITUDE
ACQUIRED WAVEFORM
FAST FOURIER TRANSFORM (FFT)
∆
T
FREQUENCY SPAN
SAMPLING PERIOD
FREQUENCY SPAN = K * ( 1/
∆
T)
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