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Fast Fourier Transforms
TDS 620A, 640A & 644A User Manual
3Ć43
Zero Phase
Reference
Waveform Record > 10 K
Waveform Record ≤ 10 K
FFT Time Domain Record =
Waveform Record
Zero Phase
Reference
FFT TimeDomain Record =10k
Figure 3Ć24:ăWaveform Record vs. FFT Time Domain Record
FFTs Transform Time Records to Frequency Records Ċ The FFT
time domain record just described is input for the FFT. Figure 3Ć25 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 3Ć25:ăFFT Time Domain Record vs. FFT Frequency Domain
Record
FFT Frequency Range and Resolution Ċ 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
frequencies displayed in this waveform is determined by the following equaĆ
tion: