Fast Fourier Transforms
TDS 620A, 640A, & 644A User Manual
3Ć45
Zero Phase
Reference
Waveform Record > 10 K
Waveform Record ≤ 10 K
FFT Time Domain Record =
Waveform Record
Zero Phase
Reference
FFT Time Domain Record = 10k
Figure 3-24: Waveform Record vs. FFT Time Domain Record
FFTs Transform Time Records to Frequency Records — The FFT
time domain record just described is input for the FFT. Figure 3-25 shows the
transformation of that time domain data record into an FFT frequency domain
record. The resulting frequency domain record is one half the length of the
FFT input because the FFT computes both positive and negative frequencies.
Since the negative values mirror the positive values, only the positive values
are displayed.
FFT Time Domain Record
FFT Frequency Domain Record
FFT
Figure 3-25: FFT Time Domain Record vs. FFT Frequency Domain
Record
FFT Frequency Range and Resolution — When you turn on an FFT
waveform, the oscilloscope displays either the magnitude or phase angle of
the FFT frequency domain record. The resolution between the discrete fre-
quencies displayed in this waveform is determined by the following equation:
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